Late Payment Interest Calculator
What an overdue invoice has cost, and the daily figure to put in the chaser. 100% free, no signup. Everything runs in your browser.
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This late payment interest calculator tells you what an overdue invoice has actually cost you. Enter the invoice amount, the due date and the date it was paid or today's date, then either a monthly late fee percentage, the common US contract term that usually sits between 1 and 1.5 percent a month, or an annual rate. You get the days overdue, the interest owed, the total now due, and a daily rate so you can quote a figure that keeps climbing.
There is also a UK mode following the statutory Late Payment of Commercial Debts rules: the Bank of England base rate plus 8 percent, plus the fixed compensation amount, with the base rate editable because it moves. The whole thing runs in your browser with nothing uploaded, which matters when the inputs are a real client and a real unpaid amount. When you are ready to follow up, the invoice generator produces the document itself.
How to use
- Enter the invoice amount, before or after tax depending on what your terms say interest applies to.
- Set the due date from the invoice, not the date you sent it.
- Set the date paid, or leave it on today if the money is still outstanding.
- Choose a monthly percentage if your terms are written that way, or switch to the annual rate field.
- For a UK business to business debt, switch to statutory mode and check the base rate is current.
- Read the days overdue, the interest owed, the daily rate and the total now due.
- Copy those figures into your reminder, and keep the terms you agreed in writing close to hand.
Why use our late payment interest calculator?
I built this after an agency sat on a $6,400 invoice for seven weeks and I realized I had no idea what to put in the reminder email. Saying the invoice is overdue gets ignored. Saying interest is accruing at about $3.16 a day and the balance stands at $6,548 today does not, because it turns a vague annoyance into a number that grows while nobody acts. That invoice was paid four days later.
The number I got wrong first was the daily rate. My instinct was to take the monthly figure and divide by 30, which is close enough to feel right and wrong enough to be embarrassing in a written demand. The tool annualizes the monthly percentage and divides by 365, so 1.5 percent a month on $6,400 is roughly $3.16 a day rather than $3.20, and the total after 47 days is defensible line by line.
That daily rate is the output people actually use. It lets you write that the balance is payable by Friday, after which it increases, without inventing anything, and it makes a payment plan negotiable because both sides can see what waiting costs. I have had a client ask me to drop the interest in exchange for paying same day, which is a trade I take every time.
In the United States there is no automatic right to charge interest on a late invoice. It comes from your contract or your invoice terms, agreed in advance, and if the terms were never stated you are asking rather than charging. State usury caps also limit what is enforceable. Put the late fee in the terms before the work starts, which is easiest at the quote generator stage rather than after the fact.
The UK statutory route is different and stronger. For business to business debts, interest at the Bank of England base rate plus 8 percent applies by law, along with a fixed compensation sum that steps up with the size of the debt, so it does not need to be written into the contract. The UK government guidance on late commercial payments sets out the entitlement. The base rate field is editable because the base rate changes.
What I refuse to add is a threatening letter template. Tools that generate legal-sounding demands push people into sending things they do not understand. This calculates interest. None of it is legal advice, and on a debt large enough to matter a real attorney is cheaper than a mistake.
Who is this tool for?
The standard case is the second reminder. The invoice was due 47 days ago, your terms say 1.5 percent a month, and you want the follow-up to contain a specific balance rather than a polite nudge. Days overdue, interest and the new total take about twenty seconds, and they make the email read like a ledger entry instead of a complaint.
Freelancers use it to price the cost of slow payers into what they charge. If a client reliably pays 60 days late, that delay is a financing cost you absorb whether or not you ever collect a penny of interest. Running the number once usually changes the rate you quote them next time, which is what the freelance rate calculator is for.
Ecommerce and wholesale sellers on net 30 terms run it across a whole aging report. Work through each overdue account, sort by interest accrued, and chase in that order rather than in the order the reminders happen to surface. It changes which phone call you make first, which is usually where the money actually is.
Frequently asked questions
Only if your contract or invoice terms say so and the client agreed to them before the work. There is no automatic statutory right the way there is in the UK, and state usury caps limit how high an enforceable rate can go, so a fee that looks punitive may not stand up.
In US business terms, 1 to 1.5 percent a month is the range you see most often, which is 12 to 18 percent a year. Higher than that runs into usury limits in some states and makes the term easier to challenge.
The monthly percentage is annualized by multiplying by 12, then divided by 365 for a daily figure, which is multiplied by the days overdue. Dividing a monthly rate by 30 is the shortcut most people reach for, and it produces a different number worth avoiding in anything you send.
For business to business debts it applies the Bank of England base rate plus 8 percent from the day after payment was due, plus a fixed compensation amount that steps up with the size of the debt. The base rate is editable because it changes, so check the current figure first.
From the due date. An invoice on net 30 terms is not late until day 31, and starting the clock at the issue date is the fastest way to have the whole calculation dismissed.
Yes, and separating it from the original amount goes down better than quietly restating a bigger figure. Show the original balance, the days overdue and the interest as its own line. Once it is settled, the receipt maker closes the loop.