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Methodology

This page explains the math behind every calculator on CalcMyCard, so you can check our results and see where each number comes from.

Interest calculation method

Calculation method: Our payoff and interest calculators use a simplified monthly-interest model (APR ÷ 12). Credit-card issuers may calculate interest using daily periodic rates and average daily balances, so your actual statement may differ.

In practice, each interest and payoff calculator takes a monthly rate equal to your card’s APR divided by 12, applies it to the balance remaining at the start of each month, subtracts the payment, and repeats month by month until the balance reaches zero. This is a common simplification for payoff planning; because it ignores mid-cycle purchases and payments, day counts, and issuer-specific compounding, results are estimates rather than statement-exact figures. The same disclosure appears above every interest and payoff calculator on the site.

Four calculators use daily interest instead, because that is how the cost they measure builds up: the Daily Periodic Rate Calculator, the Interest Charge Checker, the Cash Advance Calculator (APR ÷ 365) and the Bi-Weekly Payment Calculator (APR × 14 ÷ 365 for each two-week period).

Every payoff simulation stops at 600 months (50 years). If a balance is still shrinking at that point, the result says it takes more than 50 years; if the payment doesn’t cover the interest, the result says the balance never pays off.

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Credit Card Interest Calculator

Splits your next payment into interest (balance × APR ÷ 12, rounded to the cent) and principal, then runs the monthly model to payoff for total interest, payoff time and a month-by-month or year-by-year schedule.

Credit Card Payoff Calculator

Two modes. “I know my payment” runs the monthly model to a payoff date, adds a comparison with $50 a month more, and lists when 25%, 50%, 75%, 90% and 100% of the balance is paid. “I have a deadline” finds the payment that clears the balance in N months with the standard amortizing formula, balance × r ÷ (1 − (1 + r)−N) where r = APR ÷ 12, rounded up to the cent, then runs the same schedule. Optional new charges each month are added to the balance before interest until it is paid off; in deadline mode the payment then adds charges × (1 + r).

Minimum Payment Calculator

Estimates this month’s minimum, then simulates paying only the minimum every month, with the minimum recalculated on each new balance. It supports two formula types: a flat percentage of the balance, or interest plus a percentage of the balance (whichever of that and a flat percentage is higher), both with a dollar floor. Five issuer presets fill in each issuer’s standard formula from its published cardmember agreement:

Late fees and past-due amounts are left out. Links to each agreement are in the issuer table on the calculator page; individual cards can use a different formula. The calculator also runs this month’s minimum as a fixed payment through the monthly model, to show the time and interest if you keep paying that amount instead of letting it shrink.

Balance Transfer Calculator

Adds the transfer fee (fee % × balance) to the transferred balance, charges the promotional APR for the promo months and the go-to APR after that, and compares the total cost (fee plus interest) with keeping the balance on your current card, paying the same monthly amount in both cases.

Multi-Card Debt Payoff Calculator

Keeps your total monthly budget fixed: the sum of every card’s starting minimum plus the extra amount you enter. Each month, interest accrues on every open card (APR ÷ 12), each open card receives its minimum, and whatever is left goes to the current target (smallest balance for snowball, highest APR for avalanche; ties go in the order you entered the cards). When a card is paid off, its old minimum rolls over to the next target, and if the money left in a month is more than the target owes, the rest goes to the next target in the same month. With minimums set to shrink, each card’s minimum is recalculated monthly as the greater of the dollar floor and the card’s starting share of its balance (entered minimum ÷ starting balance) times the month-start balance; the extra amount stays fixed, and a paid-off card’s last minimum rolls to the next target.

APR Calculator

Turns an APR into dollars on a balance with no payments: monthly interest (balance × APR ÷ 12), daily interest (balance × APR ÷ 365), a year of simple interest (balance × APR), and a year with monthly compounding (balance × ((1 + APR ÷ 12)12 − 1)).

Daily Periodic Rate Calculator

Daily rate = APR ÷ 365 (or 360). Interest for the cycle = average daily balance × daily rate × days in the cycle, as simple daily interest.

Extra Payment Savings Calculator

Runs the monthly model twice, once with your base payment and once with the extra amount added, and reports the months and interest saved. A table repeats this for several common extra amounts.

0% Intro APR Calculator

The payment that clears the balance before the promo ends is the balance ÷ promo months, rounded up to the cent. If your planned payment is lower, the remainder at the end of the promo is run through the monthly model at the go-to APR. If your payment wouldn’t cover that interest, the result uses the smallest payment that still makes progress and says so. It does not model deferred-interest offers (the Deferred Interest Calculator below does).

