Calculators that show their working

18 free tools. Each one gives you a number AND the reason it is that number — the limit that applied, the rule that decided it, the document it came from. That is the part every other calculator leaves out.

Free · no sign-up · nothing you type is stored or sent · available in 8 languages

Everyday money

Free budget calculator Try the 50/30/20 rule on your own numbers, or plan a single paycheck. Nothing is stored — it all runs in your browser. Debt Snowball vs Avalanche Calculator Free debt payoff calculator. Enter your debts and an extra monthly payment, and compare the snowball and avalanche methods: months to debt-free, total interest and payoff order. Savings Goal Calculator Free savings goal calculator. Pick an amount and a date and see what to put aside weekly, biweekly, twice a month or monthly — deposit counts from the real calendar, with optional APY interest. Emergency Fund Calculator Free emergency fund calculator for irregular income. Enter 6–12 months of real income and see how many months of cover your swings call for, the target in dollars, how long you would last today and how many months until fully funded. Nothing you type is stored. HDHP vs PPO calculator: which health plan really costs less Compare two health plans by what the whole year really costs: premiums, your expected care through the deductible, coinsurance and the out-of-pocket cap, employer HSA money, and the income-tax and payroll-tax savings that premium-only comparisons skip. Shows the spending level where the plans trade places. Free, runs in your browser, nothing stored.

Self-employment and tax · United States

Quarterly Estimated Tax Calculator Free quarterly estimated tax calculator — US federal, tax year 2026. Self-employment tax, income tax, the QBI deduction, all three safe-harbour tests and the lowest legal quarterly payment. Underpayment Penalty Calculator Free Form 2210 underpayment penalty calculator — US federal, tax year 2026. What a missed estimated tax instalment costs today, and what each extra day adds. The self-employed deduction checklist that counts A deduction checklist that counts. Enter your numbers and see the amount for each write-off — and the reason it is not more. Free, no sign-up, every limit sourced. Mileage log template 2026 — self-employed, with both IRS rates Most mileage templates carry one rate for the year. In 2026 the IRS raised the business rate mid-year, so an honest log keeps every trip at the rate of its own date. The free template below does that by itself, and the calculator shows what both halves add up to. S-Corp vs Sole Proprietor Breakeven Calculator Free S-corp breakeven calculator — US federal, tax year 2026. What electing S-corporation status saves after payroll, the extra 1120-S and section 199A. Solo 401(k) contribution calculator What a self-employed person may actually contribute to a solo 401(k) for 2026, using the Publication 560 reduced rate. Free, no sign-up, every figure sourced. IFTA fuel tax calculator — by state, with the return's own rules A free IFTA calculator that follows the quarterly return's own rules: fleet MPG to two decimals, taxable gallons per jurisdiction, credits offsetting debts on the same return, and the Kentucky and Virginia surcharge computed on full gallons — owed even when the fuel line is a credit.

Self-employment and tax · United Kingdom

UK self-employed tax calculator — a Self Assessment estimate that shows its working Estimate your UK Self Assessment bill as a sole trader: income tax by band (England, Wales & Northern Ireland or Scotland), Class 4 and Class 2 National Insurance, the trading allowance decided against your real costs, HMRC flat rates for mileage and working from home, and the payments-on-account schedule. Every rate is quoted from GOV.UK.

Bookkeeping

Why your bank reconciliation is out — and which entry did it Enter your two balances and your entries. This finds the arithmetic signature of the error — a transposition, a wrong-side posting, a missing entry — and shows the amount and the proof. Free, no sign-up.

Student loans · United States

RAP Student Loan Payment Calculator Free Repayment Assistance Plan calculator for US federal student loans: your monthly RAP payment, how much the balance really falls each month once the principal match and the interest waiver are counted, and what is left to forgive.

Food business

Recipe cost calculator — from pack price to menu price Cost a dish the way a kitchen actually buys: pack price to edible-portion cost through your own yield, exact NIST unit conversions, cost per serving with plate extras, then two pricing paths — target food-cost percent and target contribution margin — and the food cost at the price you really charge. Free, in-browser, nothing stored. Menu engineering matrix — from your sales mix, thresholds included Enter each menu item's price, plate cost and units sold. The matrix computes both thresholds from your own mix — the popularity cutoff from the number of items, the margin benchmark as the weighted average — and classifies every item, with the source of the method shown. Runs in the browser, nothing stored.

Service businesses

Lawn care pricing calculator — a per-acre price built from your own costs Enter your own minutes per acre, labor rate, mower costs, travel time and overhead. The calculator builds the job price from your costs at your margin — plus the per-acre price at every lot size and the floor no chart can give you.

Invoice maker

An invoice that adds up Fill one in, print or save it as PDF — the totals and the tax compute themselves.

How these are built, and why that matters

The arithmetic that runs in your browser is held against a second, independently written model before the page ships. Where the two disagree, nothing ships.
Every limit and rate carries the document it was read from. A figure we could not verify is shown as unverified rather than printed as fact.
The first answer is computed on the server, so the page means something before any script runs — and so a machine reading it sees the same numbers you do.

When a calculator is not enough

A calculator answers one question once. The spreadsheets carry the same verified arithmetic through a whole year — income, expenses, mileage, invoices, inventory — and keep it in one file you own.

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