Find the interest on a sum at a flat rate, or work backwards to the principal, the rate or the time. Count days on a 365 or 360-day year.
Estimates for planning, not financial, tax or legal advice. Figures use US rules unless the page says otherwise.
The simple interest formula
I = P × r × t A = P + I
P is the principal, r the yearly rate as a decimal, and t the time in years. Interest is charged on the original principal only, never on earlier interest. Rearrange the same formula to find any one value:
P = I ÷ (r × t) r = I ÷ (P × t) t = I ÷ (P × r)
Worked example
$10,000 at 5% for 90 days. On a 365-day year, t = 90 ÷ 365 = 0.2466 years, so I = 10,000 × 0.05 × 0.2466 = $123.29. On a 360-day "banker's year", t = 90 ÷ 360 = 0.25, so I = $125.00. Same rate, same days, $1.71 more, because each day counts as a bigger slice of the year.
365 or 360 days?
US savings accounts must use a daily rate of at least 1/365 of the yearly rate (Regulation DD). Many commercial loans, some credit lines and money-market instruments use actual days ÷ 360, which slightly raises the interest. Check your agreement for the "day count" or "interest calculation method".
Limitations
Simple interest doesn't apply to most savings and loans:
- Savings accounts, CDs and investments compound, so interest earns interest. Use the compound interest calculator or the CD calculator.
- Loans repaid in monthly installments charge interest on a shrinking balance, so I = Prt on the full amount overstates it. Use the loan calculator.
- Fees, taxes and late charges aren't included.
Sources
How we checked this
Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:
- $5,000 at 4% for 3 years earns $600 ✓
- $10,000 at 5% for 90 days, 365-day year: $123.29 ✓
- Same on a 360-day year: $125.00 ✓
- $150 on $2,000 over 18 months is 5% a year ✓
- $90 on $1,000 at 6% takes 1.5 years ✓
- $600 at 4% over 3 years needs $5,000 ✓
- Solving for time at 0% gives a clear message ✓
Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.
Questions people ask
How do I calculate simple interest?
Multiply the principal by the yearly rate (as a decimal) and the time in years. $5,000 at 4% for 3 years: 5,000 × 0.04 × 3 = $600.
How do I calculate simple interest for days?
Divide the days by 365 (or 360 if your agreement says so) to get years. $10,000 at 5% for 90 days: 10,000 × 0.05 × 90/365 = $123.29.
How do I find the rate from the interest?
Divide the interest by principal × time. $150 earned on $2,000 over 1.5 years: 150 ÷ (2,000 × 1.5) = 0.05, so 5% a year.
How long does it take to earn a set amount?
Divide the interest you want by principal × rate. $90 on $1,000 at 6%: 90 ÷ (1,000 × 0.06) = 1.5 years.
What is the difference between simple and compound interest?
Simple interest is only on the original principal. Compound interest is also on interest already added, so it grows faster the longer the money stays.