To calculate your monthly interest rate from an APR, divide the APR by 12. If your credit card or loan has a 24% APR, your monthly periodic rate is 2% (24% ÷ 12). That monthly rate is what actually gets applied to your balance each billing cycle, and understanding it helps you see exactly how much interest you’re being charged.
The Basic Formula
APR stands for annual percentage rate, but interest on most loans and credit cards is calculated monthly or even daily. To find the monthly periodic rate, use this formula:
Monthly rate = APR ÷ 12
So for a mortgage with an 8% APR, the monthly rate is 0.08 ÷ 12 = 0.0067, or about 0.67%. On a $200,000 balance, that means roughly $1,333 in interest for that month alone. For a credit card with an 18% APR, the monthly rate is 1.5%. Carry a $5,000 balance and you’d owe about $75 in interest that month.
This division works because APR is a nominal rate, meaning it’s simply the periodic rate multiplied by the number of periods in a year. Lenders are required by the Truth in Lending Act to disclose APR this way, making it straightforward to reverse the math.
How Credit Cards Calculate It Daily
Credit card issuers typically don’t stop at a monthly rate. Most divide your APR by 365 (or sometimes 360) to get a daily periodic rate, then multiply that by the number of days in your billing cycle.
Here’s how that works in practice with a 20% APR:
- Daily periodic rate: 20% ÷ 365 = 0.0548%, or about 0.000548
- Monthly interest on a $3,000 balance (30-day cycle): $3,000 × 0.000548 × 30 = $49.32
Your credit card statement will show the daily periodic rate alongside your APR. The Consumer Financial Protection Bureau notes that whether your issuer divides by 360 or 365 depends on the card, and the difference is small but real. Dividing by 360 produces a slightly higher daily rate, which means slightly more interest over the course of a year.
If you want a quick estimate, dividing the APR by 12 gets you close enough. But if you want to match the exact charge on your statement, use the daily rate times the number of days in that billing cycle.
Applying the Monthly Rate to Your Balance
Once you have your monthly rate, calculating a single month’s interest charge is simple multiplication:
Interest charge = Balance × Monthly rate
For a $15,000 car loan at 6% APR, the monthly rate is 0.5%. In the first month, you’d pay $75 in interest ($15,000 × 0.005). After you make a payment and the balance drops to, say, $14,800, the next month’s interest is $74. This is why more of your payment goes toward the principal over time: the interest portion shrinks as the balance falls.
For credit cards, the balance used in this calculation is usually your average daily balance, not just the amount on a single day. Your issuer adds up the balance on each day of the cycle and divides by the number of days, then applies the rate to that average.
Why the Effective Rate Is Higher Than the APR
There’s a catch with the simple “divide by 12” approach. Because interest compounds, meaning each month’s interest gets added to the balance and future interest is charged on that larger amount, you actually pay more over a year than the APR alone suggests.
The true annual cost of borrowing is called the effective annual rate (EAR). Consider a bond or loan paying 6% APR compounded semiannually. After the first six months, $1,000 earns $30 in interest. In the second half of the year, interest is calculated on $1,030, producing $30.90. The total for the year is $60.90, making the effective rate 6.09%, not 6%.
The gap between APR and the effective rate grows as compounding happens more frequently. A 24% APR compounded monthly produces an effective rate of about 26.8%. That difference matters when you’re carrying a balance for a long time. The monthly periodic rate is still 2%, but because each month’s interest rolls into the next month’s balance, the total cost over a year exceeds what 24% would suggest on its own.
Going Backward: Monthly Rate to APR
If you know your monthly rate and want to find the APR, reverse the formula:
APR = Monthly rate × 12
Say a lender quotes you a monthly rate of 0.75%. Multiply by 12 and you get a 9% APR. This is useful when comparing loan offers that are quoted differently. Some lenders, particularly for short-term or small-business loans, advertise a monthly rate that sounds low but adds up to a significant annual cost. A “reasonable-sounding” 3% monthly rate translates to a 36% APR.
For loans that include fees, the APR calculation is slightly more involved. The full formula accounts for total interest paid plus fees, divided by the loan principal, then annualized over the loan term. This is why two loans with the same interest rate can have different APRs: one may charge origination fees or closing costs that push the true annual cost higher.
Quick Reference for Common APRs
- 15% APR: 1.25% monthly rate, or about $12.50 per month on a $1,000 balance
- 20% APR: 1.67% monthly rate, or about $16.70 per month on $1,000
- 25% APR: 2.08% monthly rate, or about $20.83 per month on $1,000
- 30% APR: 2.50% monthly rate, or about $25.00 per month on $1,000
Scale these up proportionally for larger balances. A $5,000 balance at 20% APR costs roughly $83.50 per month in interest. These are estimates using the simple monthly division. Your actual statement may vary slightly based on daily compounding and the number of days in the billing cycle, but the math gets you within a dollar or two for most purposes.