How to Calculate Credit Card Interest by Hand
Last updated September 2026 · Reviewed by the CalcMyCard editorial team
To calculate credit card interest by hand: divide your APR by 365 to get the daily periodic rate, multiply by your average daily balance, then multiply by the number of days in your billing cycle. The result is your interest charge for that statement.
You don't need special software to calculate credit card interest — just three numbers and a calculator (or this one). Here's the exact process issuers use, broken into steps you can follow yourself.
Step 1: Find your daily periodic rate
Take your card's APR (as a decimal) and divide by 365 — the number of days most issuers use per year (a few use 360; check your cardholder agreement if you want an exact match). A 22% APR becomes a daily rate of about 0.0603%.
Step 2: Find your average daily balance
Add up your balance for every single day in the billing cycle, then divide by the number of days. If your balance was $1,000 for the first 15 days and $1,500 for the remaining 15 days of a 30-day cycle: (15 × $1,000 + 15 × $1,500) ÷ 30 = $1,250 average daily balance.
Step 3: Multiply it all together
Interest charged = daily periodic rate × average daily balance × number of days in the cycle. Using the numbers above: 0.0603% × $1,250 × 30 days ≈ $22.61 in interest for that cycle.
Where people usually get it wrong
The most common mistake is using your current balance instead of your average daily balance, which produces the wrong number whenever your balance changed during the cycle (which it almost always does). The second most common mistake is dividing by 12 instead of 365 — that gives you a rough monthly-compounding estimate, which is close enough for planning purposes but won't match your statement exactly.
When the "simple" monthly method is good enough
For planning and comparison purposes — deciding whether to pay extra, compare a balance transfer, or budget for a payoff — a simplified monthly rate (APR ÷ 12) applied to your current balance gives a close, easy-to-follow approximation. That's the method our calculators use, since it's simpler to reason about while staying within a few dollars of the true daily-compounding result over a full payoff timeline.
Skip the manual math
For the exact daily-rate calculation on your own numbers, use the Daily Periodic Rate Calculator. For a full payoff projection using the simplified monthly method, use the Credit Card Interest Calculator.
Frequently Asked Questions
Can I calculate this in a spreadsheet?
Yes — see our guide on building a credit card interest formula in a spreadsheet for a ready-to-use approach.
Why doesn't my manual calculation match my statement exactly?
Small differences usually come from rounding, fees included in the balance, or an issuer using a slightly different day-count convention (360 vs. 365) than you assumed.
This article is for general educational purposes and is not financial advice. Rates and figures cited reflect industry data available at the time of writing and can change.