⏰ Late Fee Calculator
By ToolNimba Finance Team · Reviewed by ToolNimba Editorial Review, small-business finance content · Updated 2026-06-19
This calculator gives an estimate only. The late fee and interest you can lawfully charge are governed by your written contract and by local law, which often caps the rate, requires a grace period, or limits flat fees. The result is not legal or financial advice. Check the exact terms in your agreement and the rules in your state or country before charging a late fee.
A late fee is the extra amount a customer owes when they pay an invoice after the due date. This calculator works out that charge two common ways: a fixed flat fee, or a percentage of the overdue amount. It can also add interest for the number of days the payment is late, using a daily, monthly, or annual rate. Enter the amount due, the days overdue, and your fee terms to see the total late charge and the new balance the customer owes.
What is the Late Fee Calculator?
Late fees exist to compensate a business for the cost and risk of being paid late, and to give customers a reason to pay on time. There are two main structures. A flat fee is a single fixed amount (for example $25) added once the invoice becomes overdue, regardless of how large the bill is. A percentage fee is calculated as a share of the overdue amount, so a 5% fee on a $1,000 invoice is $50. Many businesses use whichever is larger, or a flat fee on small invoices and a percentage on large ones.
Interest is different from a one-off fee: it accrues with time, so the longer an invoice stays unpaid, the more is owed. A monthly rate (often quoted as 1.5% per month, which is 18% per year) is the most common in commercial terms, but you can also express the same idea as a daily rate or an annual rate. This tool uses simple interest, meaning the charge is the amount due times the rate times the time overdue, without compounding. For a monthly rate it treats one month as 30 days, and for an annual rate it divides by 365 days, so partial periods are handled fairly.
What you may actually charge is not unlimited. Contracts and consumer-protection laws frequently cap late fees and interest, require the fee to be stated in writing before the work, or mandate a grace period of several days after the due date. Some jurisdictions void a fee that is punitive rather than a genuine estimate of loss. The figures here show what your stated terms produce, but always confirm those terms are enforceable where you operate.
When to use it
- Working out the late fee to add to an overdue customer invoice before you send a reminder.
- Comparing a flat fee against a percentage fee to decide which to write into your payment terms.
- Estimating the interest that has built up on a bill that is weeks or months past due.
- Checking a late charge a supplier or landlord has billed you, so you can confirm it matches the contract.
How to use the Late Fee Calculator
- Enter the amount due (the overdue balance) and how many days it is past the due date.
- Choose the fee type: a flat fee, a percentage of the amount due, or no fee.
- Enter the flat fee amount or the fee percentage for the type you picked.
- Optionally tick "Add interest", then enter the interest rate and choose whether it is per day, per month, or per year.
- Read off the fee, the interest charged, the total late charge, and the new balance owed.
Formula & method
Worked examples
A $1,000 invoice is 30 days overdue. You charge a $25 flat fee plus 1.5% interest per month.
- Flat fee = $25.00 (fixed, added once)
- Time in months = 30 ÷ 30 = 1 month
- Interest = 1,000 × 1.5 ÷ 100 × 1 = $15.00
- Total late charge = 25 + 15 = $40.00
- New balance = 1,000 + 40 = $1,040.00
Result: Fee $25.00 · interest $15.00 · total charge $40.00 · new balance $1,040.00
A $500 invoice is 45 days overdue. You charge a 5% percentage fee plus 0.05% interest per day.
- Percentage fee = 500 × 5 ÷ 100 = $25.00
- Interest = 500 × 0.05 ÷ 100 × 45 days = $11.25
- Total late charge = 25 + 11.25 = $36.25
- New balance = 500 + 36.25 = $536.25
Result: Fee $25.00 · interest $11.25 · total charge $36.25 · new balance $536.25
Interest accrued on a $1,000 overdue balance at 1.5% per month (simple interest)
| Days overdue | Months (days ÷ 30) | Interest | New balance |
|---|---|---|---|
| 10 days | 0.33 | $5.00 | $1,005.00 |
| 30 days | 1.00 | $15.00 | $1,015.00 |
| 60 days | 2.00 | $30.00 | $1,030.00 |
| 90 days | 3.00 | $45.00 | $1,045.00 |
The same rate expressed three ways (all give 1.5% over 30 days)
| Period | Rate | Equivalent annual |
|---|---|---|
| Per month | 1.5% | 18% per year |
| Per day | 0.05% | 18.25% per year |
| Per year | 18% | 18% per year |
Common mistakes to avoid
- Charging a fee your contract never mentioned. A late fee is usually only enforceable if it was agreed in writing before the work or sale. Adding a fee that does not appear in your terms or invoice can be challenged, and may be uncollectable.
- Confusing a one-off fee with interest. A flat or percentage fee is charged once when the bill goes overdue. Interest accrues with time, so it keeps growing. Mixing the two up leads to either undercharging or double counting.
- Ignoring legal caps and grace periods. Many states and countries cap the maximum late fee or interest rate, and some require a grace period of several days after the due date. A fee above the cap can be reduced or voided, so check the local limit.
- Compounding when your terms say simple interest. This tool uses simple interest, which is the norm for invoices. Compounding (charging interest on prior interest) grows faster and is often not what the contract allows, so do not apply it unless your terms clearly permit it.
Glossary
- Late fee
- An extra charge added to a bill when it is paid after the due date, set as a flat amount or a percentage of the balance.
- Flat fee
- A single fixed late charge (such as $25) added once, regardless of the size of the overdue amount.
- Percentage fee
- A late charge calculated as a share of the overdue amount, for example 5% of the invoice total.
- Simple interest
- Interest charged on the original overdue amount only, with no interest charged on previously accrued interest.
- Grace period
- A short window after the due date during which a payment is accepted without a late fee, often required by contract or law.
- Amount due
- The unpaid balance of the invoice that is past its due date, before any late fee or interest is added.
Frequently asked questions
How is a late fee calculated?
A late fee is either a flat amount (such as $25) added once the invoice is overdue, or a percentage of the overdue balance (such as 5% of $1,000 = $50). If your terms also charge interest, add the amount due times the rate times the time overdue. This calculator does both and shows the total.
What is a typical late fee for an invoice?
For business invoices a common term is 1.5% interest per month (18% per year) on the overdue amount, sometimes with a small flat fee on top. Flat fees often sit between $10 and $50, or a percentage fee of 1% to 5%. The right figure depends on your contract and local law.
Can I charge interest and a late fee at the same time?
Yes, if your written terms allow both. A flat or percentage fee is a one-off charge for being late, while interest accrues over time. Many contracts combine a fixed fee with a monthly interest rate. Make sure both are stated in your agreement before you bill them.
Does this calculator use simple or compound interest?
It uses simple interest: the charge is the amount due times the rate times the time overdue, with no interest charged on earlier interest. Simple interest is the usual convention for overdue invoices and is easier to justify if a charge is ever disputed.
Is there a legal limit on late fees?
Often, yes. Many states and countries cap the maximum late fee or interest rate, require the fee to be disclosed in advance, or mandate a grace period. A fee above the cap can be reduced or thrown out. Always check the rules where you operate before charging.
How do I convert a monthly rate to a daily rate?
Divide the monthly rate by the number of days you treat as a month. This tool uses 30 days, so 1.5% per month is about 0.05% per day. For an annual rate, divide by 365. All three express the same idea over a full period, the difference is how partial periods are counted.
Sources
- How to deal with late payments , U.S. Small Business Administration
- Late Fee , Investopedia