Free tool

Late fee calculator

Use this free late fee calculator to work out what an overdue invoice owes: flat, one-time percent, simple or compounding interest, with optional grace days.

Updated Nov 5, 2026 · By PayToMe Editorial

Result

Late fee $30.00

Days late 45 days

Fee periods 2

Total now due $1,030.00

Calculated in your browser. Nothing is sent anywhere.

Read the full guide: how to use it, what to include, worked example

A late fee calculator tells you what an overdue invoice should cost the client once the due date has passed. Enter the invoice amount, the due date, the date you are checking, and the fee rule from your payment terms. The calculator above is prefilled with an example and recomputes as you type. It runs in your browser, and nothing you enter is uploaded.

How to use the calculator

Enter the invoice amount. Use the unpaid balance, not the original total, if the client has paid part of it.
Set the due date and the “as of” date. The as-of date is usually today, but you can pick any date to see what the fee will be later.
Add grace days if your terms include them. Leave this at 0 if the fee starts the day after the due date.
Choose a fee method. Pick flat amount, one-time percent, simple interest per month, compounding monthly, or an annual rate charged daily.
Read the result. You get the days late, the days or periods actually charged, and the fee in dollars and cents.

Need the due date itself first? The invoice due date calculator turns an issue date and a term like Net 30 into an exact date.

How the math works

Every method starts with the same step: count the days between the due date and the as-of date. If the as-of date is on or before the due date, nothing is late and the fee is zero. After that, the methods differ.

Flat fee. A fixed amount, such as $25, no matter how large the invoice is or how late it gets.
One-time percent. A single percentage of the balance, such as 5%, charged once.
Simple monthly interest. A percentage of the original balance for each monthly period that has started. The days late are divided by 30 and rounded up to whole periods.
Compounding monthly. The same monthly percentage, but each period’s charge is added to the balance before the next period is calculated. Each period is rounded to the cent.
Annual rate, charged daily. The yearly rate divided by 365, multiplied by the number of charged days and the balance.

Grace days come off the days late before any of this happens. They delay the fee; they do not change the rate.

Worked examples

The other two methods are straightforward. A flat fee is the $25 you set. A one-time 5% fee on $1,000.00 is $50.00.

Method	Fee on the $1,000.00 example
Simple, 1.5% per month, 45 days	$30.00
Compounding, 1.5% per month, 95 days	$61.37
Annual 18% daily, 5 grace days, 45 days late	$19.73
One-time 5%	$50.00

Common mistakes

Adding a fee that was never agreed. A late fee is much easier to defend when it is written in your contract, quote or invoice terms before the work begins. Surprise fees invite disputes.
Calculating on the wrong amount. If the client paid half, the fee applies to what is still unpaid, unless your terms say otherwise.
Counting from the invoice date. Lateness starts at the due date, not the day you issued the invoice.
Forgetting the grace period you promised. If your terms say 5 days, charging from day one undermines the whole document.
Mixing up monthly and annual rates. 1.5% per month is 18% per year before compounding. Quote the one your terms actually use.
Silent compounding. If interest compounds, say so. Many clients assume simple interest.
Skipping the reminder. A friendly note before the fee lands usually gets you paid faster than the fee itself.

Writing the fee into your terms

A good late fee clause answers four questions: when does the fee start, how is it calculated, is there a cap, and how is it billed. For example: “Invoices are due 30 days from the issue date. After a 5-day grace period, a late fee of 1.5% of the unpaid balance applies for each month or part of a month the balance remains unpaid.” Pair that with clear payment terms and a term such as Net 30, and nobody has to guess.

For a step-by-step guide to deciding whether to charge a fee at all, and how to word it, read how to charge late fees.

Frequently asked questions

Is it legal to charge a late fee on an invoice?

It depends on where you and your client are, on the contract, and sometimes on the type of customer. Some places cap fees or interest rates. Check your state or country's rules on an official source, and have the fee in writing before the work starts. This is general information, not legal advice.

Does the calculator upload my invoice details?

No. The math runs in your browser and nothing you type is sent to a server or saved.

What is a grace period?

It is a set number of days after the due date during which no fee builds up. With a 5-day grace period and an invoice 45 days late, only 40 days are charged.

Should I use simple or compounding interest?

Simple is easier to explain and is the safer default. Compounding charges interest on interest, so it grows faster and is more likely to be questioned or limited. Whichever you pick, state it in your terms.

Why does a percent-per-month fee count whole periods?

Monthly fees are usually written as a charge for each month or part of a month that the invoice is overdue. So 45 days counts as 2 monthly periods, not 1.5.

Can I add the fee to the original invoice?

Better to send a separate, clearly labeled late fee line or a new invoice that references the original. That keeps the original total intact and the paper trail clean.