Interest calculator

Estimate how interest grows your balance over time.

Last updated: January 8, 2026

Built and maintained by Aaron Jegla

Compare simple vs compound interest, add contributions, and see year-by-year growth.

Interest details

The amount you have today.

Enter the annual rate before compounding.

Use years. Decimals are ok (ex: 7.5).

Compound earns interest on interest. Simple uses the starting balance only.

Choose how often interest is applied to the balance.

Add deposits you plan to make each period.

Only used when contributions are above $0.

For fixed-rate savings, CDs, or simple-interest loans.

Not for variable-return investments. Try our other calculators.

How to use this calculator
  1. Enter your starting balance, interest rate, and time horizon.
  2. Choose simple or compound interest and a compounding frequency.
  3. Add regular contributions if you plan to keep saving.

Select Calculate to see the summary and yearly breakdown.

Summary

Results are estimates for compound interest. Total contributions include your starting balance.

Ending balance

$0

Total contributions

$0

Total interest earned

$0

Interest share

0%

Interest type: Compound | Compounding: Monthly | Contributions: Monthly

Sources: Investor.gov compound interest calculator, CFPB simple vs precomputed interest

Assumptions: Fixed annual rate for the full term. Compounded interest applies at the selected frequency. Contributions are added at the end of each contribution period and begin earning interest after they are added. Simple interest uses the starting balance only.

Disclaimer: Estimates only and not financial advice. Actual returns depend on rate changes, fees, taxes, and deposit timing.

Balance breakdown

See how much of the ending balance comes from contributions vs interest.

Total contributions $0 (0%)
Total interest $0 (0%)

Interest makes up 0% of the ending balance.

Growth summary

Totals based on your inputs.

Year Starting balance Contributions Interest earned Ending balance

How this calculator works

Simple and compound interest are calculated differently.

Simple interest uses this formula:

Interest = P * r * t

  • P is the starting balance.
  • r is the annual interest rate (as a decimal).
  • t is the time in years.

Compound interest grows based on:

Ending balance = P * (1 + r / n)^(n * t)

Contributions are added at the end of each contribution period and begin earning interest after they are added.

Interest insights and assumptions

Quick context to help you interpret the results.

Simple vs compound

Compound interest grows faster because past interest earns interest too. Simple interest grows in a straight line.

  • Compound is typical for savings and investments.
  • Simple is common for some short-term loans.

Compounding frequency

More frequent compounding yields slightly higher results, but the difference is usually modest.

  • Monthly vs daily is often a small gap.
  • Use the frequency your institution states.

APR vs APY

APR is the nominal annual rate. APY includes the effect of compounding.

  • If you only know APY, set compounding to yearly and enter the APY.
  • Check the fine print for exact compounding rules.

Contribution timing

This model adds contributions at the end of each period. Deposits made earlier can earn slightly more.

  • Adjust the contribution amount to see a range.
  • Use the yearly table to sanity-check growth.

Fixed rate assumption

The calculator assumes a constant rate for the full term.

  • Variable rates will change actual results.
  • Fees and taxes are not included.

Example inputs to try

Use these to sanity-check the math before adjusting.

  • $10,000 at 5% for 10 years
  • $5,000 at 4% with $100 monthly for 8 years
  • $20,000 at 6% for 5 years

Interest calculator FAQs

Click a question to expand the answer.

What is the difference between simple and compound interest?

Simple interest is calculated only on the starting balance. Compound interest also earns interest on the interest that has already been added.

Does this use APR or APY?

The input is an annual rate (APR). If you only know APY, set compounding to yearly and enter the APY for a close estimate.

When are contributions added?

Contributions are added at the end of each contribution period and start earning interest after they are added.

What if my interest rate is 0%?

At 0% interest, the ending balance equals your starting balance plus any contributions you add.

Does this include taxes, fees, or inflation?

No. This calculator shows pre-tax growth with no fees or inflation adjustments. Use it as a directional estimate.

Why might my bank statement differ?

Real-world balances can differ because of daily compounding rules, fee schedules, variable rates, and the exact timing of deposits and withdrawals.

Can I use this for loan interest?

This calculator is designed for savings and investment growth. For loans, use the Loan Calculator so payments and amortization are modeled correctly.

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