Compound interest calculator by days
Project a savings balance by days using the APY from your account disclosure. The calculator converts APY to an equivalent daily rate, preserving the quoted annual yield; it does not compound APY twice.
How does daily compound interest work?
If a bank quotes nominal APR r, a daily-compounding projection is A = P × (1 + r/365)365t. This calculator instead accepts an effective APY and converts it to d = (1 + APY)1/365 − 1, so A = P × (1 + d)365t = P × (1 + APY)t. Each formula uses the rate convention stated in the account disclosure; actual day-count and crediting terms can differ.
How to calculate it in three steps. First, use the APY in the account disclosure. Second, convert it to the equivalent daily rate d = (1 + APY)1/365 − 1. For a 5% APY, 365 daily periods reproduce a 5% annual yield. Third, multiply the principal by that daily factor for each day. If you start with a nominal APR instead, divide APR by 365; at 5% nominal APR, $10,000 grows to about $10,513 in one year.
At the same 5% nominal APR, daily compounding produces about $10,513 in one year on $10,000, compared with $10,500 under annual compounding. The resulting APY is about 5.127%. If both scenarios start from a quoted 5% APY, the annual yield remains 5% under either equivalent convention.
Key terms
- Daily compounding
- Interest computed and added every day; tomorrow earns on today's new balance.
- APY
- Effective annual rate after compounding. The number to compare across products.
- APR
- Nominal rate before compounding. 5.00% APR daily = 5.127% APY.
How cash-account compounding terms work
Some deposit products calculate interest from a daily balance, while others use a different convention or crediting schedule. If a product uses a nominal APR with daily periods, the illustrative calculation isbalance × (APR / 365) for each day; the provider's disclosure controls the actual method. In the US, Regulation DD requires APY disclosures for covered deposit accounts, so compare APY and read the account terms rather than inferring the schedule from the product label.
Daily vs monthly vs annual — actual numbers
$10,000 at 5% APR over 10 years:
- Annual compounding: $16,289 (APY 5.000%)
- Monthly compounding: $16,470 (APY 5.116%)
- Daily compounding: $16,486 (APY 5.127%)
- Continuous compounding: $16,487 (APY 5.127%)
The difference between annual and daily is about 1.2% over a decade. The difference between daily and continuous is essentially zero. Once you compound daily or faster, you've captured 99.99% of the available compounding benefit.
What this calculator is good for
- HYSA projections — model the provider's current APY over 1-5 years and test a lower-rate case because the APY can change. See current rates in the HYSA shopper.
- CD ladder planning — compare 12, 24, 36, 60-month terms. Try the CD ladder builder.
- Money market funds — most credit interest daily.
- Short-term goal saving — emergency fund, down payment, wedding.
What this calculator is NOT for
Long-term investing in stocks. Equity returns aren't smooth or daily-compounded — they fluctuate wildly. For projections beyond 5 years, use our compound investment calculator with a long-term equity rate (~7-10%), or the monthly compound interest calculator.
The biggest compounding mistake
Obsessing over compounding frequency while ignoring the APY itself. A 4.5% HYSA compounded daily earns less than a 5.0% HYSA compounded monthly. Always compare APY to APY — that number already accounts for the compounding frequency. Convert any APR to its true APY with our APR to APY converter.
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FAQ
What is daily compound interest?
How much more do you earn with daily vs monthly compounding?
Do HYSAs really compound daily?
What's the difference between APY and APR for compounding?
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Methodology & sources
Formula: A = P(1 + r/365)^(365t). APY conversion: (1 + APR/365)^365 − 1. The calculator accepts the disclosed effective APY and converts it once. Formula and input convention reviewed 2026-10-13; examples are assumptions, not current offers. See our editorial standards.
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Why this calculator and not the others?
Snowballr publishes eight compound-interest calculator 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.
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- 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.
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- 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.