Interest Calculator
Calculate simple or compound interest on a principal with any rate, time, and frequency.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this interest calculator
- Pick "Simple interest" for a one-shot interest calculation or "Compound interest" for reinvested growth.
- Enter the principal and choose a currency.
- Type the annual rate (%) and the time in years.
- For compound mode, pick a compounding frequency: annual, semi-annual, quarterly, monthly, or daily.
- Press Calculate to see the interest earned and the total (principal + interest).
- Use Copy to share the result, or Reset to try other numbers.
About this interest calculator
The interest calculator handles the two textbook formulas. Simple interest: I = P · r · t, where P is principal, r is the annual rate as a decimal, and t is time in years; the total is P + I. Compound interest: A = P(1 + r/n)^(nt), where n is compoundings per year; the interest portion is A − P. These are the canonical formulas in the U.S. SEC's Investor.gov "Compound Interest" primer and the U.S. Federal Reserve's Truth-in-Savings Act disclosures (Regulation DD).
Worked example (simple): P = USD 1,000, r = 5%, t = 3 years. I = 1,000 × 0.05 × 3 = USD 150; total = USD 1,150. Worked example (compound, monthly): same inputs with n = 12. A = 1,000 × (1 + 0.05/12)^(12·3) ≈ USD 1,161.47; interest ≈ USD 161.47. Notice compound interest produced about USD 11.47 more — and the gap widens dramatically over longer horizons. Currency is just a label; there is no exchange-rate conversion or live rate lookup.
FAQ
- What is the formula for simple interest?
- I = P · r · t, where P is principal, r is the annual rate as a decimal, and t is time in years. For USD 1,000 at 5% for 3 years: I = 1,000 × 0.05 × 3 = USD 150.
- What is the formula for compound interest?
- A = P(1 + r/n)^(nt). The interest earned is A − P. For USD 1,000 at 5%/yr, monthly compounding (n = 12), 3 years: A ≈ USD 1,161.47, interest ≈ USD 161.47.
- When should I use simple vs compound?
- Use simple for short-term loans, bridge financing, and bonds with non-reinvested coupons. Use compound for savings accounts, CDs, mortgages, and any interest that is reinvested or capitalized into the principal.
- Does a higher compounding frequency help much?
- Marginally. Switching from annual to monthly at 5% over 3 years adds about USD 5.97 on USD 1,000. Going from monthly to daily adds well under a dollar. The first jump matters most.
- Does this include contributions or withdrawals?
- No — it is a single-principal calculation. Use our Compound Interest Calculator if you want to model regular contributions.
- Is my data shared?
- No. All calculations happen in your browser; nothing is uploaded.