How every number is calculated. Formulas, assumptions and limits.

🚗 Auto Loan

Sales tax is applied to the price minus the trade-in value. Trade-in equity (trade-in value − amount still owed) reduces the loan; negative equity is added to the new loan, as dealers usually do. Tax and fees are either financed or paid upfront, depending on your choice.

The result is checked against the 20/4/10 rule: at least 20% down (cash plus positive equity), a term of 48 months or less, and car payment plus insurance at or below 10% of gross monthly income. A total DTI above 43% overrides everything with a red verdict. “Cost to own” adds insurance, fuel and maintenance to the payment.

🏠 Mortgage

Principal and interest use the amortization formula on price minus down payment. The housing payment adds the monthly property tax, insurance, HOA and PMI you enter. Front-end ratio = housing ÷ income; back-end ratio = (housing + other debts) ÷ income.

The 0–100 affordability score is a simple, transparent mapping of the back-end ratio: 95 at 20% or less; 95 − 2.5 points per point above 20% up to 35%; 57 − 4 points per point from 35% to 43%; and 25 − 1.5 points per point above 43% (minimum 0). It is a reading aid, not a credit score or an approval probability.

💵 Personal Loan

Many lenders deduct an origination fee from the money you receive while you repay the full amount. CheckAfford computes the payment on the full loan, then finds the real APR: the monthly rate i that makes the amount you actually receive equal to the present value of your payments, solved numerically (bisection) and multiplied by 12. Total cost = payments − amount received.

Example: $10,000 at 12% for 36 months with a 5% fee → you receive $9,500, pay $332.14 a month, and the real APR is about 15.6%, not 12%. Total cost: $2,457.

To compare with your current debt, the calculator simulates that balance month by month (interest = balance × APR ÷ 12, then your payment). If the payment does not cover the monthly interest, the debt is flagged as never paid off.

💳 Debt Payoff

A month-by-month simulation with a fixed total budget. Each month: interest is added to every open debt, every minimum is paid, and the money left over goes to one target debt. Avalanche targets the highest APR first (ties: smaller balance); snowball targets the smallest balance first (ties: higher APR). Because the budget stays fixed, the minimum of a paid-off debt automatically rolls into the next one.

A third scenario pays only the minimums, to show what the plan saves. Simulations stop at 600 months (50 years). Any debt whose minimum is below its monthly interest gets a warning, because it can never shrink on minimums alone.

📱 Buy Now, Pay Later

Each plan is turned into a dated schedule: equal installments of amount ÷ number of payments, or an annuity payment when the plan charges interest (52, 26 or 12 periods per year for weekly, biweekly or monthly plans). Every installment is assigned to the last payday on or before its due date, because that is the paycheck that has to cover it.

The verdict looks at the heaviest paycheck from the current one onward: up to 10% of take-home pay is green, 10–20% is yellow, and above 20% is red. These are CheckAfford's own guardrails for cash-flow risk, not a lender rule.

📊 DTI Calculator

Adds up every monthly debt payment you enter, divides by gross monthly income, and shows how much room is left before 36% and 43%. The “room at 36%” amount is converted into the largest loan it could support at the rate and term you choose, using the inverted payment formula above.