How Our Calculators Work
Last updated: October 6, 2026
Every calculator on TheSmartWealthTools runs on a published formula, not a black box. This page explains the math behind each tool, the assumptions it makes, and the places where its answer can differ from what your bank or card issuer shows you. If you want to check a result by hand, everything you need is here.
Principles That Apply to Every Calculator
- Monthly interest is APR divided by 12. A card or loan at 18.9% APR is charged 1.575% per month on the balance carried into that month.
- Payments are made at the end of each month, after that month's interest has been added. This is the most conservative timing, so our payoff estimates lean slightly long rather than optimistic.
- No new charges. Debt calculators assume you stop adding purchases, late fees or annual fees to the balance while you pay it down.
- Rates stay fixed unless the calculator asks you for a second rate (for example, the rate after a promo period ends).
- Results are rounded to the cent for display only. Internally, every step keeps full precision, so totals do not drift from rounding.
- The examples on this page were recomputed independently with a separate script using the same formulas, and they match what each calculator shows.
Credit Card Payoff Calculator
The credit card payoff calculator builds a month-by-month amortization schedule. Each month it does three things:
principal = payment - interest
new balance = balance + interest - payment
It repeats until the balance reaches zero. The final payment is reduced to exactly what is left, so you never "overpay" in the schedule. If your payment does not cover the first month's interest, the balance can never shrink, and the calculator tells you so instead of showing a result.
The comparison against "paying the minimum" uses a common industry formula: 2% of the current balance, with a floor of $25. Card issuers use different rules, so treat that row as a sense of scale, not a prediction of your statement.
Example: $5,000 at 18.9% APR, paying $200 a month. Month 1 interest is $78.75, so $121.25 goes to principal. The card is paid off in 33 payments with $1,405.04 of total interest.
Balance Transfer Calculator
The balance transfer calculator runs two schedules side by side with the same monthly payment:
- Stay: your current balance at your current APR, start to finish.
- Transfer: your balance plus the transfer fee (added on day one), charged at the promo APR for the promo months, then at the regular APR on whatever is left.
The difference in total cost is your real saving, with the fee included. If money is still owed when the promo ends, the calculator shows how much.
Example: $6,000 at 24% APR, paying $300 a month. Staying costs $1,739.24 in interest over 26 months. Transferring with a 3% fee to a 0% card for 15 months (24% afterwards) costs $180 in fees plus $119.52 in interest and finishes in 21 months - a saving of $1,439.72.
Avalanche vs. Snowball Calculator
The avalanche vs. snowball calculator simulates paying several debts with one fixed monthly budget. Every month, for every open debt:
- Interest is added to each balance.
- Each debt receives its minimum payment.
- Whatever is left of the budget goes to one target debt. When a debt is cleared, its money rolls to the next target.
The only difference between the two methods is the order of targets. Avalanche targets the highest APR first; snowball targets the smallest balance first. Ties are broken by the other rule. If your budget is below the sum of your minimums, or the balances stop falling, the calculator stops and says so.
Example: a $900 store card at 12.9%, a $6,500 Visa at 26.9% and a $4,000 car loan at 7.9%, with a $555 monthly budget. Avalanche finishes in 25 months with $2,234.84 of interest. Snowball finishes in 27 months with $3,153.17, but clears its first debt in month 4 instead of month 20.
Compound Interest Calculator
The compound interest calculator moves forward one month at a time. Each month your contribution is added to the balance; interest is applied at the compounding frequency you choose (monthly by default, at the annual rate divided by the number of periods). If you set a yearly increase, the monthly contribution grows by that percentage at the start of each new year.
each compounding period: balance = balance × (1 + rate ÷ periods per year)
The "today's money" column divides each year's balance by (1 + inflation) raised to the number of years, so you can see buying power instead of just nominal dollars.
Example: $10,000 to start, $200 a month, 7% a year compounded monthly, for 20 years. You deposit $58,000 in total and end with $145,180.47 - so $87,180.47 is growth you never deposited.
Mortgage Calculator
The mortgage calculator uses the standard fixed-rate payment formula, where P is the loan amount (price minus down payment), r is the monthly rate and n is the number of monthly payments:
Property tax and home insurance are entered as yearly amounts and added at one twelfth each per month. PMI, HOA dues and closing costs are not included.
Example: a $300,000 loan at 6.5% for 30 years has a principal-and-interest payment of $1,896.20 a month, with $382,633.47 of interest over the full term.
Auto Loan Calculator
The auto loan calculator uses the same fixed-payment formula as the mortgage calculator, with the term entered in months. The amount financed is the vehicle price minus your down payment or trade-in. Sales tax, registration and dealer fees are not added automatically - include them in the price if they are being financed.
Example: $30,000 financed at 7% for 60 months costs $594.04 a month and $5,642.16 in total interest.
FIRE Calculator
The FIRE calculator first finds your target number by dividing your yearly retirement spending by your safe withdrawal rate. It then grows your savings year by year until they reach that target:
each year: savings = savings × (1 + real return) + yearly contribution
The return you enter is treated as a real return, meaning after inflation, so the target stays in today's dollars. Contributions are added at the end of each year.
Example: $40,000 of yearly spending at a 4% withdrawal rate gives a target of $1,000,000. Starting from $100,000 and adding $30,000 a year at a 5% real return, you reach it in 17 years.
ROI Calculator
The ROI calculator reports two numbers. Total ROI is your profit as a share of what you put in. Annualized ROI (also called CAGR) is the steady yearly rate that would turn your starting amount into your final amount over the same period:
annualized ROI = (final ÷ initial)1 ÷ years - 1
Example: $10,000 that grows to $16,000 in 5 years is a 60.00% total return, or 9.86% a year.
Why Your Statement Might Not Match to the Cent
- Daily interest. Most card issuers charge interest daily on your average daily balance. We charge it monthly at APR ÷ 12. Over a full payoff the difference is usually around 1%.
- Payment timing. Paying earlier in the cycle lowers the balance that interest is charged on. We assume end-of-month payments, the most expensive timing.
- Fees and new purchases. Annual fees, late fees and new spending are not in our schedules unless you include them.
- Lender rounding. Banks round each payment and each interest charge to the cent, which can move the final payment by a few cents.
These effects partly cancel out, which is why our projections are usually close. They are planning estimates, not quotes.
Found a Mistake?
If you think a result is wrong, please tell us through the contact form with the numbers you entered and what you expected. We check every report and correct this page and the calculator if we find an error.
TheSmartWealthTools provides calculators for education and planning only. Nothing on this site is financial, investment, tax or legal advice. See our Terms of Service.