diff --git a/rhp/v2/merkle.go b/rhp/v2/merkle.go
index ba026283..a676295d 100644
--- a/rhp/v2/merkle.go
+++ b/rhp/v2/merkle.go
@@ -435,6 +435,29 @@ func VerifyAppendProof(numLeaves uint64, treeHashes []types.Hash256, sectorRoot,
 	return acc.root() == newRoot
 }
 
+// DiffProofSize returns the size of a Merkle diff proof for the specified
+// actions within a tree containing numLeaves leaves.
+func DiffProofSize(actions []RPCWriteAction, numLeaves uint64) (numHashes int) {
+	indices := sectorsChanged(actions, numLeaves)
+	numHashes += len(indices)
+
+	buildRange := func(i, j uint64) {
+		for i < j {
+			subtreeSize := nextSubtreeSize(i, j)
+			numHashes++
+			i += subtreeSize
+		}
+	}
+
+	var start uint64
+	for _, end := range indices {
+		buildRange(start, end)
+		start = end + 1
+	}
+	buildRange(start, numLeaves)
+	return
+}
+
 // BuildDiffProof constructs a diff proof for the specified actions.
 // ActionUpdate is not supported.
 func BuildDiffProof(actions []RPCWriteAction, sectorRoots []types.Hash256) (treeHashes, leafHashes []types.Hash256) {
diff --git a/v2/net/rhp/merkle.go b/v2/net/rhp/merkle.go
index 3547acb7..5067ac5e 100644
--- a/v2/net/rhp/merkle.go
+++ b/v2/net/rhp/merkle.go
@@ -6,6 +6,7 @@ import (
 	"io"
 	"math"
 	"math/bits"
+	"sort"
 	"unsafe"
 
 	"go.sia.tech/core/v2/internal/blake2b"
@@ -244,8 +245,61 @@ func RangeProofSize(n, start, end uint64) uint64 {
 
 // DiffProofSize returns the size of a Merkle diff proof for the specified
 // actions within a tree containing n leaves.
-func DiffProofSize(n int, actions []RPCWriteAction) int {
-	return 128 // TODO
+func DiffProofSize(n uint64, actions []RPCWriteAction) (numHashes int) {
+	indices := sectorsChanged(actions, uint64(n))
+	numHashes += len(indices)
+
+	buildRange := func(i, j uint64) {
+		for i < j {
+			subtreeSize := nextSubtreeSize(i, j)
+			numHashes++
+			i += subtreeSize
+		}
+	}
+
+	var start uint64
+	for _, end := range indices {
+		buildRange(start, end)
+		start = end + 1
+	}
+	buildRange(start, n)
+	return
+}
+
+func sectorsChanged(actions []RPCWriteAction, numSectors uint64) []uint64 {
+	newNumSectors := numSectors
+	sectorsChanged := make(map[uint64]struct{})
+	for _, action := range actions {
+		switch action.Type {
+		case RPCWriteActionAppend:
+			sectorsChanged[newNumSectors] = struct{}{}
+			newNumSectors++
+
+		case RPCWriteActionTrim:
+			for i := uint64(0); i < action.A; i++ {
+				newNumSectors--
+				sectorsChanged[newNumSectors] = struct{}{}
+			}
+
+		case RPCWriteActionSwap:
+			sectorsChanged[action.A] = struct{}{}
+			sectorsChanged[action.B] = struct{}{}
+
+		default:
+			panic("unknown or unsupported action type: " + action.Type.String())
+		}
+	}
+
+	var sectorIndices []uint64
+	for index := range sectorsChanged {
+		if index < numSectors {
+			sectorIndices = append(sectorIndices, index)
+		}
+	}
+	sort.Slice(sectorIndices, func(i, j int) bool {
+		return sectorIndices[i] < sectorIndices[j]
+	})
+	return sectorIndices
 }
 
 // nextSubtreeSize returns the size of the subtree adjacent to start that does
diff --git a/v2/net/rhp/rpc.go b/v2/net/rhp/rpc.go
index 38fe376c..c8e7745a 100644
--- a/v2/net/rhp/rpc.go
+++ b/v2/net/rhp/rpc.go
@@ -231,7 +231,7 @@ func RPCWriteRenterCost(settings HostSettings, fc types.FileContract, actions []
 		sectorStoragePrice := settings.StoragePrice.Mul64(SectorSize).Mul64(storageDuration)
 		storageCost = sectorStoragePrice.Mul64(sectorsAdded - sectorsRemoved)
 	}
-	proofSize := DiffProofSize(int(fc.Filesize/SectorSize), actions)
+	proofSize := DiffProofSize(fc.Filesize/SectorSize, actions)
 	downloadBandwidth := uint64(proofSize) * 32
 	return settings.InstrWriteBaseCost.
 		Add(settings.UploadBandwidthPrice.Mul64(sectorsAdded * SectorSize)).