People type this into search every day: invoice late fee calculator.
The usual answer is “1.5% a month” or a flat $35. Both can be in the same contract. Each one uses its own math, and a federal agency uses a third version.
This page is a contract calculation. It is not legal advice, and it does not decide whether a fee is enforceable where you work.
Simple interest, then the fixed fee
The invoice late fee calculator uses:
balance × annual rate × days late ÷ 365 + fixed fee
That is simple interest on a 365-day year. The rate does not compound inside the tool. The fixed fee is added once. The tool’s own note says to charge only what the contract and the applicable law allow.
Fig 1
Balance, days, then the fee
The annual rate is spread across 365 days
Interest is balance times the annual rate times days late, divided by 365. The fixed fee is added once.
A private contract can pick its own rate, its own day count, and its own flat fee. The calculator applies the numbers you type.
Federal agency invoices are a different rule. The Prompt Payment Act requires an agency that pays a vendor late to pay interest. The Bureau of the Fiscal Service lists the rate for July 1 through December 31, 2026 as 4.75%. 5 CFR 1315.10 says the interest calculation uses a 360-day year, and that unpaid penalty interest is added to principal at the end of each 30-day period, for up to one year. Example 6 runs that beside the calculator.
The project quote page is where the invoice amount comes from. The cash runway post is what happens to the bank balance while you wait. A booked sale that has not been paid is not cash yet. The SBA’s Manage your finances page makes the same split: the cash method records the sale when the payment arrives.
Example 1: the published $7,500 bill, 45 days late
Here are the calculator defaults:
- invoice balance: $7,500
- contract annual interest: 12%
- days overdue: 45
- fixed late fee: $35
Daily rate: 0.12 ÷ 365 = 0.000328767
Interest: 7500 × 0.12 × 45 ÷ 365 = $110.96
Fixed fee: $35.00
Total now due: 7500 + 110.96 + 35 = $7,645.96
Added cost: (110.96 + 35) ÷ 7500 = 1.9%
Calculator result
Invoice late fee
- Invoice balance
- $7,500.00
- Contract annual interest
- 12%
- Days overdue
- 45 days
- Fixed late fee
- $35.00
$7,645.96
total now due
- Interest
- $110.96
- Fixed fee
- $35.00
- Effective added cost
- 1.9%
The interest is 110.96 ÷ 145.96 = 76% of the extra money. The $35 fee is the other 24%. On a bill this size, the rate does more work than the flat charge. Example 7 flips that.
Fig 2 · 1/2
The extra $145.96
Defaults: $7,500, 12%, 45 days, $35 fee
Interest
$110.96
Fixed fee
$35.00
Interest is $110.96.
The fixed fee is $35. Together they are 1.9% of the invoice.
Example 2: an $850 invoice at 18% for 30 days
A short freelance invoice, with a smaller flat fee.
- balance: $850
- annual rate: 18%
- days: 30
- fixed fee: $25
Interest: 850 × 0.18 × 30 ÷ 365 = $12.58
Total now due: $887.58
Added cost: 4.4%
Calculator result
Invoice late fee
- Invoice balance
- $850.00
- Contract annual interest
- 18%
- Days overdue
- 30 days
- Fixed late fee
- $25.00
$887.58
total now due
- Interest
- $12.58
- Fixed fee
- $25.00
- Effective added cost
- 4.4%
Here the $25 fee is about twice the interest. The percentage looks larger than Example 1 (4.4% versus 1.9%) because the flat fee sits on a smaller balance. The clock only ran 30 days.
Example 3: a $48,000 invoice, 8%, no flat fee
- balance: $48,000
- annual rate: 8%
- days: 60
- fixed fee: $0
Interest: 48000 × 0.08 × 60 ÷ 365 = $631.23
Total now due: $48,631.23
Added cost: 1.3%
Sixty days at 8% with no flat fee is a quieter percentage than the $850 bill, and a much larger check. $631.23 is the whole penalty. If the contract also has a $35 fee, you add it yourself. The tool will not invent one.
