Compound Interest Calculator: Future Value & Initial Principal

 

Calculate compound interest from an initial principal, or work backwards from a future savings target to find the starting amount required. Choose annual, quarterly, or monthly compounding.

Calculate compound growth or starting principal

Enter a fixed nominal annual interest rate, not an APY. This model assumes no deposits or withdrawals. All amounts are in US dollars.

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For formulas and worked examples, read our guide to annual compound interest.

How to calculate compound interest

Future value = P x (1 + r / n)^(n x t)

P is the initial principal, r is the annual percentage divided by 100, n is the number of compounding periods per year, and t is the duration in years. Total interest is future value minus initial principal.

Annual compounding example

With $10,000 at a nominal annual rate of 5% for 10 years, compounded annually: $10,000 x 1.05^10 = $16,288.95. The total interest is $6,288.95.

Backwards compound interest calculator

To find the starting principal, rearrange the formula: P = future value / (1 + r / n)^(n x t). For a $10,000 target after 10 years at 5%, compounded annually, the required starting amount is approximately $6,139.13, with no additional contributions.

Frequently asked questions

Is the interest rate the same as APY?

No. Enter a nominal annual rate. APY already reflects compounding. For example, 5% nominal interest compounded monthly has an effective annual yield of approximately 5.1162%. Entering that APY as the nominal rate would overstate growth.

Can I include monthly contributions?

No. This version models a single starting amount, with no later deposits or withdrawals. A savings plan with recurring contributions needs a different formula.

Can I enter part of a year?

Yes, up to two decimal places. The formula uses fractional compounding periods for fractional years. Actual account crediting dates and partial-period rules may produce different results.

What happens at a 0% rate?

Initial principal and future value are equal, and total interest is zero.

What input limits apply?

Amounts must be greater than zero and no more than $100 million. Rates range from 0% to 100%. Duration must be greater than zero and no more than 100 years. Future values above $1 trillion are not displayed. These are calculation limits, not expected returns.

For loan repayments, use our Mortgage Payment Calculator.

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