Late payment interest calculator
Enter one constant unpaid principal, a checked due date, a calculation date and your rate. Compare the interest estimate and any separately selected charge before preparing a claim.
By Invoodle · Updated · UK references reviewed October 2026
Estimate one constant-rate, constant-principal period using simple interest and a fixed 365-day year. Check the debt, due date, rate and any charge’s authority first. The tool does not determine legal entitlement, handle part payments or fetch statutory rates. Dates start blank. No invoice draft is read or saved and nothing is sent.
Currency formatting only, without conversion or a choice of governing law. Changing it keeps the numeric principal, clears rate and charge, and returns to manual mode.
Changing illustration clears the rate and manual charge. The UK option does not establish a right to statutory interest or compensation.
One unchanged balance for the whole period. Part payments and credit notes need separate review. Use the original qualifying debt for a UK compensation-band illustration.
The due date is day zero; count calendar days after it through the calculation date. Fixed 365-day denominator, including in leap years. No grace-period or holiday adjustment.
The default 8% is fictional arithmetic, not a current statutory rate or a suggested contract term. Monthly input uses nominal ×12 conversion, without compounding or APR calculation.
Check authority and timing yourself. Included once only for a positive principal after the due date; no fee accrues per day.
Arithmetic estimate
Choose both dates and check the rate to calculate. No claim date is assumed.
Interest is rounded once at the end. The total is an arithmetic estimate, not a legal finding of an amount payable. No part payments, compounding, rate-period changes or claim submission.
A calculation link includes principal, currency, dates, rate, basis, method and charge. Recipients can read those values. Typing does not update this page’s URL.
Try a fictional 45-day estimate
These use fictional dates from 1 January to 15 February 2026 and fictional rates, without determining legal entitlement.
| Assumptions | Interest | Charge | Estimated total |
|---|---|---|---|
| 8% annual simple interest | GBP 49.32 | GBP 0.00 | GBP 5,049.32 |
| 1% monthly nominal ×12; assumed GBP 25 charge | GBP 73.97 | GBP 25.00 | GBP 5,098.97 |
Official UK references for checking a real debt
Review contract terms, eligibility, the relevant reference period and prior compensation. These UK links are not worldwide interest or fee rules. Use the official calculator for its supported statutory workflow; this page remains a manual-rate illustration.
- GOV.UK: when a commercial payment becomes late
- GOV.UK: statutory interest and the effect of contractual terms
- GOV.UK: fixed compensation bands and recovery conditions
- Office of the Small Business Commissioner: official interest calculator
- Office of the Small Business Commissioner: calculator and reference-rate guidance
- Bank of England: official Bank Rate history
Review the document before using the figures
Opening the editor does not transfer this estimate or clear an existing draft. Review document mode, currency and particulars. Copy calculation is for your review; it does not send a payment demand or create an issued interest invoice.
Open invoice editorHow simple late-payment interest is calculated
For principal P, an assumed annual rate r% and d calendar days, this tool uses P × r ÷ 100 × d ÷ 365. On a fictional GBP 5,000 balance at 8% for 45 days, the unrounded interest is about GBP 49.3151, displayed as GBP 49.32. The 8% is an arithmetic assumption, not a current statutory rate or suggested contract term.
Interest is calculated at full precision and rounded once at the end to the nearest penny or cent, with exact half-units rounded up. The daily figure is indicative and displayed to up to four decimals; multiplying that rounded display may differ from the period result.
What the monthly input actually means
Monthly input here is a nominal illustration: multiply the entered monthly percentage by 12, then apply the same annual simple-interest formula over 365 days. An assumed 1% monthly input therefore uses 12% nominal annual arithmetic. It is not an effective annual rate, APR, compounding or a charge for every completed month.
If your agreement uses actual days per month, a 360-day year, compound interest, minimum charges, grace periods or another convention, this tool does not implement it. Check the agreed method and applicable law instead of converting a contractual term by assumption.
Check the dates and the balance first
The due date is day zero. The calculation counts calendar days after it through the selected calculation date, including weekends. A calculation date on or before the due date gives zero elapsed overdue days and no illustrated interest or added charges. No holiday adjustment or grace period is applied.
Use one unchanged principal for the whole period. Partial payments, credit notes, disputed balances, rate changes and court or insolvency stages need a separate review. This tool does not split periods, determine when payment legally became due or establish the date at which interest must stop.
