Example 1 — 10% for 30 days
- Amount: $10,000
- Rate: 10% a year
- 30 days, actual/365
Interest ≈ $82.19
How much late interest at 10,000, 10%, and 30 days? About 82.19 (actual/365).
Late Payment Interest Calculator
Estimate what a late payment may cost on a U.S. invoice or contract. Enter the overdue amount, the annual rate, and how long the delay lasts. There is no single federal late-payment rate for private invoices; the preset buttons suggest common contract samples you can edit. At $10,000, 10%, and 30 days on actual/365 — calendar days divided by 365 — interest is about $82.19. This is an estimate for comparison, not legal advice.
Results
Enter the overdue amount, the annual rate, and the delay in days. The result will appear here.
Interest ≈ amount × rate × (days ÷ 365 or 360). At 10,000, 10%, and 30 days the result is about 82.19. This is not a loan installment and not an amortization schedule. For interest with compounding, use the related calculator below.
Worth knowing
A higher contract rate raises the late cost on the same invoice
There is no single U.S. federal late-payment rate for private invoices — your contract rate does the work. On the same 10,000 and 30 days (actual/365), 10% a year is about 82.19. An 18% clause is about 147.95. The gap is roughly 65.75 for one month of delay, before collection costs or state caps.
10% · 30 days · 10,000≈ 82.19
18% · same 30 days≈ 147.95
That is not a judgment and not a demand letter. It is the same simple-interest formula, used to show how much the rate in the clause changes the cost of delay.
“Same 10,000 and 30 days: about 82 at 10%, about 148 at 18%.”
How the results are calculated
Amount10,000
Rate10%
Days30
Base365
Interest≈ 82.19
Late-payment (default) interest on this page is a simple-interest estimate: interest = amount × (rate/100) × (days ÷ year base). You choose the year base: actual/365 (calendar days ÷ 365) or 30/360 (30-day months, 360-day year). There is no compounding. You enter the annual rate; the preset buttons only suggest common U.S. contract samples. A statute, a contract, or a court may count differently. This is not legal or accounting advice. The United States has no single federal late-payment rate for private invoices.
At 10,000, 10%, and 30 days on actual/365, interest is about 82.19, or about 2.74 a day, because 10,000 × 0.10 × 30/365. Total due is the amount plus that interest. Instead of entering days, you can switch to From dates and give a start and end date — the length then comes from the span. Scenario B lets you compare another day count or another rate on the same amount.
Enter the overdue amount and the annual rate. Measure the delay either as a day count or as both dates — not both at once. Choose actual/365 or 30/360. Scenario B is optional. After Calculate you see estimated interest, total due, daily interest, interest as a share of principal, and the day count used in the formula.
Paying a balance in installments belongs on a loan payment or amortization page, not here. APR is a different model. This calculator covers one delay period and one or two rates.
About 82.19 at 10% is not a court judgment. Another day-count base, contractual compounding, or a partial payment mid-period will change the figure. Scenario B does not handle mid-period installments. After the 10,000 / 10% / 30-day example, compare the 18% sample — interest is about 147.95.
Formula
interest = amount × (rate / 100) × (days / B)
B = 365 (actual/365) or 360 (30/360). daily = amount × (rate / 100) / B. total = amount + interest.
This is classic simple interest for late payment, with no compounding. In From dates mode, the day count comes from the date span (calendar or 30/360). The calculator does not load a statutory default-interest table — you enter the rate yourself.
How to use
- In Overdue amount enter the unpaid sum, for example 10,000.
- In Annual rate enter a percent or use a rate button (for example the 10% sample). The calculator does not fetch a federal table.
- Choose Day count or From dates. For days enter e.g. 30. For dates fill start and end.
- Choose actual/365 or 30/360. Optionally fill scenario B.
- Click Calculate. At 10,000, 10%, 30 days, and actual/365, interest should be about 82.19.
