diff --git a/packages/cashscript/src/TransactionBuilder.ts b/packages/cashscript/src/TransactionBuilder.ts
index bddbb2985..7e0e54368 100644
--- a/packages/cashscript/src/TransactionBuilder.ts
+++ b/packages/cashscript/src/TransactionBuilder.ts
@@ -19,9 +19,11 @@ import {
   StandardUnlockableUtxo,
   VmResourceUsage,
   isContractUnlocker,
+  isPlaceholderUnlocker,
   BchChangeOutputOptions,
   TokenChangeOutputOptions,
 } from './interfaces.js';
+import { PLACEHOLDER_P2PKH_UNLOCKING_SIZE } from './constants.js';
 import { NetworkProvider } from './network/index.js';
 import {
   calculateDust,
@@ -278,12 +280,19 @@ export class TransactionBuilder {
 
   /**
    * Build the transaction (skipping fee and burn checks) and return its encoded byte length.
+   * Inputs with a placeholder unlocker are counted at the size they take up once the wallet signs them.
    *
    * @returns The size of the transaction in bytes.
    */
   getTransactionSize(): bigint {
     const transaction = this.buildLibauthTransaction(true);
-    return BigInt(encodeTransaction(transaction).byteLength);
+    return BigInt(this.getEncodedTransactionSize(transaction));
+  }
+
+  // Placeholder unlockers serialise as an empty unlocking script, so their eventual signed size is added here
+  private getEncodedTransactionSize(transaction: LibauthTransaction): number {
+    const placeholderInputCount = this.inputs.filter((input) => isPlaceholderUnlocker(input.unlocker)).length;
+    return encodeTransaction(transaction).byteLength + placeholderInputCount * PLACEHOLDER_P2PKH_UNLOCKING_SIZE;
   }
 
   /**
@@ -561,7 +570,7 @@ export class TransactionBuilder {
   private checkFee(transaction: LibauthTransaction): void {
     const totalInputAmount = this.inputs.reduce((total, input) => total + input.satoshis, 0n);
     const totalOutputAmount = this.outputs.reduce((total, output) => total + output.amount, 0n);
-    const transactionSize = encodeTransaction(transaction).byteLength;
+    const transactionSize = this.getEncodedTransactionSize(transaction);
 
     const fee = totalInputAmount - totalOutputAmount;
     const feePerByte = Number((Number(fee) / transactionSize).toFixed(2));
@@ -623,7 +632,7 @@ export class TransactionBuilder {
   }
 
   private checkTransactionSize(transaction: LibauthTransaction): void {
-    const transactionSize = encodeTransaction(transaction).byteLength;
+    const transactionSize = this.getEncodedTransactionSize(transaction);
 
     const TX_MAX_STANDARD_SIZE = 100_000;
     if (transactionSize > TX_MAX_STANDARD_SIZE) {
diff --git a/packages/cashscript/src/constants.ts b/packages/cashscript/src/constants.ts
index 3093f1d2e..f9f9871de 100644
--- a/packages/cashscript/src/constants.ts
+++ b/packages/cashscript/src/constants.ts
@@ -1,3 +1,4 @@
 export const VERSION_SIZE = 4;
 export const LOCKTIME_SIZE = 4;
-export const P2PKH_INPUT_SIZE = 32 + 4 + 1 + 1 + 65 + 1 + 33 + 4;
+// Size of a placeholder P2PKH unlocking script using Schnorr signatures: push(signature) push(pk).
+export const PLACEHOLDER_P2PKH_UNLOCKING_SIZE = 1 + 65 + 1 + 33;
diff --git a/packages/cashscript/test/TransactionBuilder.test.ts b/packages/cashscript/test/TransactionBuilder.test.ts
index fe9b9922e..bb81be5b7 100644
--- a/packages/cashscript/test/TransactionBuilder.test.ts
+++ b/packages/cashscript/test/TransactionBuilder.test.ts
@@ -250,6 +250,41 @@ describe('Transaction Builder', () => {
     });
   });
 
+  describe('test TransactionBuilder.getTransactionSize', () => {
+    it('should size placeholder P2PKH inputs as signed inputs', async () => {
+      const aliceUtxos = (await provider.getUtxos(aliceAddress)).filter(isNonTokenUtxo);
+
+      const placeholderSize = new TransactionBuilder({ provider })
+        .addInput(aliceUtxos[0], placeholderP2PKHUnlocker(aliceAddress))
+        .addOutput({ to: aliceAddress, amount: 1000n })
+        .getTransactionSize();
+
+      const signedSize = new TransactionBuilder({ provider })
+        .addInput(aliceUtxos[0], new SignatureTemplate(alicePriv).unlockP2PKH())
+        .addOutput({ to: aliceAddress, amount: 1000n })
+        .getTransactionSize();
+
+      // the wallet signs with a 65-byte Schnorr signature, so the placeholder is sized exactly like the signed input
+      expect(placeholderSize).toBe(signedSize);
+    });
+
+    it('should pay exactly the fee rate once a placeholder input is signed', async () => {
+      const aliceUtxos = (await provider.getUtxos(aliceAddress)).filter(isNonTokenUtxo);
+
+      const builder = new TransactionBuilder({ provider })
+        .addInput(aliceUtxos[0], placeholderP2PKHUnlocker(aliceAddress))
+        .addOutput({ to: aliceAddress, amount: 1000n })
+        .addBchChangeOutputIfNeeded({ to: aliceAddress, feeRate: 1 });
+
+      const signedSize = new TransactionBuilder({ provider })
+        .addInput(aliceUtxos[0], new SignatureTemplate(alicePriv).unlockP2PKH())
+        .addOutputs(builder.outputs)
+        .getTransactionSize();
+
+      expect(builder.calculateTransactionFee().feeSats).toBe(signedSize);
+    });
+  });
+
   it('should not fail when validly spending from only P2PKH inputs', async () => {
     const aliceUtxos = (await provider.getUtxos(aliceAddress)).filter(isNonTokenUtxo);
     const sigTemplate = new SignatureTemplate(alicePriv);