diff --git a/package.json b/package.json
index 2d131c6..f8b8462 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
     "name": "@polymarket/clob-client",
     "description": "Typescript client for Polymarket's CLOB",
-    "version": "4.5.4",
+    "version": "4.5.5",
     "contributors": [
         {
             "name": "Jonathan Amenechi",
diff --git a/src/client.ts b/src/client.ts
index c13ccb7..df8abcb 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -103,6 +103,7 @@ import {
 } from "./endpoints";
 import { OrderBuilder } from "./order-builder/builder";
 import { END_CURSOR, INITIAL_CURSOR } from "./constants";
+import { calculateMarketPrice } from "./order-builder/helpers";
 
 export class ClobClient {
     readonly host: string;
@@ -507,8 +508,11 @@ export class ClobClient {
         const negRisk = options?.negRisk ?? false;
 
         if (!userMarketOrder.price) {
-            const marketPrice = await this.getPrice(tokenID, Side.BUY);
-            userMarketOrder.price = parseFloat(marketPrice);
+            userMarketOrder.price = await this.calculateMarketPrice(
+                tokenID,
+                Side.BUY,
+                userMarketOrder.amount,
+            );
         }
 
         if (!priceValid(userMarketOrder.price, tickSize)) {
@@ -824,6 +828,28 @@ export class ClobClient {
         return this.get(`${this.host}${GET_MARKET_TRADES_EVENTS}${conditionID}`);
     }
 
+    public async calculateMarketPrice(
+        tokenID: string,
+        side: Side,
+        amount: number,
+    ): Promise<number> {
+        const book = await this.getOrderBook(tokenID);
+        if (!book) {
+            throw new Error("no orderbook");
+        }
+        if (side === Side.BUY) {
+            if (!book.asks) {
+                throw new Error("no match");
+            }
+            return calculateMarketPrice(book.asks, amount);
+        } else {
+            if (!book.bids) {
+                throw new Error("no match");
+            }
+            return calculateMarketPrice(book.bids, amount);
+        }
+    }
+
     private canL1Auth(): void {
         if (this.signer === undefined) {
             throw L1_AUTH_UNAVAILABLE_ERROR;
diff --git a/src/order-builder/helpers.ts b/src/order-builder/helpers.ts
index 7d01698..ad0143a 100644
--- a/src/order-builder/helpers.ts
+++ b/src/order-builder/helpers.ts
@@ -16,6 +16,7 @@ import {
     TickSize,
     RoundConfig,
     CreateOrderOptions,
+    OrderSummary,
 } from "../types";
 import { decimalPlaces, roundDown, roundNormal, roundUp } from "../utilities";
 import {
@@ -268,7 +269,7 @@ export const buildMarketBuyOrderCreationArgs = async (
         feeRateBps,
         nonce,
         signer,
-        expiration: (userMarketOrder.expiration || 0).toString(),
+        expiration: "0",
         signatureType,
     } as OrderData;
 };
@@ -301,3 +302,15 @@ export const createMarketBuyOrder = async (
 
     return buildOrder(eoaSigner, exchangeContract, chainId, orderData);
 };
+
+export const calculateMarketPrice = (positions: OrderSummary[], amountToMatch: number) => {
+    let sum = 0;
+    for (let i = 0; i < positions.length; i++) {
+        const p = positions[i];
+        sum += parseFloat(p.size) * parseFloat(p.price);
+        if (sum >= amountToMatch) {
+            return parseFloat(p.price);
+        }
+    }
+    throw new Error("no match");
+};
diff --git a/src/types.ts b/src/types.ts
index c4fa537..15043bf 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -138,11 +138,6 @@ export interface UserMarketOrder {
      */
     nonce?: number;
 