Example 4: what if I change only the days
Keep $7,500, 12%, and the $35 fee. Change the overdue count.
| Days | Interest | Total due | Added cost |
|---|---|---|---|
| 0 | $0.00 | $7,535.00 | 0.5% |
| 1 | $2.47 | $7,537.47 | 0.5% |
| 15 | $36.99 | $7,571.99 | 1% |
| 30 | $73.97 | $7,608.97 | 1.5% |
| 45 | $110.96 | $7,645.96 | 1.9% |
| 60 | $147.95 | $7,682.95 | 2.4% |
| 90 | $221.92 | $7,756.92 | 3.4% |
| 180 | $443.84 | $7,978.84 | 6.4% |
One day of interest is $2.47. The flat fee is $35, about 14 times the first day’s interest. From day 0 to day 180 the interest grows in a straight line, because the tool does not compound.
The added cost in the chart is interest plus the $35 fee. At 15 days that is 36.99 + 35 = $71.99. At 180 days it is 443.84 + 35 = $478.84.
Fig 3 · 1/7
Added cost as the days pile up
$7,500 at 12%, plus a $35 fee on every point
Interest plus fee
- Added cost
| Point | Added cost |
|---|---|
| 0d | $35 |
| 15d | $72 |
| 30d | $109 |
| 45d | $146 |
| 60d | $183 |
| 90d | $257 |
| 180d | $479 |
Through 45 days the extra amount is $145.96.
At 180 days it is $478.84. The fee stayed $35 the whole way.
Fig 4 · 1/2
Fee stays put, interest grows
Same $7,500 bill at 12%
- Interest
- Fixed fee
| Category | Interest | Fixed fee |
|---|---|---|
| 15 days | $37 | $35 |
| 45 days | $111 | $35 |
| 90 days | $222 | $35 |
| 180 days | $444 | $35 |
Interest is $36.99 at 15 days and $110.96 at 45 days.
By 180 days interest is $443.84. The fee is still $35.
Example 5: what if I change only the rate
Keep $7,500, 45 days, and the $35 fee.
| Annual rate | Interest | Total due | Added cost |
|---|---|---|---|
| 0% | $0.00 | $7,535.00 | 0.5% |
| 8% | $73.97 | $7,608.97 | 1.5% |
| 12% | $110.96 | $7,645.96 | 1.9% |
| 18% | $166.44 | $7,701.44 | 2.7% |
| 24% | $221.92 | $7,756.92 | 3.4% |
A contract written as 1.5% per month is 1.5 × 12 = 18% a year, if the contract means a simple monthly rate and you are putting it into this annual field. Typed as 18, the interest is $166.44.
The matching mistake is to type 1.5 into the annual-rate box, because that is the number printed on the invoice. Then the interest is 7500 × 0.015 × 45 ÷ 365 = $13.87, and the total due is $7,548.87. You just billed $152.57 less interest than the 18% reading of the same clause (166.44 − 13.87).
Fig 5 · 1/4
Interest at four annual rates
$7,500, 45 days. The $35 fee is outside these bars.
- Interest
| Category | Interest |
|---|---|
| 8% | $74 |
| 12% | $111 |
| 18% | $166 |
| 24% | $222 |
8% interest is $73.97.
12% is $110.96. 18% is $166.44. 24% is $221.92.
There is a second, smaller mismatch. Some people turn 45 days into 1.5 months and do 7500 × 1.5% × 1.5 = $168.75. The calculator’s 18% line is $166.44. The gap is $2.31, because 45 ÷ 365 and 1.5 ÷ 12 are different fractions of a year.
Example 6: the 360-day year, including the federal rate
Keep the $7,500 balance and 45 days. Drop the flat fee so the day count is the only change.
Calculator, 12% and a 365-day year: 7500 × 0.12 × 45 ÷ 365 = $110.96
Same rate on a 360-day year: 7500 × 0.12 × 45 ÷ 360 = $112.50
The 360-day bill is $1.54 higher. Banks call this a banker’s year. 5 CFR 1315.10(a)(9) requires that 360-day count for Prompt Payment interest. This calculator will not switch. If the contract says 360, do that division yourself, or scale the annual rate you type.
