Financial calculators that show their work

Two things are true about almost every free calculator online: you cannot see the formula, and you cannot see where the numbers came from. FigureDeck is an attempt to fix both. Every tool here publishes its formula, ships with worked examples that were checked against a known answer, cites the primary source behind any external figure, and states who is answerable for it. It is a free reference tool: it does arithmetic on numbers you supply, and it does not advise you.

What makes these different

The formula is on the page. Not in a help article, not in a tooltip. The equation that produced your number sits directly under the calculator, with each variable defined.
Every tool has passed known-answer tests. Each calculator carries worked examples — for instance a $400,000 loan at 7.28% over 30 years must produce $2,736.85 a month — and the page does not go live until those tests pass against an independent second implementation.
External figures carry a primary source and a retrieval date. Rates and limits come from Freddie Mac, the Federal Reserve, the FDIC and the IRS, each linked with the date it was pulled.
We say what the number is not. A mortgage payment here is principal and interest only. It excludes taxes, insurance, mortgage insurance and HOA dues, and the page says so next to the result rather than in a footnote.

Where to start

The mortgage cluster is the most developed. Start with the payment calculator if you have a loan amount and a rate in mind, the affordability calculator if you are working from income instead, the extra-payment tool if you already have a mortgage and are asking whether overpaying is worth it, and the break-even tool if you are weighing a refinance.

The loan tools cover instalment credit — cars and revolving balances — where the arithmetic of stretching a term, or of paying only the minimum, produces results most people do not expect. The savings and investing tools handle compounding, effective annual yield and annualised returns.

Mortgage calculators

Mortgage Payment CalculatorWork out the monthly principal and interest on a fixed-rate mortgage, the total interest over the full term, and how much of your first payment reduces the balance. Formula shown.Extra Payment CalculatorSee how many payments a recurring extra principal payment removes and how much interest it saves, across a ladder from $50 to $1,000 a month, on your own loan figures.Refinance Break-Even CalculatorFind the month at which a lower rate has recovered the closing costs, see your cumulative position year by year, and test how small a rate cut stops being worth it.How Much House Can I Afford?Work backwards from income and existing debt to a maximum purchase price, with the result shown under four different debt-to-income bands so you can see whose judgement sets the ceiling.

Loan calculators

Auto Loan CalculatorCalculate the amount financed on a car including sales tax and trade-in credit, the monthly payment, total interest, and what stretching the term to 72 or 84 months really costs.Credit Card Payoff CalculatorSee how long a credit card balance takes to clear on minimum payments, what each extra dollar removes, and how paying a fixed 3% or 5% of the balance compares.

Investing and growth calculators

Compound Interest CalculatorSee the future value of a lump sum plus monthly contributions, how the total splits between what you paid in and what compounding added, and the year the split crosses over.Retirement Savings CalculatorProject a retirement balance from current savings and monthly contributions, then convert it to a monthly income under 3.5%, 4% and 4.5% withdrawal rates, with the IRS limits alongside.CAGR CalculatorConvert a beginning and ending value into a compound annual growth rate, see how long a given rate takes to double or triple your money, and understand what the annualised figure hides.

Savings calculators

Savings & APY CalculatorConvert a quoted savings rate into the effective annual yield, see what each compounding frequency adds, and compare the result against the FDIC national deposit rate.