diff --git a/bitcoin/test/run-tx-bitcoin_tx_2of2_input_witness_weight.c b/bitcoin/test/run-tx-bitcoin_tx_2of2_input_witness_weight.c
new file mode 100644
index 000000000..af5483956
--- /dev/null
+++ b/bitcoin/test/run-tx-bitcoin_tx_2of2_input_witness_weight.c
@@ -0,0 +1,157 @@
+#include "config.h"
+#include <assert.h>
+#include <bitcoin/privkey.c>
+#include <bitcoin/pubkey.c>
+#include <bitcoin/script.c>
+#include <bitcoin/shadouble.c>
+#include <bitcoin/signature.c>
+#include <bitcoin/tx.c>
+#include <bitcoin/varint.c>
+#include <ccan/str/hex/hex.h>
+#include <common/setup.h>
+#include <common/utils.h>
+#include <stdio.h>
+
+/* AUTOGENERATED MOCKS START */
+/* Generated stub for amount_asset_is_main */
+bool amount_asset_is_main(struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "amount_asset_is_main called!\n"); abort(); }
+/* Generated stub for amount_asset_to_sat */
+struct amount_sat amount_asset_to_sat(struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "amount_asset_to_sat called!\n"); abort(); }
+/* Generated stub for amount_sat_add */
+ bool amount_sat_add(struct amount_sat *val UNNEEDED,
+				       struct amount_sat a UNNEEDED,
+				       struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_add called!\n"); abort(); }
+/* Generated stub for amount_sat_eq */
+bool amount_sat_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_eq called!\n"); abort(); }
+/* Generated stub for amount_sat_greater_eq */
+bool amount_sat_greater_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_greater_eq called!\n"); abort(); }
+/* Generated stub for amount_sat_sub */
+ bool amount_sat_sub(struct amount_sat *val UNNEEDED,
+				       struct amount_sat a UNNEEDED,
+				       struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_sub called!\n"); abort(); }
+/* Generated stub for amount_sat_to_asset */
+struct amount_asset amount_sat_to_asset(struct amount_sat *sat UNNEEDED, const u8 *asset UNNEEDED)
+{ fprintf(stderr, "amount_sat_to_asset called!\n"); abort(); }
+/* Generated stub for amount_tx_fee */
+struct amount_sat amount_tx_fee(u32 fee_per_kw UNNEEDED, size_t weight UNNEEDED)
+{ fprintf(stderr, "amount_tx_fee called!\n"); abort(); }
+/* Generated stub for fromwire */
+const u8 *fromwire(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, void *copy UNNEEDED, size_t n UNNEEDED)
+{ fprintf(stderr, "fromwire called!\n"); abort(); }
+/* Generated stub for fromwire_fail */
+void *fromwire_fail(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_fail called!\n"); abort(); }
+/* Generated stub for fromwire_secp256k1_ecdsa_signature */
+void fromwire_secp256k1_ecdsa_signature(const u8 **cursor UNNEEDED, size_t *max UNNEEDED,
+					secp256k1_ecdsa_signature *signature UNNEEDED)
+{ fprintf(stderr, "fromwire_secp256k1_ecdsa_signature called!\n"); abort(); }
+/* Generated stub for fromwire_sha256 */
+void fromwire_sha256(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, struct sha256 *sha256 UNNEEDED)
+{ fprintf(stderr, "fromwire_sha256 called!\n"); abort(); }
+/* Generated stub for fromwire_u32 */
+u32 fromwire_u32(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_u32 called!\n"); abort(); }
+/* Generated stub for fromwire_u8 */
+u8 fromwire_u8(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_u8 called!\n"); abort(); }
+/* Generated stub for fromwire_u8_array */
+void fromwire_u8_array(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, u8 *arr UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "fromwire_u8_array called!\n"); abort(); }
+/* Generated stub for fromwire_wally_psbt */
+struct wally_psbt *fromwire_wally_psbt(const tal_t *ctx UNNEEDED,
+				       const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_wally_psbt called!\n"); abort(); }
+/* Generated stub for new_psbt */
+struct wally_psbt *new_psbt(const tal_t *ctx UNNEEDED,
+			    const struct wally_tx *wtx UNNEEDED)
+{ fprintf(stderr, "new_psbt called!\n"); abort(); }
+/* Generated stub for psbt_add_output */
+struct wally_psbt_output *psbt_add_output(struct wally_psbt *psbt UNNEEDED,
+					  struct wally_tx_output *output UNNEEDED,
+					  size_t insert_at UNNEEDED)
+{ fprintf(stderr, "psbt_add_output called!\n"); abort(); }
+/* Generated stub for psbt_append_input */
+struct wally_psbt_input *psbt_append_input(struct wally_psbt *psbt UNNEEDED,
+					   const struct bitcoin_outpoint *outpoint UNNEEDED,
+					   u32 sequence UNNEEDED,
+					   const u8 *scriptSig UNNEEDED,
