$10,000 invested at 7% for 20 years
With monthly compounding and no additional deposits, $10,000 grows to $40,387.39.
That is $30,387.39 of compound growth on the original $10,000. The 7% rate is an illustration, not a guaranteed investment return.
Project savings and investment growth
Compound Interest Calculator with Monthly Contributions
Calculate compound interest with monthly contributions and daily, monthly, quarterly, semiannual, or annual compounding. Compare nominal growth, inflation-adjusted purchasing power, and optional estimated tax drag.
Tax & inflation aware
Year-by-year projections
02 / Compare
Saved Growth Scenarios
Save up to five plans to compare rates, contribution levels, time horizons, taxes, and inflation assumptions.
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| Scenario | Final balance | Real balance | Contributions | Net interest | Tax paid |
|---|
Growth Charts
Interactive balance growth
Yearly net interestAligned with the visible year range
Ending balance comparison
Contributions vs net interest
MonkeyTactics
Compound Interest Report
Scenario graphs
Balance growth over time
Ending balance
Contributions vs net interest
Worked examples
Compound interest examples
These illustrative results use the same calculation method as the tool above. Contributions are added at the end of each compounding period, amounts are rounded to the nearest cent, and tax is set to 0% unless stated otherwise.
$200 monthly contribution for 30 years
Starting from $0 and earning an illustrative 7% compounded monthly, $200 per month grows to $243,994.20.
The deposits total $72,000; the remaining $171,994.20 is modeled compound growth.
Daily versus monthly compounding
On $10,000 at 7% for 20 years, daily compounding produces $40,546.56, compared with $40,387.39 for monthly compounding.
Under these assumptions, compounding daily adds $159.17 over 20 years. The difference depends on the balance, rate, and time period.
7% return after 3% inflation
After 20 years, $10,000 at an illustrative 7% compounded monthly reaches $40,387.39 nominally but has estimated purchasing power of $22,361.52 in today's dollars.
The real balance divides the nominal result by 20 years of cumulative 3% inflation. Taxes and fees are not included.
How long does it take $10,000 to double?
At an illustrative 7% compounded monthly, $10,000 doubles in about 9.93 years and reaches $20,096.61 after 10 full years.
The Rule of 72 gives a quick estimate of 10.3 years. The exact result varies with compounding frequency and any taxes or fees.
How to use this calculator
Enter your initial investment, annual interest rate, investment period, and optionally a monthly contribution. Choose the compounding frequency stated by the account or financial product you want to model.
Leave the optional tax rate at 0% for a pre-tax projection, or enter a flat estimated rate to model annual tax drag. The inflation rate converts the projected balance into today's purchasing power. Results include after-tax nominal and real balances, contributions, net interest, estimated tax, interactive charts, and a year-by-year table.
Save up to five scenarios to compare different rates, contribution amounts, time horizons, tax assumptions, and inflation assumptions. Select any saved scenario to edit it, print it, or export its yearly data as PDF, CSV, or plain text.
How are tax and inflation applied?
The calculator processes growth at the selected compounding frequency and summarizes the results annually:
Periodic rate = annual rate ÷ compounding periods
Periodic contribution = monthly contribution × 12 ÷ compounding periods
Periodic balance = prior balance × (1 + periodic rate) + periodic contribution
Annual tax paid = annual gross interest × tax rate
Real balance = nominal balance ÷ (1 + inflation rate)^year
The contribution convention matches Investor.gov: the annualized monthly contribution is divided evenly among the selected compounding periods and added at each period's end. Optional tax is estimated once per year from modeled gross interest. This simplified tax treatment does not account for tax brackets, allowances, account types, or jurisdiction-specific rules.
The real balance divides the after-tax nominal balance by cumulative inflation. It is an estimate of future purchasing power in today's dollars, not a forecast of actual prices or investment returns.
Frequently Asked Questions
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What is compound interest?
Compound interest is interest calculated on both the initial principal and all previously earned interest. Unlike simple interest — which earns only on the original amount — compound interest can accelerate growth over time. The outcome still depends on the rate, contribution timing, fees, taxes, withdrawals, and actual returns.
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How are monthly contributions calculated?
The calculator multiplies the monthly contribution by 12 and divides that annual amount evenly among the selected compounding periods. Each portion is added at the end of its period, matching the convention used by the Investor.gov compound interest calculator.
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Does this calculator account for taxes?
Yes. The optional tax rate is applied to each year's modeled gross interest. The result shows an after-tax nominal balance, an inflation-adjusted real balance, estimated tax paid, and gross and net interest. Set the rate to 0% when you do not want to model annual tax. Actual tax treatment depends on the account, investment, holding period, and jurisdiction.
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How does inflation affect the results?
Inflation erodes purchasing power over time. A balance of $100,000 in 20 years at 3% inflation is only worth about $55,000 in today's dollars. Enter your expected inflation rate and the "Real Balance" result — and the real column in the growth table — shows your balance expressed in today's purchasing power so you can set realistic goals.
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What is the compound interest formula?
Without contributions, the standard formula is A = P(1 + r/n)nt, where P is principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. With contributions, each periodic deposit compounds only for the time remaining after it is added.
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What is the Rule of 72?
Divide 72 by your annual gross interest rate to estimate how many years it takes to double your money. At 7% it takes roughly 10.3 years (72 ÷ 7). Note that taxes and inflation will extend the real doubling time — the calculator shows the gross Rule of 72 figure as a quick reference.
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What compounding frequency should I use?
Use the frequency stated by the account or product you are modeling. Daily and monthly compounding produce slightly different results because interest is credited at different intervals. Stocks and ETFs do not earn a guaranteed fixed rate, so any frequency and return entered for market investments is only a planning assumption.
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Why might another compound interest calculator show a different result?
Calculators can use different contribution timing, compounding, tax, inflation, rounding, and deposit conventions. For an apples-to-apples comparison, match every shared input and set tax and inflation to 0% when the other calculator does not include them.
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Is my data kept private?
Yes — calculations run locally in your browser using the shared Rust and WebAssembly finance engine. Inputs are not sent to a calculation server, and saved comparison scenarios last only for the current page session. See our Privacy Policy for details.
Methodology and limitations
This model follows the Investor.gov convention: it divides the annualized monthly contribution evenly among the selected compounding periods and applies each portion at the end of its period. Tax is an illustrative percentage of each year's calculated interest; inflation converts the resulting nominal balance into an estimated value in today's purchasing power.
Limitations: Results are estimates for planning, not investment, tax, or financial advice. Actual returns, timing of deposits, taxes, fees, inflation, and account rules can differ.
Source: U.S. Securities and Exchange Commission: Compound Interest Calculator.
Author and reviewer: Adriaan Greyling is a financial technology software engineer who has worked on transaction and credit-decisioning systems at large financial institutions since 1996. He does not hold financial planning, investment, tax, or accounting credentials. His review covers the calculator's formula implementation, calculation behavior, and alignment with the cited methodology—not personalized financial advice.
Prepared and reviewed by Adriaan Greyling for MonkeyTactics. Last reviewed .