Now the published federal rate for the second half of 2026, 4.75%, on the same bill, with no flat fee. The calculator, which always divides by 365:
7500 × 0.0475 × 45 ÷ 365 = $43.92
Total due: $7,543.92
Calculator result
Invoice late fee
- Invoice balance
- $7,500.00
- Contract annual interest
- 4.75%
- Days overdue
- 45 days
- Fixed late fee
- $0.00
$7,543.92
total now due
- Interest
- $43.92
- Fixed fee
- $0.00
- Effective added cost
- 0.6%
The regulation’s 360-day division is 7500 × 0.0475 × 45 ÷ 360 = $44.53.
The same section also adds unpaid penalty interest to principal every 30 days. On this 45-day span that means 30 days on $7,500, which accrues $29.69, then 15 days on $7,529.69. The interest comes out to $44.59. That is $0.06 past the simple 360-day figure of $44.53. The day count ($43.92 on a 365-day year versus $44.53) is the larger gap. The calculator does not perform that 30-day add-on. For a federal invoice, use the Treasury method. For a private contract, use the clause you actually signed.
Example 7: a $35 fee on a $400 bill
- balance: $400
- annual rate: 0%
- days: 21
- fixed fee: $35
Interest: $0.00
Total now due: $435.00
Added cost: 35 ÷ 400 = 8.8%
The rate never enters. Three weeks late and zero interest still costs $35, if the contract charges that fee on its own. On the $7,500 default, $35 was 0.5% before any interest. On $400 it is the whole penalty. Flat fees dominate small invoices and fade on large ones. That is Example 1 next to this one.
Example 8: charge the balance that is still open
The original invoice is $7,500. The client has already paid $5,300. The open balance is 7500 − 5300 = $2,200. The contract still says 12%, 45 days from the due date, and a $35 fee.
Interest: 2200 × 0.12 × 45 ÷ 365 = $32.55
Total now due: $2,267.55
Added cost: 3.1%
If you run the original $7,500 through Example 1 after that payment, you bill $110.96 of interest plus $35, against money that has already arrived. The tool has one balance field. Type the unpaid amount.
A zero-day row is the other edge, from Example 4. At 0 days the interest is $0.00 and the total is still $7,535.00, because the fee does not wait. Whether your clause allows a fee on the due date itself is a contract question. The calculator will add whatever fee you type, including on day 0.
The bills next to each other
| Example | Balance | Rate | Days | Fee | Interest | Total due |
|---|---|---|---|---|---|---|
| 1. Defaults | $7,500 | 12% | 45 | $35 | $110.96 | $7,645.96 |
| 2. Small invoice | $850 | 18% | 30 | $25 | $12.58 | $887.58 |
| 3. Large, no fee | $48,000 | 8% | 60 | $0 | $631.23 | $48,631.23 |
| 4. 15 days | $7,500 | 12% | 15 | $35 | $36.99 | $7,571.99 |
| 4. 180 days | $7,500 | 12% | 180 | $35 | $443.84 | $7,978.84 |
| 5. Typed 1.5% by mistake | $7,500 | 1.5% | 45 | $35 | $13.87 | $7,548.87 |
| 5. 1.5% per month as 18% | $7,500 | 18% | 45 | $35 | $166.44 | $7,701.44 |
| 6. Federal 4.75% in this tool | $7,500 | 4.75% | 45 | $0 | $43.92 | $7,543.92 |
| 7. Fee only | $400 | 0% | 21 | $35 | $0.00 | $435.00 |
| 8. Remaining balance | $2,200 | 12% | 45 | $35 | $32.55 | $2,267.55 |
Open the invoice late fee calculator with the unpaid balance, the annual rate from the clause, the day count, and the flat fee. If the clause says 1.5% a month, convert it before you type. If it is a federal Prompt Payment claim, use the 360-day rule and the current Treasury rate instead of this 365-day line. The markup versus margin post is how the invoice was priced. The freelance rate post is how the hours became that balance.