What late payment interest is
Late payment interest is the charge that builds up when a debt, an invoice, or an installment is not paid by the agreed date. It is not a flat penalty: it grows with every day of delay, because it compensates the person waiting for the money for the time it went uncollected. The later the payment, the larger the accrued interest.
In most contracts and invoices, late interest is worked out as simple interest, day by day: you take a rate, turn it into a daily rate, and multiply it by the amount owed and by the number of days late. The interest is always calculated on the original balance, not on earlier interest, so the daily charge stays constant while the principal does not change.
This calculator does all of that in your browser. Enter the amount, the rate, choose whether it is annual, monthly, or daily, and give the days late (or the due and payment dates) to see the accrued interest and the total to pay. Nothing is sent or stored.
How to use the calculator
- Amount owed: the outstanding balance the late interest runs on.
- Interest rate: the agreed percentage. Use the selector to say whether that rate is annual, monthly, or daily.
- Days late: type them in, or switch to “Dates” and enter the due date and the payment date; the tool counts the days for you.
- Pick the currency to format the result (or choose “No symbol”).
- Read the late payment interest, the total to pay, and the interest per day. Open the breakdown to see the accrued amount at 15, 30, 60, and 90 days.
The formula
It uses simple interest with a daily rate:
Interest = Amount × daily_rate × days
The daily rate comes from the period you choose:
- Annual rate:
daily_rate = rate ÷ 365 - Monthly rate:
daily_rate = rate ÷ 30 - Daily rate:
daily_rate = rate(already per day)
The total to pay is Total = Amount + Interest. With 0 days late the interest is 0 and you only owe the principal.
Worked example
You owe 10,000 at a late-payment rate of 24% per year and pay 30 days late:
- Daily rate: 0.24 ÷ 365 = 0.00065753…
- Interest per day: 10,000 × 0.00065753 = 6.58
- Late payment interest: 10,000 × 0.24 ÷ 365 × 30 = 197.26
- Total to pay: 10,000 + 197.26 = 10,197.26
If instead of typing 30 days you set the due date to 2026-01-10 and the payment date to 2026-02-09, the calculator counts those same 30 days and reaches the same result.
Reference table
Late payment interest on a debt of 100,000 at an annual rate, by days late:
| Annual rate | 30 days | 60 days | 90 days |
|---|---|---|---|
| 12% | 986.30 | 1,972.60 | 2,958.90 |
| 18% | 1,479.45 | 2,958.90 | 4,438.36 |
| 24% | 1,972.60 | 3,945.21 | 5,917.81 |
Frequently asked questions
Is the late payment rate the same everywhere?
No. The legal cap on late interest varies by country, by type of debt, and by what the contract says. Many deals set the late charge as a markup on the loan’s ordinary rate. This tool does not impose any country’s rate: you enter the one that applies to your case and it does the math.
Why divide by 365 and not 360?
It is a convention. A 365-day base (366 in a leap year) reflects the real calendar year. Some institutions use a 360-day commercial base. If your contract specifies that, convert your annual rate to monthly or daily before entering it, or adjust the rate so it matches the base you are charged on.
Can I use it for an overdue invoice?
Yes. Enter the invoice amount, the agreed late-payment rate, and the days elapsed from the due date to today or to the payment date. The result is the estimated surcharge you can add to the invoice total.
Is late payment interest simple or compound?
It is almost always simple: it is calculated on the original balance for as long as the delay lasts. Some contracts, however, capitalize the late interest (adding it to the principal periodically and charging on the new balance). In that case the cost behaves like compound interest, and this calculator, which uses simple interest, gives a conservative estimate.
Is the result an exact, legal figure?
It is an informational estimate. The exact amount depends on the applicable rate, the day-count base, any extra charges, and your country’s law. Use it as a reference and confirm the final figure with your contract or creditor before you pay.