Compound Interest Calculator 2026 — See How Money Grows | US Financial Calculators

$10,000 principal — 7% annual — Monthly compounding — $200/month — 20 years

This is a typical long-term investment scenario at stock-market-like returns.

Frequency Comparison — $10,000 at 7% for 20 years (no contributions)

Frequency	Final Value	Difference vs Annual
Annually (1×/yr)	$38,696.84	$0.00
Quarterly (4×/yr)	$40,063.92	+$1,367.07
Monthly (12×/yr)	$40,387.39	+$1,690.54
Daily (365×/yr)	$40,546.56	+$1,849.71

Compound interest is the cornerstone of long-term wealth building. This calculator shows exactly how your money grows when interest is earned on both your principal and the interest already accumulated. Adjust the rate, time, and compounding frequency to understand the true power of compounding.

Formula (lump sum only)

A = P × (1 + r/n)^(n×t)

A = future value, P = principal, r = annual rate (decimal), n = compounding periods/year, t = years

Formula (with regular contributions)

A = P(1+r/n)^(nt) + PMT × [(1+r/n)^(nt) − 1] / (r/n)

PMT = payment per compounding period (monthly contribution × 12 / n)

Compound interest means your interest earns interest. The more frequently compounding occurs — daily vs. annually — the more you earn, because each period's interest is added to the base before the next calculation. For long time horizons, this creates exponential rather than linear growth.

Rule of 72

Divide 72 by the annual interest rate to estimate years to double your money. At 7%: 72 ÷ 7 ≈ 10.3 years. At 10%: 72 ÷ 10 ≈ 7.2 years.

Assumptions

Formula: A = P(1 + r/n)^(nt) for lump sum, plus future value of annuity for contributions.
Annual rate is assumed constant throughout the period.
No taxes on growth are deducted.

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. Over long periods, compounding creates dramatically larger balances. A $10,000 investment at 7% simple interest for 20 years yields $24,000. At 7% compound interest (monthly), it yields $40,064 — $16,064 more.

What is the Rule of 72?

The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6%, money doubles in ~12 years (72/6=12). At 8%, it doubles in ~9 years. At 10%, about 7.2 years. It's an approximation but a useful one for quick comparisons.

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