Method
How Slate does the math.
Slate shows two kinds of numbers. Before you connect a card, and in the calculators on this site, everything is an estimate from a balance, an APR and a payment. After you connect, the interest, APRs and minimums your bank reports through Plaid are facts from your statement, and they replace the estimates. This page shows exactly how each estimate is made, and where a real card can differ.
What a day costs3
balance × APR ÷ 365
$5,000 × 24.99% ÷ 365 = $3.42 a day
$5,000 × 24.99% ÷ 12 = $104.13 a month
The APR is the card's purchase APR, and a day's cost is simply the year's interest spread over 365 days. Card companies apply a daily periodic rate to an average daily balance over the statement period, so a statement's interest can differ a little from a flat month, and a balance that moves during the month changes it. Slate rounds to the cent at each step, as the app does.
A purchase's second price3
amount × APR ÷ 12
$64 × 24.99% ÷ 12 = $1.33, so the dinner costs about $65.33
This is one month of interest on the purchase if it rides a balance that carries interest. It is an estimate at the card's reported APR, not a line your issuer itemizes, and three things can make the real figure different: a grace period, which can make the interest on a purchase zero if you pay the statement in full; a second APR on the same card, for a cash advance or a promotion; and how long the purchase actually stays on the balance. Slate labels it as an estimate in the app for exactly these reasons.
Your date
the same payment every month, until zero
$5,000 at 24.99%, paying $155 a month: 4 years and 7 months, $3,378.05 in interest
Paying $205 a month instead: done 20 months sooner, with $1,324.52 less interest
Each month, interest is added to what's left, then the payment comes off, until the balance reaches zero. The assumptions are: a fixed APR, the same payment every month, interest added monthly, and no new purchases. "Interest the bank doesn't get" is the difference between the total interest on the two paths. A payment that doesn't cover the first month's interest never pays off, and Slate says so instead of showing a date. The horizon is 50 years; beyond it Slate shows no date rather than a misleading one.
A typical minimum4
1% of the balance + the month's interest, at least $35
On $5,000 at 24.99%: $50 + $104.13 = $154.13
On $5,000 at the 22.15% average, paying only the minimum: 16 years and 5 months and $7,730.52 in interest
This is a common issuer formula, not yours in particular: some use 2% of the balance, some add fees, and most have a floor between $25 and $40. The minimum falls as the balance falls, which is why paying only minimums takes so long. Once you connect, the minimum your statement reports is used instead.
What you've already paid
the interest charged on each statement, added up
No estimate here: this is the interest your bank reports having charged, statement by statement, per card and in total. Where a statement is missing, Slate leaves the month out rather than guessing, and shows an unknown as a dash, never as $0.00.
Where a real card differs from the estimate
Grace periods
Pay the statement balance in full by the due date and most cards charge no interest on new purchases. The second price assumes the purchase rides a balance that is already carrying interest.
Average daily balance
Issuers charge a daily rate on the average balance across the statement period, so a payment made early in the cycle lowers that month's interest more than the same payment made late.
More than one APR
Purchases, cash advances, balance transfers and promotions can each carry their own rate, and the issuer applies your payment across them under rules set by law and your card agreement. Slate's estimates use one APR per card.
Promotional rates
A 0% or reduced rate that expires changes the whole picture on its end date. Slate treats a 0% APR as costing nothing while it lasts, and says so.
New purchases and fees
Every projection assumes no new spending and no fees. Real statements include both.
The Consumer Financial Protection Bureau explains the daily-rate and average-daily-balance method3 and what the minimum-payment box on a statement means4. Slate's job is to put those mechanics in dollars, beside your balance, and to label an estimate as an estimate until your statement says otherwise.
What Slate doesn't do
- It doesn't pull your credit report. Connecting a card is a read-only data connection, not an inquiry.
- It doesn't make or schedule payments, and can't move money.
- It doesn't give financial advice. The coach answers in arithmetic on your numbers; for a big decision, talk to a qualified professional.
Try the arithmetic
- Debt-free date calculatorBalance, APR and a monthly payment in. The month you're done, the interest on the way, and what a little extra changes, out.
- Credit card interest calculatorWhat a balance costs a day, a month and a year at your APR, how much of a typical minimum the bank keeps, and the second price on a purchase.
Questions about a figure? Write to the address on the support page.
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.