Mortgage payments, take-home pay, percentages, paint coverage, running pace, subnet ranges — and how long a bearing lasts at 1,450 rpm. The numbers you'd otherwise dig out of a textbook or a forum post, worked out properly, in your browser, free.
Rule of the day · The Rulebook
A field heuristic, then the exact calculator
135 rules of thumb - each one linked to the tool that does the job properly.
Open the full RulebookFifteen chains through the real calculators - and every link opens the next tool with the numbers already in it.
Same verified physics as the full tools. Click a mini to feature it above - open the simulator for controls and scenarios.
The earth, the sky, and the clock - moist-air physics, solar geometry, and time across the globe.
Structural and mechanical reference - load, stress, and the standard checks that go with them.
Circuits and magnetics - Ohm's law, power factor correction, transformer turns ratio, and wire sizing.
Everyday money math - what your money does while it sits, and what a loan really costs over time.
How things respond and how we steer them - feedback control and the limits of digital sampling.
The machines themselves - storage layout, networks, and the trade-offs of keeping data safe.
Real, live simulations you can nudge and watch respond - oscillation, chaos, motion under friction, and heat transfer.
Hands-on reference for around the house - paint, fixings, concrete, tea, coffee, eggs, and precision timing.
Trigonometry, a full 2D/3D geometry suite, statistics with a genuinely random Galton board, and a graphing calculator.
BMI on a full colour-coded chart, running/cycling/swimming pace, and calorie needs from a validated formula.
Stored on this device only. Nothing is sent anywhere.
Ranked by how many people starred each tool — one vote per browser, no account, and nothing about what you calculated.
Yes - every tool, no account, no paywall. There are no ads and no analytics running on this site today; see the privacy policy for exactly what that means and what would change first if that ever changes.
Each one is built from a real, cited formula or dataset - a physics equation, an engineering standard, published statistics - not a guess. Every tool's footer names its source, and most are verified against the source's own worked examples before publishing. See Sources for the full list.
No first-party cookies and no analytics. Calculator inputs stay in your browser. The optional Favourite star on a tool saves it for you locally and can add +1 to the public popular ranking - never the numbers you typed. See Privacy.
Yes, after your first visit. The site is installable as an app on desktop and mobile, and previously visited tools keep working offline.