Invoice Late Fee & Interest Calculator
Work out what an overdue invoice now owes. Enter the amount, due date, grace period, interest rate and any flat late fee to get the interest, the total, the daily charge and how overdue it is.
Written by Suhaib Hassan. Figures are estimates you can edit. Our methodology.
It counts the days past the due date, removes the grace period, and applies simple or compound interest on the days left, then adds any flat fee once.
Chasing late payments is awkward when you are not sure what to ask for. This shows the figures so you can decide and explain them.
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Late fees and interest can usually only be charged if your contract or invoice terms say so, or a law gives you the right. Check the terms and the rules where you operate. This is not legal advice.
Late payment rules in a few places
A starting reference only. Rates change, and terms of business, the type of customer and local law all matter. Always check the current rule.
| Where | Statutory interest | Fixed fee | Law |
|---|---|---|---|
| US federal government | Treasury-set rate (Prompt Payment Act) | Interest only | 31 U.S.C. 3901 to 3907 |
| California | 10% a year where the contract sets no rate | Per contract | Civil Code 3289 |
| New York | 9% a year on most judgments and claims | Per contract | CPLR 5004 |
| United Kingdom | Bank of England base rate + 8% | £40 (under £1,000), £70 (£1,000 to £9,999.99), £100 (£10,000+) | Late Payment of Commercial Debts (Interest) Act 1998 |
| European Union | ECB reference rate + at least 8% | At least €40 | Directive 2011/7/EU |
Formulas
How the numbers are worked out.
Days charged interest
Days past the due date, less any grace period. Interest only runs after the grace period.
days late = as-of date − due date
charged days = max(0, days late − grace)Simple interest
Amount times the yearly rate times the fraction of a year.
interest = amount × rate × days ÷ 365Compound interest
Interest added to the balance monthly or yearly.
monthly: amount × ((1 + rate ÷ 12)12 × days ÷ 365 − 1)
yearly: amount × ((1 + rate)days ÷ 365 − 1)Worked examples
Illustrative figures, each checked by hand. The first is the default setup.
Default
$8,500 due 15 August, calculated to 14 September, 5 grace days, 18% simple, $50 fee.
interest = 8,500 × 0.18 × 25 ÷ 365 = $104.79
total = 8,500 + 104.79 + 50 = $8,654.79
Per-day interest
Same invoice.
Monthly compounding
$8,500 at 18% for 90 charged days (no grace period).
How to calculate a late fee
- Enter the invoice amount and its due date.
- Set the date to calculate up to and any grace period from your terms.
- Enter the yearly interest rate and whether it compounds, plus any flat fee.
- Read the total owed and copy the summary for your reminder.
Not legal advice. Late fees and interest are only enforceable where your contract or the law allows them, and some places cap the rate. Check the terms and local rules.
By Suhaib Hassan · Reviewed 29 September 2026 · How CalculatePilot verifies formulas →
Sources
- UK Government, Late Payment of Commercial Debts (Interest) Act 1998 and the statutory fixed sums.
- European Union, Directive 2011/7/EU on combating late payment in commercial transactions.
- U.S. Government, Prompt Payment Act, 31 U.S.C. 3901 to 3907.
Frequently asked questions
Can I charge a late fee on any invoice?
Only if your contract or invoice terms say so, or a law gives you the right, such as the UK and EU late payment rules for business-to-business debts.
What interest rate is reasonable?
Many businesses use 1% to 1.5% a month (12% to 18% a year) if their terms allow it. Some places cap the rate, and statutory rates differ.
What is a grace period?
A number of days after the due date during which no interest or fee is charged. It is set in your terms.
Simple or compound interest?
Simple interest is charged on the original amount only and is the norm for invoices. Compound interest adds unpaid interest to the balance.
Is the day the invoice is paid included?
This counts the days between the due date and the date you choose, not including the due date itself.
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