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Interest Calculator | Compare Simple and Compound Interest

Last updated: June 2026 · Free · No sign-up required

Results

Enter values above and click Calculate to see your result instantly.

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Quick reference

Type	Formula
Simple interest	I = P × r × t
Annual compound	A = P(1+r)^t
Monthly compound	A = P(1+r/12)^(12t)

savings Plan with confidence

Finance decisions get a lot easier when you can see the full picture. Enter your numbers above to see total payments, interest paid, and the long-term cost of every choice — so you can compare options side by side before signing anything.

percent How the math works

We use the standard amortization, compound-interest and present-value formulas published by the Consumer Financial Protection Bureau and the Federal Reserve. The methodology block below shows every variable and rounding step we apply, so the answer is never a black box.

shield_lock Your data stays private

Every calculation happens in your browser with JavaScript — your income, balances, and loan numbers are never sent to our servers, logged, or shared. Close the tab and the inputs vanish. No sign-up, no tracking pixels on the form, no spreadsheet emailed to you later.

lightbulb Pro tip

Save the URL after you calculate — your inputs aren't stored, so write down the headline number plus the breakdown. Then come back and edit one variable at a time (down payment, rate, term) to see exactly which lever moves your monthly figure the most. That's where the real planning happens.

Interpretation guide

Use case	Best metric
Loans	Total interest paid
Savings	Future balance
Compare rates	Use APY/EAR when compounding differs

lightbulb Example — simple interest on $2,000 at 5% for 3 years

Simple interest is principal × rate × time = 2,000 × 0.05 × 3.

Result: Interest = $300; total = $2,300.

What this means: Simple interest is linear; with compound interest the total would be slightly higher because interest earns interest.

Formula & methodology

Formula: Simple: I = PRT; compound: A = P(1+r/n)^(nt)

The Interest Calculator is built on a well-established calculation method. It uses the formula Simple: I = PRT; compound: A = P(1+r/n)^(nt) to turn your inputs into a reliable result. Enter the principal, rate, and time, and this calculator returns the simple interest earned or owed, plus the total balance, with the formula (I = PRT) shown step by step. It also calculates compound interest on the The steps are shown on the page so you can follow the reasoning from input to output.

Simple interest pays only on principal. Compound interest pays on principal plus previously earned interest, so compounding frequency changes the effective return.

Authoritative source: Consumer Financial Protection Bureau

Frequently asked questions

How do I use the interest calculator?

Enter the principal, annual rate, and time, then choose simple or compound interest. Simple uses I = PRT, so $2,000 at 5% for 3 years earns $300. Compound uses A = P(1 + r/n)^(nt) and grows faster the more frequently it compounds. The tool returns the interest earned and the total balance for both methods side by side.

What formula does the interest calculator use?

The Interest Calculator uses the standard Simple: I = PRT; compound: A = P(1+r/n)^(nt). You enter your values, and the calculator applies the formula step by step, showing the working so you can verify the math and understand how the result is derived rather than trusting a black box.

Is the interest calculator accurate?

The Interest Calculator applies the standard formula: Simple: I = PRT; compound: A = P(1+r/n)^(nt). It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

Why do my results differ from another calculator?

The Interest Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Enter the principal, rate, and time, and this calculator returns the simple interest earned or owed, plus the total balance, with the formula (I = PRT) shown step by step. It also calculates compound interest on the same inputs so you can compare the two side There is no limit on usage, and it works on any device.

Can the interest calculator replace professional advice?

No — it computes interest with the standard simple and compound formulas, but it does not model fees, rate changes, penalty terms, or tax treatment, and it is not financial advice. Use it to understand how interest works on a loan or deposit, then confirm the actual terms with your lender or bank, and consult a professional for major financial decisions.

What does the Interest Calculator do?

Enter the principal, rate, and time, and this calculator returns the simple interest earned or owed, plus the total balance, with the formula (I = PRT) shown step by step. It also calculates compound interest on the same inputs so you can compare the two side. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.

Is the Interest Calculator free and private?

Yes — the Interest Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.

How accurate is the Interest Calculator?

The calculations are exact: simple interest I = PRT and compound interest A = P(1 + r/n)^(nt) are applied with full precision for the inputs you enter. Real-world accuracy depends on the inputs matching your actual terms — banks may compound daily, charge fees, or vary rates, which the standard formulas do not capture. For typical fixed-rate deposits, the estimate is within a few dollars.

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