Compound interest calculator
Free compound interest calculator with monthly contributions. Works for HYSAs, CDs, index funds, and 401(k) projections — daily, monthly, or annual compounding. Results update live as you type.
Key terms (used throughout this page)
- Compound interest
- Interest calculated on principal + previously accumulated interest. The reason $1 invested at 10% becomes $17.45 over 30 years instead of $4 with simple interest.
- APY (Annual Percentage Yield)
- The effective annual return after compounding. 5% APR compounded daily becomes 5.127% APY. Compare APYs, not APRs, when shopping for savings products.
- Principal
- The initial sum you start with, before any interest is earned. In a loan context, the amount borrowed (excluding interest).
- Future Value (FV)
- The projected nominal balance at a future date. The separate purchasing-power result divides this balance by (1 + inflation rate) raised to the number of years.
How to use this compound interest calculator
- Enter your starting balance. Use $0 if starting from scratch — the math still works.
- Add monthly contribution. Deposits are added at each month-end in this calculator, so each one compounds from its deposit date.
- Set the annual rate. See the rate guidance section below for HYSA / CD / index fund / portfolio defaults.
- Choose years. Match your goal horizon — 1-3 short, 10-15 medium, 25-40 retirement.
- Read the result. Future value, total contributions and total interest earned; the point where growth exceeds contributions depends on the amount, rate and time horizon.
The compound interest formula
With contributions added at the end of each period, future value equals:
q = (1 + r/n)^(n/12) - 1 FV = P(1 + q)^(12t) + PMT × [((1 + q)^(12t) − 1) / q] At r = 0: FV = P + PMT × 12t P = principal (initial deposit) r = annual interest rate (decimal) n = compoundings per year (1, 12, or 365) t = number of years PMT = contribution at each month-end q = equivalent monthly growth rate
The first term is your principal compounding. The second is the future value of an ordinary annuity — every contribution you make also compounds, from the day it's deposited.
What rate should I use?
Use a nominal annual rate with the matching compounding frequency. If your bank quotes APY, use annual compounding so you do not count compounding twice, or convert APY to the matching nominal rate first.
For deposit accounts, check the dated HYSA comparison and official provider links or your CD agreement. Rates and conditions vary. For investments, compare several hypothetical rates: a fixed-growth calculator cannot model market volatility or guarantee a future return.
Compound interest examples
Nominal annual rates, monthly compounding and deposits at each month-end. Values are rounded to whole dollars; taxes and fees are excluded.
| Starting | Monthly | Rate | Years | Final value |
|---|---|---|---|---|
| $0 | $100 | 7% | 40 | $262,481 |
| $1,000 | $100 | 7% | 30 | $130,114 |
| $5,000 | $250 | 7% | 30 | $345,575 |
| $10,000 | $500 | 7% | 25 | $462,290 |
| $10,000 | $500 | 10% | 30 | $1,328,618 |
| $25,000 | $1,000 | 8% | 30 | $1,763,753 |
| $50,000 | $2,000 | 8% | 25 | $2,269,062 |
Walked example: $10,000 at 7% with $500/month, year by year
To make compounding concrete, here's the same starting scenario tracked at key milestones. Monthly compounding, $500 deposited at the end of each month, 7% nominal annual rate:
| Year | Balance | Contributed | Interest | Interest as % of balance |
|---|---|---|---|---|
| 1 | $16,919 | $16,000 | $919 | 5% |
| 5 | $49,973 | $40,000 | $9,973 | 20% |
| 10 | $106,639 | $70,000 | $36,639 | 34% |
| 15 | $186,971 | $100,000 | $86,971 | 47% |
| 20 | $300,851 | $130,000 | $170,851 | 57% |
| 25 | $462,290 | $160,000 | $302,290 | 65% |
| 30 | $691,150 | $190,000 | $501,150 | 73% |
| 40 | $1,475,521 | $250,000 | $1,225,521 | 83% |
Compare the interest column with your contributions to see how much of the final balance comes from growth. These are fixed-rate projections, not a forecast of market returns.
Keep rate assumptions separate from current offers
The examples on this page use hypothetical fixed rates. The dated HYSA table has reviewed account offers and official sources. Check a provider's current rate and qualification rules before depositing. A past market return does not establish the rate your investments will earn.
