💹 Simple Interest Calculator

How the Simple Interest Calculator Works

Simple interest is charged or earned only on the original amount (the principal), never on interest that has already built up. It is used for many short-term loans, some car loans, certain bonds and savings certificates, and late-payment charges. Enter the principal, the annual interest rate, and the time period; the calculator returns the interest and the total amount at the end.

For comparison, it also shows what the same rate would produce with annual compounding, so you can see how much difference compounding makes over your time period. The calculator works in any currency — just enter amounts in the currency you use.

Formula

Interest (I) = P × r × t
Total amount (A) = P + I = P × (1 + r × t)
P = principal, r = annual rate as a decimal (5% = 0.05), t = time in years
Months → t = months ÷ 12      Days → t = days ÷ 365

How to Use It

1. Enter the principal amount.
2. Enter the annual interest rate in percent.
3. Enter the time and choose years, months, or days.
4. Read the interest, the total amount, and the compound-interest comparison.

Frequently Asked Questions

What is the formula for simple interest?

Simple interest = principal × annual rate × time in years (I = P × r × t). For example, 10,000 at 6% for 3 years earns 10,000 × 0.06 × 3 = 1,800 in interest.

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal. Compound interest is also calculated on interest already earned, so it grows faster over time — the longer the period, the bigger the gap.

How do I calculate simple interest for days?

Convert the days to years by dividing by 365 (some lenders use 360), then apply I = P × r × t. For 90 days, t = 90 ÷ 365 ≈ 0.2466.

Related Calculators

• 📈 Compound Interest Calculator — See how your money grows with compound interest and regular monthly contributions.
• 💳 Loan Calculator (EMI) — Works for any loan type — find your monthly payment and total interest.
• ⚖️ Flat vs. Reducing Balance Rate Calculator — Compare flat and reducing-balance loans and find the true effective rate.
• 🎈 Inflation Calculator — Future cost and buying power of money for any inflation rate.

Formula and content last reviewed: September 2026. All calculations run in your browser — nothing you enter is sent to or stored on our servers.