Both dates start blank so a build date or device clock cannot silently become your claim date. The explicit Use today button uses your device’s local calendar date. The supported date gap is at most 36,525 days, a calculator limit rather than a legal limitation period.
Using the UK compensation illustration
GOV.UK describes statutory interest for qualifying business-to-business debts as 8 percentage points above the Bank of England base rate, and says a different contractual interest rate affects that remedy. Check the contract, applicable rules and correct reference rate. This tool does not fetch Bank Rate, select a statutory period or determine entitlement.
The Small Business Commissioner publishes reference-rate guidance and an official calculator. Use those sources to check the relevant dates and treatment. Do not automatically use today’s Bank Rate or enter only the 8-point margin. Enter the full annual percentage you have checked; switching to this illustration clears the rate for review.
For a qualifying debt, GOV.UK lists fixed compensation of GBP 40 below GBP 1,000, GBP 70 from GBP 1,000 to GBP 9,999.99 and GBP 100 at GBP 10,000 or more. Its guidance limits the fixed sum to once for each payment. This illustration is GBP-only, requires an overdue positive principal and does not stack a separate manual charge.
Use the original qualifying principal for a single unchanged debt, not a remaining balance after part payment. Previously claimed compensation, recovery costs, multiple invoices, consumer debts and special contractual or legal cases are outside this illustration. A computed band is not confirmation that it can be claimed.
A calculation is not a demand or an invoice
Manual mode accepts an assumed fixed charge separately from interest. Check its authority, amount and timing before using it. The tool includes it once only when the entered principal is positive and the calculation date is after the due date. It does not decide that any charge is enforceable.
The result is arithmetic, not “what the client owes” as a legal conclusion. Review the agreement, applicable jurisdiction, dates, balance, rate and prior charges before drafting a reminder or claim. Currency selection changes display only; it does not choose a legal system or convert amounts. Tax treatment is not determined here.
Opening Invoodle’s editor does not transfer the estimate or clear an existing draft. Check document mode, currency and required particulars yourself. The calculator does not create or send a reminder, add an issued interest invoice, record a payment or file a recovery claim.
UK references reviewed on 21 October 2026. The tool estimates one constant-rate, constant-principal simple-interest period using a fixed 365-day denominator. It does not establish legal entitlement or maintain worldwide interest-rate limits. Verify the actual agreement and jurisdiction before using the figures.
Frequently asked questions
How do I calculate daily interest on a late invoice?
For this simple-interest model, principal × annual rate ÷ 100 ÷ 365 is the daily amount before rounding. Multiply at full precision by the elapsed calendar days, then round the final interest. A fictional GBP 5,000 at 8% for 45 days gives GBP 49.32.
Does 1% per month mean the calculator compounds interest?
No. Monthly input is multiplied by 12 for nominal annual simple-interest arithmetic over 365 days. It does not implement compounding, APR, an effective annual rate or billing for each completed month. Use another method if the agreement requires it.
Can the calculator tell me what rate I am legally allowed to charge?
No. Check the contract, jurisdiction, parties, debt and applicable rules. The input range of 0 to 100 with up to three decimals is a calculator bound, not a statement of permitted rates. Changing currency does not choose the governing law.
Does the UK option automatically add the correct Bank of England rate?
No. Enter the full annual percentage you have checked, not just an 8-point margin. Use the linked official guidance, rate history and Small Business Commissioner calculator to check the reference date and eligibility. This tool does not fetch or determine the statutory rate.
When is the UK fixed-compensation illustration added?
Only in the GBP UK illustration, with a positive principal and at least one elapsed day. It uses the GBP 40, 70 and 100 bands and excludes a manual fixed charge. Confirm the debt qualifies and that the fixed sum has not already been claimed; partial payments and multiple debts need separate review.
What happens if the due date has not passed?
The day count is zero when the calculation date is on or before the due date. Interest, the manual fixed charge and UK compensation are all zero in that result. This is the tool’s illustration behavior, not a legal due-date determination.
Does this save or send a reminder?
No. Copy calculation copies the assumptions and breakdown for your review. Copy calculation link explicitly includes amount, currency, dates, rate, basis, method and charge; recipients can read them. Typing does not update the URL or read or write invoice drafts. Nothing is sent automatically.