Credit Card Interest Calculator

How credit card interest works

Credit card interest is not a monthly percentage of your balance, it is a daily rate applied to your average daily balance and then summed over the billing cycle. The mechanics are dull, but the size of the number is not.

• Daily rate. The APR divided by 365. A 24.99% APR is 0.0685% per day.
• Monthly equivalent. 0.0685% × 30.4 days is about 2.08% a month. That is why a 24.99% APR behaves like a 2% monthly charge.
• Compounding. Unpaid interest joins the balance, and next month you pay interest on the interest. Left alone, a balance can grow even while you are making payments.
• Grace period. Usually 21 to 25 days interest-free on new purchases, but only if you paid last month's statement in full. Once you carry a balance, the grace period is gone.

Because the rate is variable and tied to the prime rate, it can move without your doing anything. The Consumer Financial Protection Bureau's explanation of credit card interest rates is the plain-English version of the same rules.

Worked example: $5,000 at 24.99%, paying $150

This is the classic debt trap, and the numbers are worse than most people expect.

• Balance: $5,000
• APR: 24.99% (about 2.08% a month)
• Payment: $150 a month, unchanged

Month one: interest of $104.12 is added, leaving only $45.88 of your $150 to reduce the balance. By the end:

• Months to clear: 58 (nearly five years)
• Total interest: about $3,622
• Total paid: about $8,622 for a $5,000 balance

Raise the payment to $300 and the same balance clears in 21 months for about $1,206 in interest. Paying $150 more a month saves roughly $2,400 and three years of your life. That single comparison is the most useful thing this calculator does: the payment is the lever, and it moves the answer far more than shopping for a slightly better APR.

The minimum-payment trap

Minimum payments are designed to keep the account current, not to clear it. A typical minimum is 2% of the balance with a $35 floor. On $5,000 at 24.99% that is:

• Minimum payment: $100
• Interest added that month: $104.12
• Result: the balance grows by about $4

You paid $100 and owed more than you did last month. On a $1,000 balance at 29.99% the same thing happens at a smaller scale: monthly interest is $24.99, so a $25 minimum covers interest and nothing else. This is why the calculator warns you when your payment is at or below the monthly interest, if that message appears, no amount of patience will pay the card off.

The escape is boring and effective: fix a payment comfortably above the interest, put it on autopay so it happens, and stop adding new charges to the card. If the balance feels impossible, a nonprofit credit counsellor can usually negotiate a lower rate, the CFPB explains how to find a legitimate one and what to avoid.

Payoff time and total interest at a glance

The same arithmetic across common balances and payments. Interest assumes a fixed payment every month and no new charges:

Read it down the page and the pattern is obvious: doubling the payment roughly cuts the interest by two thirds. Halving the balance halves the interest. Lowering the APR helps, but it is the slowest lever of the three, dropping 24.99% to 19.99% on a $5,000 balance at $150 a month saves about $500 of interest over the life of the debt, while paying $300 a month saves about $2,400.

Four moves that cut the cost

In rough order of how much difference they make:

• 1. Raise the payment. Nothing else comes close. Even $25 a month extra goes straight to principal and compounds in your favour every month after.
• 2. Attack the highest APR first. If you have several cards, put every spare dollar on the one with the highest rate while paying the minimums elsewhere. That is the avalanche method, and it costs the least in interest. The snowball method (smallest balance first) costs slightly more but keeps people motivated, the best method is the one you finish.
• 3. Move the balance to a 0% card. A 0% balance transfer for 15 to 21 months can wipe out the interest entirely if you clear the balance inside the window. Expect a 3% to 5% transfer fee, and know exactly what the rate becomes when the promotion ends.
• 4. Pay twice a month. Because interest is charged on the average daily balance, paying half on the 1st and half on the 15th trims a little off every month. On $5,000 at 24.99% it saves roughly $25 to $40 a year, small, but free.

What does not work: hoping the rate falls, and paying the minimum while using the card. A card with a balance and active charges has no grace period, so new purchases start costing interest on day one.

Where your APR comes from, and when it changes

Most credit card APRs are variable: the card's margin plus the prime rate. When the Federal Reserve moves rates, the prime rate follows and your APR usually moves within a statement or two. The Federal Reserve publishes the monthly consumer credit figures that show the average rate being charged across accounts, which is a useful sanity check on whether your own rate is typical.

Three things push a rate up on their own:

• Penalty APR. Triggered by a payment more than 60 days late or a returned payment. It is commonly around 29.99% and can apply to the existing balance as well as new charges. Federal rules generally require your rate to be restored after six consecutive on-time payments.
• Promotional periods ending. A 0% introductory rate becomes the standard rate on a set date. If you have not cleared the balance by then, the clock starts and the interest is calculated on whatever is left.
• Cash advances. A higher rate, no grace period, and often an immediate fee of 3% to 5% of the amount. A $500 cash advance can cost $20 in fees and start accruing interest that day.

One last thing worth saying plainly: this calculator is an estimate from the numbers you enter, not financial advice. Real statements use your actual average daily balance, fees and payment timing. Use the result to compare choices, $150 versus $300 a month, this card versus a balance transfer, and confirm the exact figures with your issuer before you make a decision.