Deferred Interest Calculator

For “no interest if paid in full” promotions. The payment that avoids deferred interest is the purchase ÷ promo months, rounded up to the cent. Each promo month, interest of promo balance × APR ÷ 12 is added to a separate deferred-interest total (not to the balance), then your payment reduces the balance. If the balance reaches $0 within the promotion, that total is waived; otherwise it is added to the balance left, and the result is run through the monthly model at the card’s APR at the same payment (or the smallest payment that still makes progress, as the result says). Issuers usually accrue deferred interest daily, so statements can differ by a few dollars. This monthly APR ÷ 12 accrual is a simplification we chose so the result matches the rest of the site’s monthly model.

Credit Utilization Calculator

Utilization = balance ÷ credit limit, per card and overall. Ratings: 10% or less Excellent, 30% or less Good, 50% or less Fair, above 50% High. It also shows how much to pay down to reach 30% and 10% overall.

Debt Consolidation vs. Balance Transfer Calculator

Transfer side: the fee is added to the balance, and a flat payment sized to clear it within the promo period ((balance + fee) ÷ promo months) is applied, with the go-to APR on anything left after the promo. Loan side: the origination fee is either added to the loan or, if you choose, taken out of the loan money (you then borrow balance ÷ (1 − fee %) so what’s left clears the balance), and the fixed payment is principal × r ÷ (1 − (1 + r)−n) with r = loan APR ÷ 12 and n = the term in months. The cost of each is everything paid beyond the original balance.

Cash Advance Calculator

Fee = the greater of fee % × amount and the minimum fee, added to the balance. Interest = (amount + fee) × cash advance APR ÷ 365 × days until repaid, as simple daily interest from day one, because cash advances usually have no grace period. An optional ATM fee is paid up front and not added to the card balance.

Bi-Weekly Payment Calculator

The monthly plan charges APR ÷ 12 and then applies the payment. The bi-weekly plan charges APR × 14 ÷ 365 for each two-week period and then applies half the payment. Twenty-six half-payments a year add up to 13 monthly payments, which is where most of the saving comes from. This is a simplification: 26 two-week periods cover 364 days, so the bi-weekly plan is charged slightly less interest than a full year, and card issuers actually bill monthly on your average daily balance, so the real saving can be a little smaller.

Interest Charge Checker

Expected charge = average daily balance × (APR ÷ 365 or 360) × days in the cycle. Implied APR = the charge on your statement ÷ (average daily balance × days) × 365. Small differences usually come from daily compounding or a different day count.

Fed Rate Change Calculator

Variable card APRs are the prime rate plus a fixed margin, and the prime rate moves with the federal funds target, so the card’s APR moves by the same amount as the Fed change. The calculator runs the monthly model at the old and new APR with the same payment and shows the difference. Fixed-rate cards don’t move with the Fed.

How we test the calculators

An automated test suite loads each calculator’s live code in a real browser, enters inputs, and checks the displayed results against hand-worked examples and a separate reference implementation of these formulas. It covers all 16 calculators with 157 test cases, including edge cases such as $0 balances, 0% and 100% APR, payments equal to or below the monthly interest, minimum-payment floors, 0% promos and fees, equal APRs or balances, and $0 credit limits. It also fails if a result shows “NaN” or “Infinity.” Last full run: October 25, 2026 (157 of 157 passed). The suite runs automatically before every update to the site goes live, and a separate daily check of the live site confirms every page is reachable, indexable and listed in the sitemap.

What the 157 test cases check:

The suite is written and run by CalcMyCard; it is not an outside audit. See the About page for what each run found and fixed.

What our calculators don’t account for

Our tools don’t model card-specific quirks like exact grace period rules, issuer-specific deferred-interest terms beyond what the Deferred Interest Calculator asks for, or fees beyond what you explicitly enter. They’re built for clear, general-purpose estimation — always confirm exact figures against your official statement or cardholder agreement before making a financial decision.

Privacy of your inputs

Every calculation runs entirely in your browser using JavaScript. The numbers you enter are never sent to our servers, stored, or shared with any third party. If you use a calculator’s share link, your inputs are written into that link’s web address, so anyone you send it to (and our server, when the link is opened) can see them.

Last reviewed

This methodology was last reviewed by Ali Ahmad on October 16, 2026.