Simple interest, actual/365, and 30/360
Simple interest is amount × rate × (days ÷ 365 or 360). 10,000 at 10% for 30 days on actual/365 is about 82.19, or about 2.74 a day.
- amount
- The overdue balance from the form. Interest and total due are based on it.
- rate
- The annual percent you enter. The buttons suggest common U.S. contract samples, not a federal statutory table.
- actual/365
- Calendar days divided by 365. 10,000 × 0.10 × (30/365) ≈ 82.19.
- 30/360
- A banking convention: 30-day months and a 360-day year. The same amount and 30 days then give a larger year fraction than actual/365.
- scenario B
- A comparison of another delay or rate on the same amount. It shows the interest difference versus scenario A.
Examples
Example 2 — days from dates
- Amount: $12,000
- 11.25% a year
- Jan 1–Mar 15, 2026 (73 days)
Interest = $270.00
How much interest at 12,000, 11.25%, and a 73-day date range? Exactly 270.00.
Example 3 — 10% vs 18%
- A: $10,000 @ 10%, 30 days
- B: same 30 days @ 18%
A ≈ $82.19 · B ≈ $147.95
Δ ≈ +$65.75
How does scenario B compare at a higher rate? A ≈ 82.19; B ≈ 147.95; Δ ≈ +65.75.
Related calculators
FAQ
How does the simple late-interest formula work?
Interest = amount × (rate/100) × (days ÷ year base). The base is 365 (actual/365) or 360 (30/360). At 10,000, 10%, and 30 days on actual/365 the result is about 82.19, because 10,000 × 0.10 × 30/365.
How is actual/365 different from 30/360?
Actual/365 divides calendar days by 365. 30/360 treats each month as 30 days and the year as 360. The same amount, rate, and 30 days come out a little higher on 30/360 because the year fraction is larger.
Do I need to enter days when I have start and end dates?
No. In From dates mode you enter both dates, and the calculator takes the span (calendar or 30/360). In Day count mode the date fields are hidden.
Does the calculator insert a statutory or Fed rate?
No. It does not download a federal table. The rate buttons (10%, 12%, 18%) only fill the field with common U.S. contract samples you can edit. There is no single nationwide private-invoice late rate — your contract usually sets it.
How is this different from a loan installment or APR?
Here you have one delay and simple interest on an amount. A loan installment and APR pay principal down over time with a different formula and usually a schedule. This page does not build a schedule.
Why is there a scenario B?
So you can compare another delay length or another rate on the same amount. The result shows interest for B and the difference versus A. It does not cover a partial payment mid-period.
Does the interest compound?
No. The model is simple: no interest on interest. If a contract uses compounding, that result is outside this page.
Is the result legal advice or a judgment?
No. It is an estimate from the numbers you enter. A statute, a contract, and a court may count differently, including another day-count base.
Can I add sales tax or VAT to the interest here?
No. Invoice tax belongs on the VAT calculator. Here you only get late cost from amount, rate, and days.
Does this calculator use Bank of England base rate plus 8% automatically?
No. You enter the annual rate yourself. UK statutory late-payment interest is often base rate plus 8%, but this page does not fetch the BoE rate or split periods when the reference rate changes.
From which day to which day is late interest counted?
In date mode you enter the start and end of the range; the calculator takes the length from that range (calendar days or 30/360). It does not decide whether your contract or statute includes the due date itself.
Does the calculator split the period when the rate changes mid-delay?
No. One rate and one period (or scenario B with a second rate or length). If the applicable rate changed during the overdue stretch, calculate each segment separately and add them, or use a tool that splits automatically.
Is this the same as UK statutory late payment interest with compensation?
It only computes simple interest from amount, rate, and days. Fixed recovery compensation bands (£40 / £70 / £100) and automatic BoE-linked statutory rates are outside this model.
Sources
The model is simple interest. You enter the rate; the page does not load a statutory default-interest table.
- Wikipedia — simple interest
- Investopedia — simple interest
- Federal Reserve — selected interest rates (H.15) (context only; not auto-filled here)
Page updated October 21, 2026.