Interest Calculator
See how a balance grows with simple or compound interest, any compounding frequency, and optional monthly contributions — with the final balance, total interest, and a year-by-year curve.
$10,000 at 5% compounded monthly for 10 years grows to $16,470 — $6,470 of it interest, an effective 5.12% APY.
$16,470
$10,000 at 5% compounded monthly for 10 years
5.12% APY
Effective annual yield of a 5% nominal rate compounded monthly
$6,470
Interest earned in that example — pure growth on top of deposits
Why compound interest builds wealth
Three forces — compounding, contributions, and time — drive the final number.
Compounding is interest on interest
Each period, interest is added to your balance and the next period's interest is calculated on the larger total. Over years, this snowball is what separates compound from simple growth.
Small contributions compound too
Adding a fixed amount each month is one of the most reliable ways to grow a balance. Every deposit starts earning its own interest from the moment it lands.
Time matters more than rate
Because growth compounds, the number of years often moves the final balance more than a small change in rate. Starting earlier usually beats chasing a slightly higher return.
How the Interest Calculator Works
Formula
Compound:
monthlyRate = (1 + rate ÷ n)^(n ÷ 12) − 1
balance = balance × (1 + monthlyRate) + contribution
Simple:
interest += deposits × (rate ÷ 12) each month
balance = deposits + interest
final balance = principal + contributions + interest
total interest = final balance − total deposited
APY = (1 + monthlyRate)^12 − 1Choose simple or compound
Compound pays interest on interest; simple pays only on deposits.
Enter your balance, rate, and term
The rate is yours to set — use any savings, CD, or investment assumption.
Pick a compounding frequency
Annually through daily; more frequent compounding raises the effective yield.
Add optional contributions
A recurring monthly deposit compounds alongside the starting balance.
Review the growth curve
See the final balance, total interest, APY, and a year-by-year schedule.
This is a planning model that assumes a constant rate and end-of-month contributions. Real accounts can change rates, charge fees, or tax interest, all of which affect your actual return.
Compound interest works for you when you are saving and against you when you are borrowing — the same math drives loan and credit-card balances, just in reverse.
Frequently Asked Questions
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Pair this with adjacent savings and growth tools.