diff --git a/coinlib/lib/src/tx/inputs/input.dart b/coinlib/lib/src/tx/inputs/input.dart
index 882d776..e5178d1 100644
--- a/coinlib/lib/src/tx/inputs/input.dart
+++ b/coinlib/lib/src/tx/inputs/input.dart
@@ -21,6 +21,12 @@ abstract class Input with Writable {
   /// True when the input is fully signed and ready for broadcast
   bool get complete;
 
+  /// The maximum total size when fully signed via the default hash type
+  /// including any witness data of the input. If this is unknown, this is
+  /// null. The actual signed size may be lower according to the data being
+  /// encoded.
+  int? get signedSize => null;
+
   Input();
 
   /// Given a [RawInput] and witness data, the specific [Input] subclass is
diff --git a/coinlib/lib/src/tx/inputs/p2pkh_input.dart b/coinlib/lib/src/tx/inputs/p2pkh_input.dart
index 2b15836..d9bbcf0 100644
--- a/coinlib/lib/src/tx/inputs/p2pkh_input.dart
+++ b/coinlib/lib/src/tx/inputs/p2pkh_input.dart
@@ -23,6 +23,8 @@ class P2PKHInput extends LegacyInput with PKHInput {
   final ECPublicKey publicKey;
   @override
   final ECDSAInputSignature? insig;
+  @override
+  final int? signedSize = 147;
 
   P2PKHInput({
     required OutPoint prevOut,
diff --git a/coinlib/lib/src/tx/inputs/p2sh_multisig_input.dart b/coinlib/lib/src/tx/inputs/p2sh_multisig_input.dart
index 2bcccb6..91df1e1 100644
--- a/coinlib/lib/src/tx/inputs/p2sh_multisig_input.dart
+++ b/coinlib/lib/src/tx/inputs/p2sh_multisig_input.dart
@@ -200,4 +200,17 @@ class P2SHMultisigInput extends LegacyInput {
   @override
   Script get script => super.script!;
 
+  int get _signedScriptSize
+    => 1 // Extra 0
+    + program.threshold*73 // Add 73 bytes per signature
+    // Determine the length of the program pushdata by actually compiling it.
+    // Not the most efficient but the simplest solution.
+    + ScriptPushData(program.script.compiled).compiled.length;
+
+  @override
+  int? get signedSize
+    => 40 // Outpoint plus sequence
+    + _signedScriptSize
+    + (_signedScriptSize < 0xfd ? 1 : 3); // Varint size
+
 }
diff --git a/coinlib/lib/src/tx/inputs/p2wpkh_input.dart b/coinlib/lib/src/tx/inputs/p2wpkh_input.dart
index c4998d2..c00d64c 100644
--- a/coinlib/lib/src/tx/inputs/p2wpkh_input.dart
+++ b/coinlib/lib/src/tx/inputs/p2wpkh_input.dart
@@ -22,6 +22,8 @@ class P2WPKHInput extends LegacyWitnessInput with PKHInput {
   final ECPublicKey publicKey;
   @override
   final ECDSAInputSignature? insig;
+  @override
+  final int? signedSize = 147;
 
   P2WPKHInput({
     required OutPoint prevOut,
diff --git a/coinlib/lib/src/tx/inputs/taproot_key_input.dart b/coinlib/lib/src/tx/inputs/taproot_key_input.dart
index 0cb919f..4408d00 100644
--- a/coinlib/lib/src/tx/inputs/taproot_key_input.dart
+++ b/coinlib/lib/src/tx/inputs/taproot_key_input.dart
@@ -16,6 +16,10 @@ class TaprootKeyInput extends TaprootInput {
 
   final SchnorrInputSignature? insig;
 
+  @override
+  // 64-bit sig plus varint with default sighash type
+  final int? signedSize = 41 + 65;
+
   TaprootKeyInput({
     required OutPoint prevOut,
     this.insig,
diff --git a/coinlib/test/tx/transaction_test.dart b/coinlib/test/tx/transaction_test.dart
index 86cb1ac..1d59935 100644
--- a/coinlib/test/tx/transaction_test.dart
+++ b/coinlib/test/tx/transaction_test.dart
@@ -40,6 +40,10 @@ void main() {
       expect(mapToHex(tx.outputs), mapToHex(vec.obj.outputs));
     }
 
+    expectInputSignedSize(Input input) => expect(
+      input.size, lessThanOrEqualTo(input.signedSize!),
+    );
+
     test("valid txs", () {
       for (final vec in validTxVecs) {
 
@@ -331,6 +335,7 @@ void main() {
             key: keyVec.privateObj,
             hashType: hashType,
           );
+          expectInputSignedSize(signed.inputs[i]);
         }
 
         expect(tx.complete, false);
@@ -426,6 +431,7 @@ void main() {
         inputN: 1, key: keyVec.privateObj, value: BigInt.from(3000000),
       );
       expect(signed.complete, true);
+      expectInputSignedSize(signed.inputs[1]);
 
       expect(
         signed.toHex(),
@@ -479,6 +485,9 @@ void main() {
       );
 
       expect(signed.complete, true);
+      for (final input in signed.inputs) {
+        expectInputSignedSize(input);
+      }
 
       expect(
         signed.toHex(),
@@ -602,6 +611,8 @@ void main() {
         outputs: [exampleOutput, exampleOutput],
       );
 
+      final signedSizeFromUnsigned = tx.inputs[1].signedSize;
+
       // Sign first P2PKH input
       var signed = tx.sign(inputN: 0, key: privkeys[0]);
       expect(signed.complete, false);
@@ -638,6 +649,8 @@ void main() {
 
       // Check final tx
       expect(signed.complete, true);
+      expectInputSignedSize(signed.inputs[1]);
+      expect(signed.inputs[1].signedSize, signedSizeFromUnsigned);
       expectMultisigSigs(
         signed, [
           SigHashType.single(),