Yield Theory
Subscribe

Calculator

Compound Interest Calculator

See how an investment grows when returns compound over time.
Future value
$343,778
Total contributed
$130,000
Interest earned
$213,778

Compound interest is the engine behind long-term investing: your returns earn returns of their own. This calculator projects how a starting balance plus regular contributions can grow at a given annual rate, compounded over time. Adjust the inputs to see how starting earlier or contributing more changes the outcome.

Formula reviewed 2026-08-021 primary sourceMethodology and disclosures →

How this compound interest calculator works

The calculator grows your starting balance and each end-of-month contribution at the same assumed monthly rate. It then separates the money you contributed from the projected growth so you can see what compounding is doing.

Formula

FV = P(1 + r/12)12t + C × [((1 + r/12)12t − 1) ÷ (r/12)]

This is a projection, not a forecast. Real investment returns are uneven and can be negative, especially over shorter periods.

Primary specifications

Before you use the result

Assumptions

  • • The annual return is steady and compounds monthly.
  • • Contributions are added at the end of each month.
  • • Taxes, fees, inflation, and changes in return are not included.

Quick start

  1. 1. Enter the amount already invested.
  2. 2. Add the amount you expect to contribute each month.
  3. 3. Test a conservative annual return and time horizon.

Frequently asked questions

What is compound interest?

Compound interest is interest calculated on both your original principal and the interest already added to it. Over long periods this creates exponential, rather than linear, growth.

How often should returns compound?

More frequent compounding produces slightly higher results, but the annual rate and time horizon matter far more. For long-term stock investing, annual compounding is a reasonable approximation.

What return rate should I assume?

Historically, broad US equity indices have returned roughly 7–10% annualized over long horizons before inflation, but future returns are not guaranteed. Use a conservative estimate and test a range.

Does the result include fees, tax, or inflation?

No. It is a nominal projection before taxes, investment fees, and changes in purchasing power.

More calculators

AI Capex, Depreciation & Return ModelModel how AI infrastructure vintages flow through depreciation, replacement spending, free cash flow, and the revenue required to earn a return.AI Power Exposure & Earnings ModelTranslate announced data-center megawatts into probability-adjusted load, electricity spend, rate-base opportunity, and supplier exposure.Company FX Revenue & EPS Sensitivity ModelSeparate currency translation from operating growth and estimate revenue, margin, and EPS sensitivity by currency exposure.AI Silicon Break-Even MonitorEstimate when custom AI silicon repays design and software costs—and what that could mean for GPU spend and supplier economics.AI Model-to-HBM Demand CalculatorEstimate whether a model fits, the accelerator count required, and how model architecture translates into deployment HBM demand.Dollar-Cost Averaging CalculatorProject the outcome of investing a fixed amount at a regular interval.

Go deeper

Investing glossaryUnderstand the conceptsStock researchApply it to real names

The math is easy. The macro is hard.

Compound Interest Calculator shows the arithmetic. Members get the monthly call that decides which inputs matter, plus written breakpoints when the facts move. $15/month or $150/year.

Get this month’s call for $15

$15/mo or $150/yr · cancel future renewals anytime · sources included