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Fixed rate · monthly periods · lump-sum projection

Monthly compound interest calculator

Project a starting balance at a fixed nominal annual rate, divided into monthly periods. Set recurring deposits to zero to isolate compounding frequency; use the inputs to explore other assumptions.

By Snowballr Research Team
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Coverage: Compound interest · Retirement · FIRE · Debt payoff · Mortgages · Fraud prevention
Built from: IRS · FINRA · SEC · BLS · Federal Reserve · Freddie Mac30+ primary sources verified
Illustration · $10,000 at a fixed 8% nominal annual rate

With monthly compounding and no recurring deposits, the balance is about $22,196 after 10 years and $109,357 after 30 years, before fees and taxes. This is a mathematical example, not a return forecast.

Key terms

Monthly compounding
Interest added every month; next month earns on the new balance.
Nominal annual rate
The entered annual rate before dividing by 12 for monthly periods. It is an assumption, not a promised return.
Effective annual yield (APY)
A yearly yield that already includes compounding. Convert it before entering it as a nominal annual rate.

How this monthly-compounding model works

For a lump sum with no deposits, the formula is A = P × (1 + r/12)12×t. Here, P is the starting balance, r is a nominal annual rate written as a decimal, and t is the number of years. The model applies the same rate each period; real account rates and investment returns can change.

The calculator also includes a monthly-deposit input. Leave it at zero for a pure lump-sum comparison. To focus on regular deposits, see the compound interest calculator with monthly contributions.

Monthly vs annual and daily compounding

For the same $10,000 starting balance, fixed 7% nominal annual rate, 20 years, and no deposits, the shared calculation model gives:

  • Annual compounding: $38,697
  • Monthly compounding: $40,387
  • Daily compounding (365 periods): $40,547

These are mathematical estimates under a constant nominal rate and the selected compounding convention. Fees, taxes, changing rates, and actual investment performance are not included. For daily-only projections, see the daily compound interest calculator; for APY conversion, see the APY calculator.

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FAQ

What is monthly compound interest?

It is a calculation where interest is added to the balance every month. The next month’s interest is then calculated on the updated balance. This calculator models that schedule using a nominal annual rate divided by 12.

How much does $10,000 grow with monthly compounding?

$10,000 grows to $22,196 after 10 years and $109,357 after 30 years at a fixed 8% nominal annual rate compounded monthly, with no recurring deposits, fees, or taxes. These are mathematical illustrations, not return forecasts.

What is the monthly compound interest formula?

For one starting balance, A = P × (1 + r/12)^(12 × t), where P is the principal, r is the nominal annual rate as a decimal, and t is years. An effective APY must be converted before using this nominal-rate formula.

How does monthly compounding compare with annual compounding?

At the same nominal annual rate, monthly compounding produces a slightly higher balance than annual compounding. For $10,000 at 7% over 20 years with no deposits, the estimates are $40,387 monthly and $38,697 annually, before fees and taxes.

How do I model recurring monthly deposits?

Use the separate compound-interest calculator with monthly contributions. This page defaults recurring deposits to zero so the calculation isolates the compounding frequency.

Calculation method

The lump-sum projection uses A = P × (1 + r/12)^(12t), with a fixed nominal annual rate divided into 12 monthly periods. Monthly deposits, when entered, are added at each month end according to the calculator's model. Examples exclude fees and taxes and are not return forecasts. Updated 2026-09-22. See editorial standards.

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Why this calculator and not the others?

Snowballr publishes six compound-interest variants because the math is the same but the conventions, defaults, and product context differ. Here's where this one fits and when to switch to another.

You're using:
Monthly compound interest calculator
Best for: Standard US brokerage, 401(k), IRA modeling — the convention used by Fidelity, Vanguard, Schwab projections. Monthly is the practical default for retirement math.
A = P(1 + r/12)^(12t)
Switch away if: Daily-compounded bank products (use Daily) or one-off lump-sum modeling where you'd rather just use annual.
Backed by our research: 10,000 deterministic Monte Carlo scenarios — methodology, percentile distribution, and the year interest beats contributions.
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