Compound Interest Calculator (Monthly Contributions)

What Rate Should You Use?

The interest rate input represents your expected annual return. For a high-yield savings account, use the current APY, typically 4 to 5 percent in 2026. For a broad stock market index fund (like a total market ETF), historical average real returns after inflation run around 6 to 7 percent annually over long periods. For retirement projections, 6 percent is a conservative and commonly used assumption. Avoid using past performance of individual stocks or recent bull market returns as your baseline.

Lump Sum vs Monthly Contributions

Set the principal to model a one-time investment — savings you already have, an inheritance, a bonus. Add a monthly contribution to model ongoing saving, like a pension contribution or automated transfer. The combination shows how both streams compound together. Even a small monthly amount added to a lump sum dramatically changes the outcome over 20 or 30 years because each contribution starts compounding immediately.

Compounding Frequency

Daily compounding produces slightly more than monthly, which produces slightly more than annual, but the difference is smaller than most people expect. At 7 percent, the difference between annual and daily compounding on $10,000 over 30 years is around $800. The rate and time horizon matter far more than frequency. Use monthly to match most real investment accounts.

The Compound Interest Formula

The future value of an investment with compounding and optional recurring contributions is:

FV = P × (1 + r ÷ m)^{m × t} + C × [((1 + r ÷ m)^{m × t} − 1) ÷ (r ÷ m)]

where P is your starting principal, r is the annual return as a decimal, m is the number of compounding periods per year, t is years invested, and C is your recurring monthly contribution (set C to 0 to model a lump sum only). The first term grows your principal on itself; the second term compounds each new contribution from the moment you make it.

A Worked Example

Invest $1,000 at an 8% annual return compounded monthly for 20 years, with no extra contributions. Monthly compounding means m = 12, so the math is:

FV = 1,000 × (1 + 0.08 ÷ 12)^{12 × 20} = 1,000 × 4.9268 ≈ $4,926.80

Your $1,000 grows nearly five-fold without adding a dollar. Now add $100 per month to the same inputs — those contributions, plus their own compounding, push the 20-year balance to roughly $64,000. That jump is the entire case for automating a monthly transfer instead of waiting to invest a larger lump sum later.

How Long to Double? The Rule of 72

To estimate doubling time, divide 72 by your annual return rate. At 8%, 72 ÷ 8 = 9 years to double; at 6%, it takes about 12 years. On a $10,000 principal, the difference between 6% and 8% flips a 30-year outcome from roughly $57,000 to over $100,000 — a gap created entirely by the rate, not your contributions. The Rule of 72 is approximate, so use the calculator for exact figures.

Common Scenarios to Model

Retirement: Set years to your target retirement age minus your current age. Use 6 to 7 percent. Add your monthly 401(k) or IRA contribution. The result shows your projected pot.

House deposit: Set years to 3 to 5, rate to current savings account APY, add your monthly savings target. See if the timeline is realistic.

Child education fund: Set years to 18 minus the child's current age. A modest monthly contribution started at birth compounds for 18 years into a meaningful sum.

Reading the Year-by-Year Table

Expand the table to see how the interest portion grows each year while contributions stay flat. In early years, most growth comes from your contributions. In later years, interest earned in a single year can exceed your total annual contributions. That crossover point — when compounding does more work than you do — is why starting early matters more than starting with a large amount.