How we calculate and check our numbers
Every DollarShuffle calculator uses official government data and published formulas, and every result is checked against an independent calculation before it ships. This page explains where the numbers come from, how they are verified and how they stay current.
Figures last reviewed for the 2026 tax year.
Where the numbers come from
| Data | Source | Refreshed |
|---|---|---|
| Mortgage rates | Freddie Mac Primary Mortgage Market Survey | Every 12 hours |
| Inflation (CPI-U) | U.S. Bureau of Labor Statistics | Every 6 hours; full history rebuilt at each deploy |
| Savings and CD rates, Treasury yields | FDIC national rates and rate caps | Every 6 hours (published monthly) |
| Credit card APRs | Federal Reserve G.19 Consumer Credit | Every 12 hours (published quarterly) |
| Savings bond values, I bond rates | U.S. Treasury Fiscal Data | Daily (published twice a year) |
| Federal income tax and withholding | IRS Revenue Procedure 2025-32, Publication 15-T, Form 1040 instructions | Each tax year |
| State income tax and withholding | Each state's revenue department (51 jurisdictions) | Each tax year and when laws change |
| Loan limits and mortgage insurance | FHFA, HUD (FHA), VA, USDA | Each year |
| Property tax, home insurance | U.S. Census Bureau American Community Survey | Each year |
| Household spending | BLS Consumer Expenditure Survey | Each year |
If a live source cannot be reached, the calculator uses the last values it saved and says so. Each calculator's own "How this is calculated" section lists the exact formulas and rules it applies.
How results are verified
Independent calculations
Before a formula is used, we work examples by hand or in separate code and compare them with official worked examples where agencies publish them, such as IRS withholding examples, state withholding guides and Treasury savings bond values to the cent.
Automated tests
Hundreds of automated tests check the math on every change: tax brackets and credits for all 50 states and DC, loan and investment formulas, and the parsers that read each official data file.
Real-browser checks
Each calculator is also exercised in a real browser on desktop and phone, confirming that the numbers on the page match the tested engines and that CSV imports flag bad rows instead of guessing.
Keeping it current
Live data
Rates that change often (mortgage, savings, CD, credit card, inflation, savings bonds) refresh automatically on the schedule above.
Twice-yearly review
Every June and December we review each tax year figure, state rule, loan limit and data set against its source, after the agencies publish their annual updates, and update the date at the top of each page.
Estimates are labeled
Where an official figure is not yet published or a state has not released a formula, the calculator says the figure is an estimate and explains why.
What the calculators do not do
Results are estimates for planning and education, not financial, tax or legal advice. They cannot see your full situation, and some rules are simplified, which each calculator notes. Check important decisions with a qualified professional, and see the About page for who we are.