Interest Calculator

What is Interest Calculator?

The Interest Calculator works out how money grows over time under two methods. Simple interest applies the rate only to the original principal, while compound interest applies it to the running balance so earlier interest itself earns interest. Switch between the two with the Simple / Compound toggle. Savers and students use it to project a deposit or a CD, while borrowers use it to understand the cost of a loan. The compounding-frequency setting (annual, semiannual, quarterly, monthly, daily or continuous) controls how often interest is added in compound mode, which is why the same nominal rate can produce different effective annual yields. The APY field shows that true yearly return as a percentage.

How to use Interest Calculator

1. Choose Simple or Compound interest with the segmented toggle at the top.
2. Enter the starting principal — the amount you deposit or borrow.
3. Enter the annual interest rate as a percentage (for example 5 for 5%).
4. Enter the term in years; fractional years like 2.5 are allowed.
5. For compound interest, open Settings and pick the compounding frequency (yearly through daily, or continuous).
6. Read the future value, total interest and effective annual yield (APY); they recalculate automatically.

Examples

Mode: Compound, frequency: Monthly, principal = 10000, rate = 5, years = 10

Future value ≈ 16,470.09, interest ≈ 6,470.09, APY ≈ 5.116%

Mode: Simple, principal = 10000, rate = 5, years = 10

Future value = 15,000.00, interest = 5,000.00, APY = 5%

Mode: Compound, frequency: Continuous, principal = 1000, rate = 8, years = 5

Future value ≈ 1,491.82, interest ≈ 491.82, APY ≈ 8.329%

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal: future value = P × (1 + r × t). Compound interest is calculated on principal plus all interest accumulated so far, so earlier interest also earns interest: future value = P × (1 + r ÷ n)^(n × t), where n is the compounding frequency.

What does APY mean here?

APY is the effective annual yield — the real percentage your balance grows in one year once compounding is taken into account. A 5% nominal rate compounded monthly gives an APY of about 5.116%, because interest is added twelve times rather than once.

How does the compounding frequency change the result?

The more often interest is added, the larger the future value for the same nominal rate. Daily compounding beats monthly, which beats annual; continuous compounding (e^(r × t)) is the theoretical upper limit. The frequency setting only affects compound mode — simple interest ignores it.

Can I enter fractional years or zero values?

Yes. You can enter fractions like 2.5 years, and a rate or term of zero simply returns the principal with no interest. If a field is blank or not a number, the results show a dash (—).

Are my numbers sent anywhere?

No. The entire calculation runs client-side in your browser with no upload and no server communication, so the principal, rate and term you type never leave your device.