-    /**
-     * Timestamp after which the order is expired.
-     */
-    expiration?: number;
-
     /**
      * Address of the order taker. The zero address is used to indicate a public order
      */
diff --git a/tests/order-builder/helpers.test.ts b/tests/order-builder/helpers.test.ts
index ce0be73..925b2e5 100644
--- a/tests/order-builder/helpers.test.ts
+++ b/tests/order-builder/helpers.test.ts
@@ -1,6 +1,6 @@
 import "mocha";
 import { expect } from "chai";
-import { UserOrder, Side, Chain, UserMarketOrder } from "../../src/types";
+import { UserOrder, Side, Chain, UserMarketOrder, OrderSummary } from "../../src/types";
 import {
     buildOrderCreationArgs,
     buildOrder,
@@ -10,6 +10,7 @@ import {
     getOrderRawAmounts,
     getMarketBuyOrderRawAmounts,
     ROUNDING_CONFIG,
+    calculateMarketPrice,
 } from "../../src/order-builder/helpers";
 import { OrderData, SignatureType, Side as UtilsSide } from "@polymarket/order-utils";
 import { Wallet } from "@ethersproject/wallet";
@@ -2475,7 +2476,6 @@ describe("helpers", () => {
                     amount: 100,
                     feeRateBps: 111,
                     nonce: 123,
-                    expiration: 50000,
                 };
                 const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                     "0x0000000000000000000000000000000000000001",
@@ -2494,7 +2494,7 @@ describe("helpers", () => {
                     feeRateBps: "111",
                     nonce: "123",
                     signer: "0x0000000000000000000000000000000000000001",
-                    expiration: "50000",
+                    expiration: "0",
                     signatureType: SignatureType.EOA,
                 });
             });
@@ -2506,7 +2506,6 @@ describe("helpers", () => {
                     amount: 100,
                     feeRateBps: 111,
                     nonce: 123,
-                    expiration: 50000,
                 };
                 const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                     "0x0000000000000000000000000000000000000001",
@@ -2525,7 +2524,7 @@ describe("helpers", () => {
                     feeRateBps: "111",
                     nonce: "123",
                     signer: "0x0000000000000000000000000000000000000001",
-                    expiration: "50000",
+                    expiration: "0",
                     signatureType: SignatureType.EOA,
                 });
             });
@@ -2537,7 +2536,6 @@ describe("helpers", () => {
                     amount: 100,
                     feeRateBps: 111,
                     nonce: 123,
-                    expiration: 50000,
                 };
                 const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                     "0x0000000000000000000000000000000000000001",
@@ -2556,7 +2554,7 @@ describe("helpers", () => {
                     feeRateBps: "111",
                     nonce: "123",
                     signer: "0x0000000000000000000000000000000000000001",
-                    expiration: "50000",
+                    expiration: "0",
                     signatureType: SignatureType.EOA,
                 });
             });
@@ -2568,7 +2566,6 @@ describe("helpers", () => {
                     amount: 100,
                     feeRateBps: 111,
                     nonce: 123,
-                    expiration: 50000,
                 };
                 const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                     "0x0000000000000000000000000000000000000001",
@@ -2587,7 +2584,7 @@ describe("helpers", () => {
                     feeRateBps: "111",
                     nonce: "123",
                     signer: "0x0000000000000000000000000000000000000001",
-                    expiration: "50000",
+                    expiration: "0",
                     signatureType: SignatureType.EOA,
                 });
             });
@@ -2602,7 +2599,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
                     const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                         "0x0000000000000000000000000000000000000001",
@@ -2621,7 +2617,7 @@ describe("helpers", () => {
                         feeRateBps: "111",
                         nonce: "123",
                         signer: "0x0000000000000000000000000000000000000001",
-                        expiration: "50000",
+                        expiration: "0",
                         signatureType: SignatureType.EOA,
                     });
                 });
@@ -2788,7 +2784,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
                     const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                         "0x0000000000000000000000000000000000000001",
@@ -2807,7 +2802,7 @@ describe("helpers", () => {
                         feeRateBps: "111",
                         nonce: "123",
                         signer: "0x0000000000000000000000000000000000000001",
-                        expiration: "50000",
+                        expiration: "0",
                         signatureType: SignatureType.EOA,
                     });
                 });
@@ -2974,7 +2969,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
                     const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                         "0x0000000000000000000000000000000000000001",
@@ -2993,7 +2987,7 @@ describe("helpers", () => {
                         feeRateBps: "111",
                         nonce: "123",
                         signer: "0x0000000000000000000000000000000000000001",
-                        expiration: "50000",
+                        expiration: "0",
                         signatureType: SignatureType.EOA,
                     });
                 });
@@ -3160,7 +3154,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
                     const orderData: OrderData = await buildMarketBuyOrderCreationArgs(
                         "0x0000000000000000000000000000000000000001",
@@ -3179,7 +3172,7 @@ describe("helpers", () => {
                         feeRateBps: "111",
                         nonce: "123",
                         signer: "0x0000000000000000000000000000000000000001",
-                        expiration: "50000",
+                        expiration: "0",
                         signatureType: SignatureType.EOA,
                     });
                 });
@@ -3350,7 +3343,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3372,7 +3364,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("200000000");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3386,7 +3378,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3408,7 +3399,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("178571400");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3422,7 +3413,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3444,7 +3434,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("1785714280");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3458,7 +3448,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3480,7 +3469,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("17857142857");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3489,7 +3478,7 @@ describe("helpers", () => {
             });
         });
 
