Interest Calculator
Four calculators in one: simple interest, compound interest, savings with monthly deposits, and loan repayment with a full amortization schedule. Loans support equal payments (annuity, 元利均等) and equal principal (元金均等). All math uses exact decimals.
What does the Interest Calculator do?
It answers "how much will this grow?" and "how much will I pay?". Pick a tab, enter the amounts and the annual rate, and the result and a year-by-year (or month-by-month) table appear. Nothing leaves your browser.
Tabs
- Simple: interest = principal × annual rate × years. Interest is never added to the principal.
- Compound: interest is added to the principal yearly, half-yearly, quarterly, monthly or daily, and then earns interest itself.
- Savings: an initial amount plus a deposit at the end of every month, compounded as you choose. Shows the total deposited and the interest earned each year.
- Loan: the monthly payment and the schedule of principal, interest and remaining balance for every month.
How to use
- Choose a tab and enter the amounts. Set Currency for a symbol and Decimals for how amounts are rounded (0 for yen).
- Enter the Annual rate in percent and the term.
- Read the summary and scroll the table.
Loan repayment methods
| Method | Monthly payment | Total interest |
|---|---|---|
| Equal payment (annuity, 元利均等) | Same every month: P × r / (1 − (1 + r)^−n) | Higher |
| Equal principal (元金均等) | Starts high and falls each month | Lower |
Here P is the loan amount, r the monthly rate (annual rate ÷ 12) and n the number of months. Each month's interest is rounded to the chosen decimals, and the last payment settles whatever remains.
Examples
- 1,000,000 at 5% compounded yearly for 10 years grows to ≈ 1,628,895.
- Saving 10,000 a month at 3% compounded monthly for a year gives ≈ 121,664.
- A 30,000,000 loan at 1% over 35 years with equal payments costs 84,686 a month.
Features
- Simple, compound, savings and loan calculations
- Compounding yearly, half-yearly, quarterly, monthly or daily
- Equal-payment and equal-principal loans with a month-by-month schedule
- Exact decimal arithmetic, any currency symbol, 0–2 decimals
- Inputs are remembered in this browser
FAQ
Are taxes or fees included?
No. Results are before tax and fees. Real banks may also round or count days differently, so treat the result as an estimate.
Why does the last loan payment differ slightly?
Payments are rounded, so the last month pays exactly what is left. That makes the principal add up to the loan amount.