diff --git a/test/functional/test_framework/util.py b/test/functional/test_framework/util.py
index 9f5bca6884d..0678e2e6fe2 100644
--- a/test/functional/test_framework/util.py
+++ b/test/functional/test_framework/util.py
@@ -36,12 +36,12 @@ def assert_approx(v, vexp, vspan=0.00001):
 
 def assert_fee_amount(fee, tx_size, feerate_BTC_kvB):
     """Assert the fee is in range."""
-    feerate_BTC_vB = feerate_BTC_kvB / 1000
-    target_fee = satoshi_round(tx_size * feerate_BTC_vB)
+    target_fee = get_fee(tx_size, feerate_BTC_kvB)
     if fee < target_fee:
         raise AssertionError("Fee of %s BTC too low! (Should be %s BTC)" % (str(fee), str(target_fee)))
     # allow the wallet's estimation to be at most 2 bytes off
-    if fee > (tx_size + 2) * feerate_BTC_vB:
+    high_fee = get_fee(tx_size + 2, feerate_BTC_kvB)
+    if fee > high_fee:
         raise AssertionError("Fee of %s BTC too high! (Should be %s BTC)" % (str(fee), str(target_fee)))
 
 
@@ -218,6 +218,18 @@ def str_to_b64str(string):
     return b64encode(string.encode('utf-8')).decode('ascii')
 
 
+def ceildiv(a, b):
+    """Divide 2 ints and round up to next int rather than round down"""
+    return -(-a // b)
+
+
+def get_fee(tx_size, feerate_btc_kvb):
+    """Calculate the fee in BTC given a feerate is BTC/kvB. Reflects CFeeRate::GetFee"""
+    feerate_sat_kvb = int(feerate_btc_kvb * Decimal(1e8)) # Fee in sat/kvb as an int to avoid float precision errors
+    target_fee_sat = ceildiv(feerate_sat_kvb * tx_size, 1000) # Round calculated fee up to nearest sat
+    return satoshi_round(target_fee_sat / Decimal(1e8)) # Truncate BTC result to nearest sat
+
+
 def satoshi_round(amount):
     return Decimal(amount).quantize(Decimal('0.00000001'), rounding=ROUND_DOWN)