-        describe("CTF Exchange", () => {
+        describe("Neg Risk CTF Exchange", () => {
             describe("buy order", async () => {
                 it("0.1", async () => {
                     const order: UserMarketOrder = {
@@ -3498,7 +3487,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3520,7 +3508,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("200000000");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3534,7 +3522,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3556,7 +3543,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("178571400");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3570,7 +3557,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3592,7 +3578,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("1785714280");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3606,7 +3592,6 @@ describe("helpers", () => {
                         amount: 100,
                         feeRateBps: 111,
                         nonce: 123,
-                        expiration: 50000,
                     };
 
                     const signedOrder = await createMarketBuyOrder(
@@ -3628,7 +3613,7 @@ describe("helpers", () => {
                     expect(signedOrder.makerAmount).equal("100000000");
                     expect(signedOrder.takerAmount).equal("17857142857");
                     expect(signedOrder.side).equal(UtilsSide.BUY);
-                    expect(signedOrder.expiration).equal("50000");
+                    expect(signedOrder.expiration).equal("0");
                     expect(signedOrder.nonce).equal("123");
                     expect(signedOrder.feeRateBps).equal("111");
                     expect(signedOrder.signatureType).equal(SignatureType.EOA);
@@ -3637,4 +3622,96 @@ describe("helpers", () => {
             });
         });
     });
+
+    describe.only("calculateMarketPrice", () => {
+        describe("BUY", () => {
+            it("empty orderbook", () => {
+                expect(() => calculateMarketPrice([], 100)).to.throw("no match");
+            });
+            it("not enough", () => {
+                const positions = [
+                    { price: "0.5", size: "100" },
+                    { price: "0.4", size: "100" },
+                ] as OrderSummary[];
+                expect(() => calculateMarketPrice(positions, 100)).to.throw("no match");
+            });
+            it("ok", () => {
+                let positions = [
+                    { price: "0.5", size: "100" },
+                    { price: "0.4", size: "100" },
+                    { price: "0.3", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.3);
+
+                positions = [
+                    { price: "0.5", size: "100" },
+                    { price: "0.4", size: "200" },
+                    { price: "0.3", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.4);
+
+                positions = [
+                    { price: "0.5", size: "120" },
+                    { price: "0.4", size: "100" },
+                    { price: "0.3", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.4);
+
+                positions = [
+                    { price: "0.5", size: "200" },
+                    { price: "0.4", size: "100" },
+                    { price: "0.3", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.5);
+            });
+        });
+        describe("SELL", () => {
+            it("empty orderbook", () => {
+                expect(() => calculateMarketPrice([], 100)).to.throw("no match");
+            });
+            it("not enough", () => {
+                const positions = [
+                    { price: "0.4", size: "100" },
+                    { price: "0.5", size: "100" },
+                ] as OrderSummary[];
+                expect(() => calculateMarketPrice(positions, 100)).to.throw("no match");
+            });
+            it("ok", () => {
+                let positions = [
+                    { price: "0.3", size: "100" },
+                    { price: "0.4", size: "100" },
+                    { price: "0.5", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.5);
+
+                positions = [
+                    { price: "0.3", size: "100" },
+                    { price: "0.4", size: "300" },
+                    { price: "0.5", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.4);
+
+                positions = [
+                    { price: "0.3", size: "100" },
+                    { price: "0.4", size: "200" },
+                    { price: "0.5", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.4);
+
+                positions = [
+                    { price: "0.3", size: "300" },
+                    { price: "0.4", size: "100" },
+                    { price: "0.5", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.4);
+
+                positions = [
+                    { price: "0.3", size: "334" },
+                    { price: "0.4", size: "100" },
+                    { price: "0.5", size: "100" },
+                ] as OrderSummary[];
+                expect(calculateMarketPrice(positions, 100)).equal(0.3);
+            });
+        });
+    });
 });