Where do I find my APR?

On your statement, in the interest charge calculation section, and in your card's online account. Use the purchase APR. Cash advances and balance transfers usually carry their own rates.

Why does my statement's interest differ from this?

Card companies apply a daily periodic rate to an average daily balance over the statement period, so payments and purchases made mid-cycle change the figure. This calculator spreads a flat month's interest over the days. The pattern is the same; the cents can differ.

Does every purchase really cost interest?

No. If you pay your statement balance in full by the due date, most cards charge no interest on new purchases thanks to the grace period. The second price applies when a purchase rides a balance that is already carrying interest, which is the situation Slate is built for.

Is this financial advice?

No. It is arithmetic on the numbers you entered: information and education, not financial, legal or tax advice. For a big decision, talk to a qualified professional.

Calculator

Credit card interest calculator

Enter a balance and its APR. See what the bank charges each day, each month and each year, how much of a typical minimum payment it keeps, and the second price on any purchase. The same arithmetic as the app.

Balance

APR

A purchase

Your APR is on your statement, usually under "Interest charge calculation". Use the purchase APR.

Each month: $104.13
Each year, if the balance stays: $1,249.56
Second price on a $64 purchase: +$1.33
Same balance at the 22.15% average, each month: $92.29

Each day, the bank charges

Holding $5,000.00 at 24.99% costs about $3.42 a day, $104.13 a month, and $1,249.56 a year if the balance stays where it is.

A typical minimum here is $154.13. The bank takes $104.13 of it in interest before a cent reaches the balance.

Estimates at the APR you enter: a day is balance × APR ÷ 365, a month is ÷ 12, and the second price is one month of interest on the purchase if it rides the balance. Issuers charge on an average daily balance and a grace period can make a purchase's interest zero. A typical minimum is 1% of the balance plus the month's interest, at least $35. Not financial advice. How Slate does the math

How to read it

Each day, the bank charges: The balance times the APR, divided by 365. It's the clearest way to feel an APR: 24.99% on $5,000 is $3.42 every day the balance stays.
A typical minimum, split: A common issuer formula: 1% of the balance plus the month's interest, at least $35. The cyan part is what the bank keeps as interest before anything reaches the balance.
The second price on a purchase: One month of interest on that purchase at the card's APR, if it rides a balance that carries interest. An estimate: a grace period can make it zero, and a card can carry more than one APR.
The national average: The Federal Reserve's average APR on card accounts that were charged interest, so you can see whether your card charges more or less than typical.

Slate does this for your real cards.

Connect a card, read-only through Plaid, and these figures come from your own statements: each card you track, every purchase, and a date you can move. In development for iPhone. The App Store listing goes up when it ships.

Questions

Where do I find my APR?: On your statement, in the interest charge calculation section, and in your card's online account. Use the purchase APR. Cash advances and balance transfers usually carry their own rates.
Why does my statement's interest differ from this?: Card companies apply a daily periodic rate to an average daily balance over the statement period, so payments and purchases made mid-cycle change the figure. This calculator spreads a flat month's interest over the days. The pattern is the same; the cents can differ.
Does every purchase really cost interest?: No. If you pay your statement balance in full by the due date, most cards charge no interest on new purchases thanks to the grace period. The second price applies when a purchase rides a balance that is already carrying interest, which is the situation Slate is built for.
Is this financial advice?: No. It is arithmetic on the numbers you entered: information and education, not financial, legal or tax advice. For a big decision, talk to a qualified professional.

Sources

1Average interest rate on credit card accounts assessed interest, 22.15%, second quarter of 2026. G.19 Consumer Credit, Terms of Credit table, released September 8, 2026. Federal Reserve Board. Checked October 3, 2026.
3How a card company calculates the interest you owe: a daily periodic rate applied to an average daily balance, with separate APRs for purchases, cash advances and balance transfers. Consumer Financial Protection Bureau. Checked October 3, 2026.
4What the minimum-payment box on a statement means, and why paying more than the minimum shortens the payoff and lowers the interest. Consumer Financial Protection Bureau. Checked October 3, 2026.

Every other figure on this page is an example, computed with the same arithmetic the app uses. Cited figures are re-read from their release each time it updates.