Free Private Calculators

Loan, pricing, unit, date, engineering, and fitness calculators that run entirely in your browser. Every page explains the formula, its assumptions, and where it breaks down.

Nothing you enter is transmitted

Every calculator on this site runs in the page. Your inputs are read into browser memory, computed by JavaScript, and rendered back to you. There is no request to a server at any point, which you can verify by opening the network tab while you work.

That matters more here than it does for most utilities, because of what people put into calculators. A mortgage calculation contains your income assumptions and how much house you are considering. A debt payoff plan contains every balance and rate you carry. A salary conversion contains your compensation. A BMI or TDEE calculation contains your height, weight, and age. None of that should become a row in someone else's analytics table, and on this site it does not.

The formula is the smaller half of the answer

Any of these calculations can be done in a spreadsheet. What is harder to find is an honest account of what the formula assumes and where it stops being right, so that is what each page is written around.

A mortgage payment is straightforward arithmetic; the useful part is that your first payment is 86 percent interest, that escrowed taxes and insurance add 30 to 40 percent to the quoted figure, and that shortening the term saves more than shaving the rate. A tow rating is a published number; the useful part is that payload, not tow rating, is what actually stops you, and that tongue weight consumes it faster than anything else. BMI is one division; the useful part is what it cannot distinguish and what to measure instead.

Where an estimate has a known error band, the page states it. Mifflin-St Jeor is accurate to about 10 percent for 80 percent of people. An Epley one-rep max is good to about 5 percent from a set of three to six reps and considerably worse above ten. A month count between two dates has no single correct answer because months are not a fixed length. Presenting those numbers without their uncertainty would make the tool look more authoritative and less useful.

Guides for each area

Alongside the calculators there are seven longer articles, one per category, covering the arithmetic that spans several tools: how amortisation front-loads interest, why margin and markup are different numbers, why percentage changes do not cancel, why temperature differences convert without the offset, why a stored UTC offset is a scheduling bug, why squared and cubed relationships defeat intuition, and how far to trust a population-derived health formula.

Loans & interest

How loans actually work — Why your first payment is almost all interest, and why the term costs more than the rate.

Pricing & pay

Pricing and pay — Margin is not markup, a 20 percent discount can halve your profit, and a 40-hour week does not contain 40 billable hours.

Everyday math

Everyday math — A 50 percent gain and a 50 percent loss leave you down 25 percent, and that is the least surprising thing here.

Units & conversion

Unit conversion done properly — Most conversions are exact multiplications. The ones that are not cause the expensive mistakes.

Dates & time

Working with dates and time — Days are exact, months are a convention, and two hours a year do not exist or happen twice.

Engineering & vehicles

Engineering and vehicle math — Squared and cubed relationships explain most of it: heat, scaling, and why your truck runs out of payload before tow rating.

Health & fitness

Health and fitness math — Every formula here is an estimate with a known error band. Knowing the band is what makes them useful.