Compound Interest Calculator with Monthly Contributions
Enter a starting balance, a monthly contribution, an annual rate, and a time span to project future value with the formula A = P(1 + r/n)^(nt) plus the future value of your deposits. Seven compounding frequencies, a year-by-year schedule, free and 100% client-side — your 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.
Compound Interest Calculator: Project Savings With Monthly Contributions
A compound interest calculator projects how a balance grows when interest is earned on both the principal and the interest already accrued. Enter a starting balance, a monthly contribution, an annual rate, and a number of years; it returns the future value using A = P(1 + r/n)^(nt) for the principal plus the future value of your deposits. It supports seven compounding frequencies, shows a year-by-year schedule, and runs 100% in your browser — free, with no upload.
How to use the compound interest calculator
- Enter your Initial Principal — the lump sum you already have invested (enter 0 if you are starting from scratch).
- Enter your Monthly Contribution — the amount you add every month, such as a 401(k) deferral or automatic transfer.
- Set the Annual Return (%) — use a realistic figure; US stocks average about 6.5% real, not the 10%+ some tools default to.
- Set the Years you plan to keep the money invested, from 1 to 100.
- Pick a Compounding Frequency — monthly is standard; annual through continuous are available.
- Read the Future Value, total contributions, and interest earned, then expand the Year-by-Year Balance to see the crossover where interest outpaces deposits.
What is compound interest and how is it calculated?
The U.S. Securities and Exchange Commission defines compound interest as “interest paid on principal and on accumulated interest.” In plain terms, each period's interest is added to the balance, and the next period earns interest on that larger balance. This calculator combines two standard finance formulas: compound growth on your starting balance, and the future value of an ordinary annuity for your recurring deposits.
“Compound interest is interest paid on principal and on accumulated interest.”— U.S. SEC, Investor.gov glossary
The exact formulas this tool computes
1. Compound growth on the initial principal:
A = P(1 + r/n)^(nt)
2. Future value of regular deposits (ordinary annuity):
(1 + r/n)^(nt) − 1
FV = PMT · ───────────────────
r / n3. Continuous compounding (principal):
A = P·e^(rt)
Total future value: A + FV
- A — future value of the starting balance after t years
- P — initial principal (the lump sum you start with)
- PMT — deposit per compounding period (your monthly amount × 12 ÷ n)
- FV — future value of all the deposits combined
- r — annual interest rate as a decimal (7% = 0.07)
- n — compounding periods per year (12 for monthly, 365 for daily)
- t — time in years
- e — Euler's number (≈ 2.71828), used for continuous compounding
Deposits use ordinary annuity timing (end of period) because that matches how most retirement accounts and savings apps credit contributions. When the rate is 0%, the tool skips division and returns the linear sum P + PMT · 12 · t, avoiding a divide-by-zero.
Worked example: $10,000 + $500/month at 7% for 10 years
These are the exact figures this calculator returns with monthly compounding, so you can verify the math by hand.
Inputs: P = $10,000, PMT = $500/month, r = 0.07, n = 12, t = 10
Step 1. r/n = 0.07 / 12 = 0.0058333
Step 2. nt = 12 × 10 = 120 periods
Step 3. (1.0058333)^120 = 2.009661
Step 4. Principal: $10,000 × 2.009661 = $20,096.61
Step 5. Deposits: $500 × [(2.009661 − 1) / 0.0058333] = $86,542.40
Future value: $20,096.61 + $86,542.40 = $106,639.02
Total contributed: $10,000 + ($500 × 120) = $70,000
Interest earned: $106,639.02 − $70,000 = $36,639.02
Plug the same inputs into the tool above and you will see $106,639.02 — the content and the calculator agree to the cent.
How compounding frequency changes the result
The same $10,000 at 7% over 30 years with no deposits, computed at each frequency this tool offers. The jump from annual to monthly is meaningful; beyond monthly the gains shrink fast.
| Compounding | Periods/Year (n) | Future Value |
|---|---|---|
| Annually | 1 | $76,122.55 |
| Quarterly | 4 | $80,191.83 |
| Monthly | 12 | $81,164.97 |
| Daily | 365 | $81,645.26 |
| Continuous | ∞ | $81,661.70 |
Monthly to continuous differs by under $500 over 30 years. The rate is the real lever — 7% versus 8% on the same $10,000 over 30 years is $81,164.97 versus $109,357.30.
Realistic returns by asset class
Pick a rate you can defend. Real (inflation-adjusted) long-term returns sit well below the double-digit defaults some calculators use.
| 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 starting early beats saving more later
Two savers, both retiring at 65, both investing $200/month at 7% annual.
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 finished at the same balance. The 30 extra years of compounding on that first decade of deposits made up the entire difference. This is why advisors say to start now, even with small amounts.
The Rule of 72: mental math for doubling time
A quick shortcut for how long a balance takes 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 approximates ln(2)/ln(1+r) and stays within about 1% accuracy for rates between 4% and 12%. Use it to sanity-check any projection: if a tool claims $10K becomes $40K in 10 years at 6%, the rule (12 years just to reach $20K) tells you it is wrong.
The limitation most calculators hide
This result is a nominal, pre-tax, pre-fee estimate. It does not subtract inflation, income tax on gains, capital-gains tax, expense ratios, or advisory fees — and on a 30-year horizon those quietly erase a large share of the headline number. A 1% expense ratio plus 3% inflation effectively cuts a 7% nominal return to about 3% real, which roughly halves the future value versus the unadjusted figure. To approximate real, after-cost growth, subtract your expected inflation and fees from the rate before entering it: enter ~3% instead of 7% to see today's-dollars purchasing power.
One more nuance: continuous compounding here uses a closed-form formula for the principal (P·e^(rt)) but approximates deposit growth with fine-grained hourly steps, so its deposit figure is an extremely close approximation rather than a single closed-form value.
Estimates only — not financial advice
This calculator is for general informational purposes and produces estimates, not financial advice. Actual returns vary with markets, fees, and taxes, and past performance does not guarantee future results. For figures you can act on, consult a licensed financial professional or your account provider. Your data never leaves your device — principal, deposits, rate, years, and the full schedule are computed in your browser with no upload.
Last reviewed: June 2, 2026
Frequently asked questions
What compound interest formula does this calculator use?
It sums two parts: principal growth as A = P(1 + r/n)^(nt) and deposit growth as the future value of an ordinary annuity, FV = PMT · [((1 + r/n)^(nt) − 1) / (r/n)]. Continuous mode uses A = P·e^(rt) for the principal.
Does the monthly contribution earn interest too?
Yes. Each deposit is added at the end of its period and compounds for every remaining period. The tool converts your monthly figure to the chosen frequency before applying the annuity formula, so earlier deposits earn more than later ones.
Does it account for inflation, taxes, or fees?
No — the output is nominal, pre-tax, and pre-fee. To approximate real, after-cost growth, subtract your expected inflation and fees from the rate before entering it. This is an estimate, not financial advice.
Is my financial data sent to a server?
No. Every value and the full year-by-year schedule are computed in your browser with JavaScript. There are no network requests tied to your figures.
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Last reviewed: June 2, 2026 · Estimates only, not financial advice · Runs 100% in your browser — no uploads, nothing leaves your device.
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