Free tool

Simple interest calculator

Work out interest on a fixed amount, at a fixed annual rate, for the dates you choose.

UK small business guides · Sources checked 18 October 2026

Your calculation
The fixed amount earning or attracting interest.
Enter the full annual rate, for example 8 for 8%.

Counts calendar days after the start date up to and including the end date. Uses a 365-day year, including in leap years. The amount and rate stay fixed; no compounding, payments, fees or tax adjustments.

Free to use. No account needed. Calculation amounts, rates and dates stay in your browser and are not sent to FindCompany.

How to calculate simple interest

Interest = amount × (annual rate ÷ 100) × days ÷ 365

Worked example: £2,500 at 8% for 30 days

  1. Convert the annual percentage to a decimal: 8 ÷ 100 = 0.08.
  2. Work out a full year’s interest: £2,500 × 0.08 = £200.
  3. Work out the daily interest: £200 ÷ 365 = £0.547945… per day.
  4. Multiply by 30 days: (£200 ÷ 365) × 30 = £16.438356….
  5. Round the final interest to the nearest penny: £16.44.

In one calculation: £2,500 × (8 ÷ 100) × 30 ÷ 365 = £16.44, rounded.

Add the interest to the original amount: £2,500 + £16.44 = £2,516.44.

Keep the full precision of the daily interest until the final step. This is an illustration, not a quoted lending or savings rate.

Simple interest is calculated on the original amount. Compound interest also earns interest on previous interest. This tool calculates simple interest only; it is not an APR or AER calculator and does not determine what a contract allows you to charge.

Common questions

Can I use this for an overdue invoice?

You can use it to check arithmetic at a rate you already know. For qualifying UK commercial debts, our late payment calculator also explains reference rates and includes the fixed recovery sum.

What if my agreement uses another day-count basis?

Use the method in your agreement. Some contracts use a different denominator or count dates differently, so this fixed 365-day calculation may not match.

Are leap days included?

Yes. Actual calendar days between the dates are counted, including 29 February where it falls in the period. The annual denominator remains 365.

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