Linear, not exponential
Simple interest grows in a straight line—double the time, double the interest. No snowballing like compound interest.
Flat curve
Simple Interest Calculator (I = P·r·t)
Enter your values
Starting balance (P)
APR as a percent (e.g., 5 = 5%)
Shortcuts: Enter to calculate • Esc to clear.
Results
How simple interest works
Simple interest grows linearly with time. It’s common in short-term loans and some bonds.
- Interest: \(I = P\cdot r \cdot t\)
- Total amount: \(A = P(1+rt)\)
- \(P\) = principal, \(r\) = annual rate as a decimal (5% → 0.05), \(t\) = time in years
Tip: 18 months = 1.5 years; 90 days ≈ 90/365 years (actual day count can vary by contract).
Simple Interest: FAQ
What is the simple interest formula?
Simple interest uses \(I = P\cdot r \cdot t\) where \(P\) = principal, \(r\) = annual rate (decimal), \(t\) = years. The total is \(A = P(1+rt)\).
Does this calculator compound interest?
No—this tool uses simple interest only. For compounding, try our Compound Interest Calculator.
Are my inputs private?
Yes. Everything runs in your browser—no uploads, no accounts.
What units should I use for time?
Enter years directly, or choose months/days and we’ll convert to years automatically (months ÷ 12; days ÷ 365).
5 Fun Facts about Simple Interest
The Roman “usura” vibe
Ancient Romans often used flat-rate “usura” charges—today’s simple interest echoes that straightforward math.
History rhymes
APR ≠ always simple
Some loans quote an APR but calculate payments with compounding. Simple interest loans amortize more evenly.
Check the fine print
Day count tweaks results
Using 365 vs 360 days changes daily interest slightly—common in mortgages and corporate notes.
Day-count quirks
Grace periods quietly matter
A short grace period before interest starts can cut total costs—zero interest until disbursement is underrated.
Borrower bonus