Finance calculator

Compound Interest Calculator, Growth Projection with Monthly Contributions

See how your money grows over time with compound interest and regular contributions. Visualize exponential growth by rate and time horizon.

Published by · Reviewed against primary-source formulas

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years

Future Value

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Total balance at end

Effective Annual Rate

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With compounding

Total Contributions

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Your money in

Total Interest Earned

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Growth from compounding

What makes up your final balance

Live split of your ending balance into contributions and compound growth.

Enter values above to see the breakdown.

How CalcMesh calculates compound interest

This is a compound interest calculator with monthly contributions: enter a starting balance, a recurring monthly deposit, an annual rate, and a compounding frequency, and it projects your future value, total contributions, and interest earned. Contributions are credited at the end of each month using the equivalent monthly rate, so they compound correctly at any selected frequency.

Compound growth follows A = P(1 + r/n)^(nt), where interest is earned on prior interest. A useful shortcut, the Rule of 72, estimates doubling time as 72 divided by the annual percentage return, about 10 years at 7%.

We apply the nominal rate at the selected compounding frequency. Monthly contributions are credited at the end of each month using the equivalent monthly rate, then separated from earned interest; the assumptions are documented in our methodology.

Why Compound Interest Matters

Compound interest lets your money earn money on its earnings, creating exponential growth over time. Unlike simple interest (calculated only on principal), compound interest accelerates growth because each interest payment becomes part of the base for the next period.

The Rule of 72

To quickly estimate how long it takes to double your money, divide 72 by your annual return:

  • 6% return: ~12 years to double
  • 8% return: ~9 years to double
  • 10% return: ~7.2 years to double
  • 12% return: ~6 years to double

Starting Early vs. Starting Late

Consider two investors, both earning 7% annually:

  • Investor A starts at 25, contributes $200/mo for 40 years = ~$525,000
  • Investor B starts at 35, contributes $400/mo for 30 years = ~$489,000

Investor A contributes $96,000 total. Investor B contributes $144,000. Starting 10 years earlier with half the monthly amount produces a larger result.

Compounding Frequency

Interest can compound daily, monthly, quarterly, or annually. More frequent compounding produces slightly higher returns, but the effect is modest. The biggest factors are your rate of return and time horizon.

When to Use This Calculator

  • Savings account comparison: Compare 4.5% vs 5.0% APY over 5 years to see the dollar difference.
  • Investment planning: Model a Roth IRA or taxable brokerage account growing at a historical market rate.
  • Goal setting: Find how much monthly contribution is needed to reach a specific future value.

Real-World Examples

Example 1, College fund: $5,000 initial, $200/month for 18 years at 7% compounded monthly. Future value: ~$103,700. Total contributed: $48,200. Interest earned: ~$55,500.

Example 2, Retirement growth: $50,000 at age 40, $800/month for 25 years at 7%. Future value at 65: ~$934,300. Total contributions: $290,000. Interest earned: ~$644,300, more than double the contributions themselves.

Data Sources

Formula: FV = P(1 + r/n)^(nt) + PMT × ((1 + r/n)^(nt) − 1) / (r/n). Historical 7% average return reflects S&P 500 inflation-adjusted returns per Vanguard Long-Term Investment Returns data.

Related Guides

Related Data

Plan your retirement contributions alongside compound growth, see salary benchmarks for 831 occupations at BLS OEWS. Compare savings rates by state and metro at SSA retirement planner.

Methodology & Assumptions

This calculator implements standard formulas drawn from primary-source authorities. Values are point-in-time estimates; consult a licensed professional for high-stakes decisions. See the per-input definitions and source citations below.

How this works

Computations are deterministic and run client-side, no inputs leave your browser. Formulas are derived from standard published formulas for the calculator's domain (mortgage, taxes, energy, conversions, etc.). When the underlying agency publishes updated rates or thresholds we refresh defaults and update the page's lastmod timestamp.

Frequently Asked Questions

What is compound interest?
Compound interest is interest earned on both your initial investment and on previously earned interest. Unlike simple interest (calculated only on the principal), compound interest accelerates growth because each interest payment becomes part of the base for the next calculation. This creates an exponential growth curve over time.
How often should interest compound?
More frequent compounding produces slightly higher returns. Daily compounding yields more than monthly, which yields more than annually. However, the differences are relatively small. For example, $10,000 at 7% for 20 years grows to $38,697 with annual compounding versus $40,387 with daily compounding. The real driver of growth is time in the market, not compounding frequency.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by the annual interest rate to get the approximate number of years. For example, at 8% interest, your money doubles in about 9 years (72 / 8 = 9). At 6%, it takes about 12 years.
Is it better to start early or invest more later?
Starting early is almost always more powerful. Someone who invests $200/month from age 25 to 65 at 7% will have about $525,000. Someone who waits until 35 and invests $400/month (double the amount) for 30 years at the same rate will have only about $489,000. The 10 extra years of compounding beat the doubled contributions.

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This page identifies the inputs, method, and limitations behind its estimates. Calculator outputs are not professional advice and should be checked against the relevant primary source for a consequential decision. This calculator's formula and defaults are drawn from standard published sources for its domain, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Inputs, defaults, and authoritative sources
Input Default Source / authority
All inputs Domain-typical defaults Editorial methodology, CalcMesh 2026