Credit Card Reality Engine: Annual Fees, Rewards vs Interest and the Deferred-Interest Trap
Card offers are built to be compared at their best case. This engine runs the three calculations the offers leave out: whether an annual fee pays for itself at your spending, what carrying a balance does to your rewards, and what a deferred-interest promotion really puts at risk. It names no cards and earns nothing from any issuer.
Pure math on your own numbers. The default APR, 22.15%, is the Federal Reserve's average for credit card accounts charged interest in the second quarter of 2026. No card names, no issuer data, no affiliate links.
The card and your spending
What the card earns on your spending
Statement credits
Your balance and habits
The promotion
| Month | Payment | Balance after | Deferred interest building up |
|---|
The last column is waived only if the promotional balance is paid in full by the deadline. On a true 0% card it never applies.
How the Engine Works
Annual fee. The engine multiplies each month's spending in a category by the card's points per dollar and by what a point is worth to you, adds the statement credits at the share you would really use (a $100 credit you would only half use counts as $50), and subtracts the fee. It then compares that with what a no-fee card paying a flat percentage back would earn on the same spending. The break-even point value is the cents per point at which the two cards come out even; a sign-up bonus counts in year one only.
Rewards vs interest. It runs 12 statement cycles. Each month's interest is the APR divided by 12, charged on the balance carried into the cycle and, when a balance was carried, on half a month of new purchases: without a grace period, purchases accrue interest from the day they post, and spread through the month they average half of it. The payment is taken at the end of each cycle, either the amount you enter or the full statement balance. The grace-period loss is the interest charged on new purchases that would have been interest-free if you paid in full.
Deferred interest vs 0% APR. A deferred-interest promotion charges nothing if the promotional balance is paid in full by the deadline; if any of it is left, the issuer adds all the interest that built up from the purchase date at the regular APR. The engine tracks that build-up month by month on the balance you still owe. A true 0% promotion charges nothing during the promotion and interest only on what is left afterward. The safe payment divides the purchase by one month fewer than the promotion lasts, so one late or misapplied payment does not cost you.
If the card carries other balances, payments above the minimum must go to the highest-rate balance first, except during the last two billing cycles before a deferred-interest promotion ends, when they go to the deferred balance first. Issuers may also apply extra payments to a deferred-interest balance when you ask.
Reading the Result
With the default numbers, $2,200 a month of spending and a $95 card that earns 3 points per dollar on groceries and dining, the card loses to a no-fee 2% card by $57 a year when a point is worth 1 cent and wins by $72 when it is worth 1.25 cents. The verdict turns on the value of a point, which is why the engine shows the break-even value: about 1.11 cents here.
Carrying a balance changes everything. Carrying $3,000 at 22.15% while paying $1,600 a month on $1,500 of new purchases costs about $791 in interest over a year against $360 of rewards, and $166 of that interest falls on new purchases that would have been free with a grace period. The balance is still about $2,591 after 12 months.
Deferred interest is a cliff, not a slope. A $2,400 purchase on a 12-month deferred-interest plan at 29.99% builds up $472 of interest by the deadline if you pay $150 a month, and the $600 left over triggers all of it. Paying $219 a month clears the balance in 11 months. On a true 0% card the same $600 would cost about $14.99 a month in interest from then on.
What the Engine Leaves Out
Sign-up bonuses usually require a minimum spend within the first few months, and the engine counts them in year one only. It leaves out foreign transaction fees, balance transfer fees, the credit-score effect of opening or closing an account, and rate changes: most card APRs are variable and move with the prime rate. Issuers compute interest daily on the average daily balance, so a real statement can differ from the engine's monthly figure by a few dollars, and some restore the grace period only after two full payments in a row. Point values are whatever you enter; the engine does not know any program's award chart, and it names no cards on purpose.