A simple interest calculator that works out the interest and total amount on a deposit or loan using the classic I = P × r × t formula. Ideal for short-term loans, bridging finance, bonds and any product where interest is charged only on the original principal rather than compounding.
How it works
The calculation is the textbook simple-interest formula:
I = P × r × t
where P is the principal, r is the annual rate as a decimal (your percentage ÷ 100) and t is the time in years. Months are converted to a fraction of a year (t = years + months ⁄ 12). The total amount is simply P + I. Because the interest is always based on the original principal — never on interest already added — the amount earned each year stays constant.
Example
Put £1,000 at 5% for 3 years:
- Interest: £1,000 × 0.05 × 3 = £150
- Total amount: £1,150
Over the same 3 years, compound interest at 5% would yield about £158 — the gap widens sharply over longer terms.
| Principal | Rate | Time | Interest | Total |
|---|---|---|---|---|
| £1,000 | 5% | 1y | £50 | £1,050 |
| £1,000 | 5% | 3y | £150 | £1,150 |
| £5,000 | 8% | 2y 6m | £1,000 | £6,000 |
| £10,000 | 3.5% | 5y | £1,750 | £11,750 |
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