+					   const u8 *input_wscript UNNEEDED,
+					   const u8 *redeemscript UNNEEDED)
+{ fprintf(stderr, "psbt_append_input called!\n"); abort(); }
+/* Generated stub for psbt_elements_input_set_asset */
+void psbt_elements_input_set_asset(struct wally_psbt *psbt UNNEEDED, size_t in UNNEEDED,
+				   struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "psbt_elements_input_set_asset called!\n"); abort(); }
+/* Generated stub for psbt_final_tx */
+struct wally_tx *psbt_final_tx(const tal_t *ctx UNNEEDED, const struct wally_psbt *psbt UNNEEDED)
+{ fprintf(stderr, "psbt_final_tx called!\n"); abort(); }
+/* Generated stub for psbt_finalize */
+bool psbt_finalize(struct wally_psbt *psbt UNNEEDED)
+{ fprintf(stderr, "psbt_finalize called!\n"); abort(); }
+/* Generated stub for psbt_input_get_amount */
+struct amount_sat psbt_input_get_amount(const struct wally_psbt *psbt UNNEEDED,
+					size_t in UNNEEDED)
+{ fprintf(stderr, "psbt_input_get_amount called!\n"); abort(); }
+/* Generated stub for psbt_input_set_wit_utxo */
+void psbt_input_set_wit_utxo(struct wally_psbt *psbt UNNEEDED, size_t in UNNEEDED,
+			     const u8 *scriptPubkey UNNEEDED, struct amount_sat amt UNNEEDED)
+{ fprintf(stderr, "psbt_input_set_wit_utxo called!\n"); abort(); }
+/* Generated stub for towire */
+void towire(u8 **pptr UNNEEDED, const void *data UNNEEDED, size_t len UNNEEDED)
+{ fprintf(stderr, "towire called!\n"); abort(); }
+/* Generated stub for towire_secp256k1_ecdsa_signature */
+void towire_secp256k1_ecdsa_signature(u8 **pptr UNNEEDED,
+			      const secp256k1_ecdsa_signature *signature UNNEEDED)
+{ fprintf(stderr, "towire_secp256k1_ecdsa_signature called!\n"); abort(); }
+/* Generated stub for towire_sha256 */
+void towire_sha256(u8 **pptr UNNEEDED, const struct sha256 *sha256 UNNEEDED)
+{ fprintf(stderr, "towire_sha256 called!\n"); abort(); }
+/* Generated stub for towire_u32 */
+void towire_u32(u8 **pptr UNNEEDED, u32 v UNNEEDED)
+{ fprintf(stderr, "towire_u32 called!\n"); abort(); }
+/* Generated stub for towire_u8 */
+void towire_u8(u8 **pptr UNNEEDED, u8 v UNNEEDED)
+{ fprintf(stderr, "towire_u8 called!\n"); abort(); }
+/* Generated stub for towire_u8_array */
+void towire_u8_array(u8 **pptr UNNEEDED, const u8 *arr UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "towire_u8_array called!\n"); abort(); }
+/* Generated stub for towire_wally_psbt */
+void towire_wally_psbt(u8 **pptr UNNEEDED, const struct wally_psbt *psbt UNNEEDED)
+{ fprintf(stderr, "towire_wally_psbt called!\n"); abort(); }
+/* AUTOGENERATED MOCKS END */
+
+int main(int argc, const char *argv[])
+{
+	struct privkey p;
+	struct pubkey pub;
+	struct bitcoin_signature sig;
+	struct sha256_double h;
+	u8 **wit;
+	size_t weight;
+
+	common_setup(argv[0]);
+	chainparams = chainparams_for_network("bitcoin");
+
+	memset(&h, 0, sizeof(h));
+	memset(&p, 1, sizeof(p));
+	sign_hash(&p, &h, &sig.s);
+	sig.sighash_type = SIGHASH_ALL;
+	pubkey_from_privkey(&p, &pub);
+
+	wit = bitcoin_witness_2of2(tmpctx, &sig, &sig, &pub, &pub);
+
+	/* 1 byte for num witnesses, one per witness element */
+	weight = 1;
+	for (size_t i = 0; i < tal_count(wit); i++)
+		weight += 1 + tal_bytelen(wit[i]);
+	assert(bitcoin_tx_2of2_input_witness_weight() == weight);
+
+	common_shutdown();
+	return 0;
+}
diff --git a/bitcoin/tx.c b/bitcoin/tx.c
index 76d47b173..215d2e25f 100644
--- a/bitcoin/tx.c
+++ b/bitcoin/tx.c
@@ -884,6 +884,16 @@ size_t bitcoin_tx_simple_input_weight(bool p2sh)
 				       bitcoin_tx_simple_input_witness_weight());
 }
 
+size_t bitcoin_tx_2of2_input_witness_weight(void)
+{
+	/* witness[0] = ""
+	 * witness[1] = sig
+	 * witness[2] = sig
+	 * witness[3] = 2 key key 2 CHECKMULTISIG
+	 */
+	return 1 + (1 + 0) + (1 + 72) + (1 + 72) + (1 + 1 + 33 + 33 + 1 + 1);
+}
+
 struct amount_sat change_amount(struct amount_sat excess, u32 feerate_perkw,
 				size_t total_weight)
 {
diff --git a/bitcoin/tx.h b/bitcoin/tx.h
index 5bccaf7b6..694cc4a75 100644
--- a/bitcoin/tx.h
+++ b/bitcoin/tx.h
@@ -271,6 +271,9 @@ size_t bitcoin_tx_simple_input_witness_weight(void);
 /* We only do segwit inputs, and we assume witness is sig + key  */
 size_t bitcoin_tx_simple_input_weight(bool p2sh);
 
+/* The witness for our 2of2 input (closing or commitment tx). */
+size_t bitcoin_tx_2of2_input_witness_weight(void);
+
 /**
  * change_amount - Is it worth making a P2WPKH change output at this feerate?
  * @excess: input amount we have above the tx fee and other outputs.