What the simple interest calculator does
Simple interest is interest charged or earned on the original amount only — it never compounds. Enter a principal, an annual rate and a time in years, and the calculator returns the interest and the total. It is the arithmetic behind short-term loans, many bonds' coupon payments, some savings certificates, late-payment penalties, and the way lenders are often required to quote daily interest on car loans and personal loans in the US.
The formula
Interest = P × r × t
Total = P × (1 + r × t)
P = principal r = annual rate (as a decimal) t = time in years
$10,000 at 5% for 3 years:
Interest = 10,000 × 0.05 × 3 = $1,500
Total = $11,500For periods shorter than a year, convert to a fraction: 9 months is 0.75, 45 days is 45/365 = 0.123. Daily interest on the example is 10,000 × 0.05 ÷ 365 = $1.37 per day.
Simple vs compound interest
| Year | Simple interest (5%) | Compound interest (5% yearly) |
|---|---|---|
| 1 | $10,500 | $10,500 |
| 2 | $11,000 | $11,025 |
| 3 | $11,500 | $11,576 |
| 5 | $12,500 | $12,763 |
| 10 | $15,000 | $16,289 |
| 20 | $20,000 | $26,533 |
They start together and diverge steadily: simple interest grows in a straight line, compound interest in a curve. For anything over a year or two, make sure you know which applies — savings accounts, mortgages and credit cards all compound. The guide on simple vs compound interest covers the difference in depth; the Compound Interest Calculator handles the compounding case.
Where simple interest is actually used
- Auto loans and personal loans in the US are usually “simple interest” loans: interest accrues daily on the outstanding balance, so paying early or extra reduces the total.
- Bonds and notes — a 4% coupon on a $1,000 bond pays $40 a year, with no compounding within the bond itself.
- Short-term loans between individuals or businesses, invoice financing and bridging loans commonly quote a flat rate per month.
- Late payment penalties and statutory interest on overdue invoices (8% + base rate in the UK, for example) are calculated as simple daily interest.
- Treasury bills and commercial paper are priced on a simple discount basis.
Rearranging the formula
Find the rate: r = I ÷ (P × t) → $600 interest on $8,000 over 2 years = 3.75%
Find the time: t = I ÷ (P × r) → how long for $10,000 to earn $2,000 at 4%: 5 years
Find the principal: P = I ÷ (r × t) → what earns $500 a year at 2.5%: $20,000The Interest Rate Calculator solves for the rate on amortizing loans, which is a different (iterative) problem; the rearrangements above are for genuinely simple interest.