Who this compound interest calculator is for
Simple vs compound interest: the dramatic gap
Simple interest: $10,000 at 8% for 40 years = $42,000 (principal × rate × time + principal).
Compound interest: $10,000 at 8% for 40 years = $217,245.
Same rate, same time, same starting amount. The only difference: letting interest compound on itself instead of paying out. Compound interest is the single most important mathematical concept in personal finance. Get this right and the rest is detail.
Daily vs monthly vs quarterly vs annual compounding
More frequent compounding earns slightly more at the same nominal rate, but the gap is small. Same money, four frequencies, on $10,000 at 5% APR:
| Frequency | Effective APY | 1 year | 10 years | 30 years | Gap vs annual (30 yr) |
|---|---|---|---|---|---|
| Annual (1×/yr) | 5.000% | $10,500 | $16,289 | $43,219 | — |
| Quarterly (4×/yr) | 5.094% | $10,509 | $16,436 | $44,402 | +$1,183 (+2.7%) |
| Monthly (12×/yr) | 5.116% | $10,512 | $16,470 | $44,677 | +$1,458 (+3.4%) |
| Daily (365×/yr) | 5.127% | $10,513 | $16,486 | $44,812 | +$1,593 (+3.7%) |
| Continuous (e^rt) | 5.127% | $10,513 | $16,487 | $44,817 | +$1,598 (+3.7%) |
Daily beats annual by 3.7% over 30 years — real money, but rate and contribution size matter far more. Don't chase compounding frequency; chase higher APY. Use our APR to APY calculator to convert between them precisely.
Why the rate matters far more than the frequency
$10,000 at 5% daily for 30 years grows to ~$44,812. At 7% annual it's ~$76,123. This illustrates why the rate assumption can matter more than the compounding frequency. This is why investing fee selection matters: 1% in fees costs roughly 25-30% of final balance over 30 years. Use our mutual fund fee analyzer to model the exact impact.
The most-cited (and misattributed) quote on compounding
"Compound interest is the eighth wonder of the world." Often credited to Albert Einstein — but there's no evidence he said it. The sentiment is correct; the attribution is folklore. See the full fact-check.
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Compound interest calculator FAQ
How does a compound interest calculator work?
What is the formula for compound interest with monthly contributions?
How much will $10,000 be worth in 20 years with compound interest?
Is daily compounding better than monthly?
Does this compound interest calculator account for inflation?
Is this compound interest calculator free?
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Does compound interest really make a difference?
Methodology & sources
Methodology: the calculator and example tables use a shared month-end contribution calculation, shown above. Purchasing power is the nominal final balance divided by the cumulative inflation factor. Calculation examples reviewed 2026-09-14. Current deposit offers and their sources are kept in the separate dated HYSA comparison.
Popular scenarios — click to run the math
Related calculators
Why this calculator and not the others?
Snowballr publishes six compound-interest variants because the math is the same but the conventions, defaults, and product context differ. Here's where this one fits and when to switch to another.
- Compound investment calculator (solve-for-X)When you know your goal and need to solve for the missing variable — 'how much/month to hit $1M', 'what rate gets me there', 'how many years'. Five interactive solver tabs.
- Daily compound interest calculatorHYSAs, CDs, money market accounts — products that explicitly state daily compounding. Tiny mathematical edge over monthly (≈0.04% at 5% APY), but it's what your bank actually quotes.
- Monthly compound interest calculatorStandard US brokerage, 401(k), IRA modeling — the convention used by Fidelity, Vanguard, Schwab projections. Monthly is the practical default for retirement math.
- UK compound interest calculatorGBP-denominated savings: Cash ISA, Stocks & Shares ISA, easy-access savings. Defaults assume UK Bank Rate context (2026 BoE base 4.25%) and £20,000 annual ISA allowance.
- Australia compound interest calculator (AUD)AUD-denominated savings: superannuation, high interest savings accounts, ETFs (VAS, A200, IVV). Defaults assume RBA cash rate context (2026) and AUD formatting.
- Canada compound interest calculator (CAD)CAD-denominated savings: TFSA, RRSP, RESP, high interest savings, Canadian ETFs (XIC, VCN, VFV). Defaults reflect Bank of Canada policy rate (2026) and CAD formatting.
- SBI compound interest calculator (India)State Bank of India FD, RD, PPF, and savings — quarterly compounding is the SBI convention. Defaults reflect 2026 SBI FD rates and 7-year PPF lock-in.