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Compound Interest Calculator

Calculate compound interest on investments

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What is Compound Interest Calculator

Written by Giorgos Kostas. Last reviewed:

Compound Interest Calculator computes how a principal amount grows over time when interest compounds at a chosen frequency, using the textbook formula A = P × (1 + r/n)^(nt), plus an optional annuity term for recurring contributions.

The result is the future value, total interest earned, and an amortization table showing the balance at the end of each compounding period — useful for verifying a bank's disclosure or sanity-checking an investment projection.

Why use it

  • Project how a savings account, CD, or brokerage deposit grows under a given interest rate.
  • Compare the effect of changing compounding frequency (monthly vs daily) on the same principal and rate.
  • Figure out the required monthly contribution to reach a target future value.

Features

  • Daily, monthly, quarterly, semiannual, and annual compounding frequencies
  • Optional regular contribution (monthly or yearly) with 'beginning-of-period' or 'end-of-period' timing
  • Year-by-year and period-by-period amortization tables showing interest earned each step
  • Chart of the balance over time, with a separate overlay for total contributions vs interest earned
  • All math is shown with the substituted formula so you can reconcile the result in Excel

How to use Compound Interest Calculator

  1. Enter the principal. Type the starting amount — the initial deposit that will compound. Any currency symbol is cosmetic; the math is identical for USD, EUR, GBP, etc.
  2. Set the annual rate and term. Enter the annual interest rate as a percent (e.g. 5.25) and the term in years (fractions allowed, e.g. 2.5).
  3. Pick compounding frequency. Choose daily (n=365), monthly (n=12), quarterly (n=4), semiannual (n=2), or annual (n=1). Daily is most common for savings accounts; monthly for mortgages.
  4. Add regular contributions (optional). Enable the annuity term and enter a contribution amount plus frequency. The calculator supports both end-of-period and beginning-of-period deposits.
  5. Review the output. The future value, total interest, and amortization table update live. Hover the chart to see the balance at any year.

Example (before/after)

Calculation inputs

Enter the values needed to calculate compound Interest in Compound Interest Calculator for your current scenario.

Calculated result

Review the resulting compound Interest numbers from Compound Interest Calculator and adjust inputs to explore different scenarios.

Common errors

Missing required values

Calculators cannot return meaningful results when one or more required inputs are empty.

Fix: Fill in every required field before calculating.

Wrong units or scales

Using the wrong units or mixing percent and decimal values can skew the result.

Fix: Double-check that each field uses the expected unit, scale, or percentage format.

Unrealistic ranges

Extreme or inconsistent inputs can produce output that looks broken even when the formula is correct.

Fix: Review the assumptions behind the numbers and correct any out-of-range values.

FAQ

What's the difference between compound interest and APY?

Compound interest is the mechanism — interest computed on principal plus previously accrued interest. APY (Annual Percentage Yield) is the result: the effective annual rate after compounding. For a 5% rate compounded monthly, APY is about 5.116%. Our APY calculator converts between the two explicitly.

Does the calculator account for taxes on the interest?

No — the result is the pre-tax gross. Tax rates vary by country, income bracket, and account type (tax-advantaged like IRA/ISA vs taxable). Apply your marginal tax rate to the 'total interest earned' line to get the post-tax figure.

Can the calculator handle negative or variable interest rates?

Negative rates are supported — the math is the same, just with a negative r. Variable rates aren't supported directly; for a variable schedule, run the calculator multiple times (one per rate period) using the ending balance of each run as the next run's principal.

Why do monthly contributions 'beginning of period' give a higher result?

A beginning-of-period contribution earns interest for one more period than an end-of-period contribution. Over 30 years of monthly contributions, the difference can be 8–10% of the total — which is why choosing the right timing matters for annuity math.

How accurate is the year-by-year amortization table?

Exact — the table uses the compound-interest formula at each period boundary and shows the interest earned in that period, not an average. It'll match Excel's FV() and NPER() functions to the rounding digit.

Is the math the same across currencies?

Yes. Compound interest is a pure ratio calculation — dollars, euros, pounds, yen all use the same formula. The currency symbol in the UI is cosmetic; pick the one that matches your data.

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