How it works

Gas: miles a year / 12 / miles per gallon x price per gallon. Electric: miles a year / 12 x kWh per 100 miles / 100 x your blended power price. Exact fractions are used, so a result of exactly half a cent rounds up (see the example below).

Balance point

balance point = your limit − insurance − fuel − upkeep − yearly registration / 12

Example: limit $900, insurance $180, fuel $140, upkeep $90, registration $240 a year. $900 − $180 − $140 − $90 − $20 = a $470 payment.

price that a payment covers = (payment x (1 − (1 + r)⁻ⁿ) / r + down payment + trade-in equity − fees) / (1 + tax rate)

Example: $450 a month at 7% for 72 months with $3,000 down and 6% tax covers a price of about $27,730.66. This simple version taxes the full price.

The limit buttons offer 10%, 15% or 20% of take-home pay. These rules of thumb are widely repeated in personal-finance writing, and writers differ on whether they count only the payment or all car costs. We did not find one official source. They are starting points for you to change, not advice.

So a first answer needs only five numbers, the used-car loan rate, months, sales tax, insurance, fuel economy, upkeep and registration start as stand-ins marked “example” (for a used car: 11%, 60 months, 7%, $160 a month, 30 mpg, $60 a month, $240 a year). The answer lists every stand-in it uses, right under the verdict, and they go away as you type yours. Another path shows a figure only once its own numbers are typed: a new car needs its price; a lease needs its price (or sticker), its months, its end value and its rate (or money factor), and never borrows the used car’s price; keeping your car needs at least one of its costs, and counts it as paid off until you type a payment. Once you type a payment above $0 for the car you keep, the page also asks for the payments left and shows no figure until you type them. If you clear a box that has a stand-in or a public starting value, that number comes back when you leave the box, marked “example” or with its source; type 0 to leave a cost out. A cost box with nothing to put back (an electric car’s kWh per 100 miles, or public charging when not all charging is at home) that stays empty is left out of the total, and the answer says so three times: in the sentence, as “not counted” in the list under it, and as an empty dashed slot under the stack. Blank down payment, trade-in and fee boxes count as $0, and the list says “none typed” for each.

All money is kept in whole cents. Each month's interest is rounded half-up to the cent (a half cent rounds up). The payment is rounded to the cent. The last payment is whatever clears the balance, so the loan ends in exactly the months you chose. Without this rule, a payment rounded down leaves a cent or two and adds an extra month. On the answer screen the total is rounded half up to whole dollars ($947.50 shows as $948). The amount over or to spare is that rounded total minus your rounded limit, so $700.50 against $900 shows $701 and $199 to spare. The five slabs are rounded so they add up to the rounded total, so one slab can be a dollar off its cents. Every place that shows a path’s total or gap uses these same figures. “Show the math” gives the cents.

• Interest. Interest builds monthly at the yearly rate divided by 12. The first payment is one month after the loan starts. We model no odd first period and no day counts.
• Sales tax. Tax is on the price minus the trade-in value (not minus what you still owe on it), plus the dealer document fee. Title and registration fees are not taxed. Some states tax the full price; untick "My state taxes the price minus the trade-in value" for that.
• Owing more than the trade-in is worth. The gap is added to the new loan, unless you tick that you will pay it in cash. Then it is cash on day one.

The lease cost as an APR-style number is the money factor x 2,400. That is a common approximation, not the exact rate. You choose how tax works: on each payment, on all payments at signing, or on the price at signing. Due at signing = cash down + first payment (with its tax) + any start fee you pay up front + fees at signing + security deposit + tax paid up front. Lease tax rules vary by state and are not checked against a real dealer worksheet here. Confirm yours.

• Card minimum. When you compare with "only the card minimum", the minimum is the larger of $25 or 1% of the balance plus that month's interest, and never more than the balance plus interest. The first payment is in month 1. Real cards differ: check your statement.
• Rebalance. A fee to move a balance is its own line. It is never counted as interest. The total change = interest change + fees.