diff --git a/src/commands/mod.rs b/src/commands/mod.rs
index 23c7b2bfb..1d794a2bb 100644
--- a/src/commands/mod.rs
+++ b/src/commands/mod.rs
@@ -15,7 +15,7 @@ pub use crate::database::{CoinStatus, LabelItem};
 use bdk_coin_select::InsufficientFunds;
 use utils::{
     deser_addr_assume_checked, deser_amount_from_sats, deser_fromstr, deser_hex,
-    select_coins_for_spend, ser_amount, ser_hex, ser_to_string,
+    select_coins_for_spend, ser_amount, ser_hex, ser_to_string, unsigned_tx_max_vbytes,
 };
 
 use std::{
@@ -1111,6 +1111,12 @@ impl DaemonControl {
         if !is_cancel {
             candidate_coins.extend(&confirmed_cands);
         }
+        let max_sat_weight: u64 = self
+            .config
+            .main_descriptor
+            .max_sat_weight()
+            .try_into()
+            .expect("it must fit");
         // Try with increasing feerate until fee paid by replacement transaction is high enough.
         // Replacement fee must be at least:
         // sum of fees paid by original transactions + incremental feerate * replacement size.
@@ -1145,25 +1151,8 @@ impl DaemonControl {
                     return Err(e);
                 }
             };
-            rbf_vsize = {
-                // TODO: Make this a function (can be used also in `sanity_check`).
-                let witness_factor: u64 =
-                    descriptors::WITNESS_FACTOR.try_into().expect("it must fit");
-                let max_sat_weight: u64 = self
-                    .config
-                    .main_descriptor
-                    .max_sat_weight()
-                    .try_into()
-                    .expect("it must fit");
-                let num_inputs: u64 = rbf_psbt.psbt.inputs.len().try_into().expect("it must fit");
-                let rbf_weight =
-                    rbf_psbt.psbt.unsigned_tx.weight().to_wu() + (max_sat_weight * num_inputs);
-                rbf_weight
-                    .checked_add(witness_factor - 1)
-                    .unwrap()
-                    .checked_div(witness_factor)
-                    .unwrap()
-            };
+            rbf_vsize = unsigned_tx_max_vbytes(&rbf_psbt.psbt.unsigned_tx, max_sat_weight);
+
             println!("replacement vsize estimated at {}", rbf_vsize);
             if rbf_psbt
                 .psbt
diff --git a/src/commands/utils.rs b/src/commands/utils.rs
index ed2301d4c..d5f5e561a 100644
--- a/src/commands/utils.rs
+++ b/src/commands/utils.rs
@@ -5,7 +5,7 @@ use bdk_coin_select::{
 use log::warn;
 use std::{convert::TryInto, str::FromStr};
 
-use miniscript::bitcoin::{self, consensus, hashes::hex::FromHex};
+use miniscript::bitcoin::{self, consensus, constants::WITNESS_SCALE_FACTOR, hashes::hex::FromHex};
 use serde::{de, Deserialize, Deserializer, Serializer};
 
 use crate::database::Coin;
@@ -244,3 +244,26 @@ pub fn select_coins_for_spend(
         change_amount,
     ))
 }
+
+/// An unsigned transaction's maximum possible size in vbytes after satisfaction.
+///
+/// This assumes all inputs are internal (or have the same `max_sat_weight` value).
+///
+/// `tx` is the unsigned transaction.
+///
+/// `max_sat_weight` is the maximum weight difference of an input in the
+/// transaction before and after satisfaction. Must be in weight units.
+pub fn unsigned_tx_max_vbytes(tx: &bitcoin::Transaction, max_sat_weight: u64) -> u64 {
+    let witness_factor: u64 = WITNESS_SCALE_FACTOR.try_into().unwrap();
+    let num_inputs: u64 = tx.input.len().try_into().unwrap();
+    let tx_wu: u64 = tx
+        .weight()
+        .to_wu()
+        .checked_add(max_sat_weight.checked_mul(num_inputs).unwrap())
+        .unwrap();
+    tx_wu
+        .checked_add(witness_factor.checked_sub(1).unwrap())
+        .unwrap()
+        .checked_div(witness_factor)
+        .unwrap()
+}