Free Compound Interest Calculator with Regular Contributions
Project future value of any savings or investment account. Initial principal plus regular monthly contributions, seven compounding frequencies including continuous, and a year-by-year balance schedule. 100% client-side — figures never leave your browser.
Regular Contributions
Initial principal AND a recurring monthly contribution — the realistic savings pattern. Most online calculators handle only one or the other.
Seven Compounding Frequencies
Annual, semi-annual, quarterly, monthly, weekly, daily, and continuous. The difference between “monthly” and “daily” on a 30-year portfolio is real (thousands of dollars).
Year-by-Year Schedule
Watch your balance grow year by year. Contributions vs interest earned breakdown surfaces the moment compounding takes over — usually around year 15–20.
100% Client-Side
Retirement projections, college-fund plans, savings goals — all stay in your browser. No upload, no save, no analytics tied to your financial figures.
The Compound Interest Calculator That Models Real Savings Behavior
Most online compound-interest calculators handle either an initial lump sum OR regular contributions — never both at once. Real savings doesn't work that way: people start with some money in the bank, add a monthly contribution from each paycheck, and watch the combination compound for decades. Our Free Online Compound Interest Calculator handles both, across seven compounding frequencies (including continuous), and shows the year-by-year balance so you can see exactly when compounding takes over from your contributions as the primary growth engine.
Pair this calculator with our Mortgage Calculator (the inverse problem — debt amortization), the Age Calculator (for retirement-date projections), and the Tip Calculator (everyday financial math).
The Two Formulas Behind the Calculator
1. Compound growth on initial principal:
A₁ = P · (1 + r/n)^(n·t)
2. Future value of regular contributions (ordinary annuity):
(1 + r/n)^(n·t) − 1
A₂ = PMT · ───────────────────
r / nTotal: A = A₁ + A₂
- P — initial principal (lump-sum balance)
- PMT — periodic contribution (per period, not per year)
- r — annual interest rate (decimal — 7% = 0.07)
- n — compounding periods per year (12 for monthly, 365 for daily)
- t — time in years
Historical Real Returns by Asset Class
| Asset Class | Real Return (Inflation-Adj.) | Context |
|---|---|---|
| US large-cap stocks (S&P 500) | ~6.5% | 1926-2024, inflation-adjusted |
| International developed stocks | ~5.0% | MSCI EAFE since 1970 |
| US 10-year Treasury bonds | ~2.0% | Long-term real return |
| High-yield savings account | ~0-1% | Best-case post-inflation |
| Cash under the mattress | ~-3% | Inflation erosion only |
Source: Ibbotson SBBI Yearbook, Vanguard historical returns. Real returns subtract long-term inflation. For nominal (pre-inflation) returns, add about 3 percentage points to the values above.
Why “Start Early” Beats “Save More Later”
Two friends. Both want to retire at 65. Both invest $200/month at 7% annual return.
Person A (Early Start)
Invests: $200/month, age 25–35 (10 years)
Total contributed: $24,000
Stops at 35, lets it grow
Balance at 65: ~$244,000
Person B (Late Start)
Invests: $200/month, age 35–65 (30 years)
Total contributed: $72,000
Contributes for 30 years straight
Balance at 65: ~$245,000
Person A contributed one-third as much and ended at the same balance. The 30 extra years of compounding on the first decade's contributions made up the entire difference. This is why every personal-finance advisor leads with “start now, even small amounts.”
The Rule of 72 (Mental Math for Compounding)
A handy shortcut for estimating how long it takes a balance to double at a given rate:
Years to double ≈ 72 / interest rate %
- At 4%: 72 / 4 = 18 years to double
- At 6%: 72 / 6 = 12 years to double
- At 8%: 72 / 8 = 9 years to double
- At 10%: 72 / 10 = 7.2 years to double
- At 12%: 72 / 12 = 6 years to double
The Rule of 72 is mathematically exact for ln(2)/ln(1+r), which equals 72/r% to within 1% accuracy for rates between 4% and 12%. Useful for sanity-checking any financial projection in your head — if a calculator tells you $10K becomes $40K in 10 years at 6%, you can immediately tell it's wrong (the rule says $10K → $20K in 12 years at 6%).