About

Planimeter

A pen name. I build and validate numerical models, and these are the same habit at hobby scale.

Every page names its model and what that model assumes.
The maths sits in its own file, with unit tests on it.
Nothing here is advice, and no page tells you what to do.

I make small interactive tools here. You move the parameter that matters, a chart moves, and you get to see the shape of the thing instead of one number.

Most of that work is finding out where a model stops being trustworthy, which is more or less what this whole site is about.

A planimeter is a mechanical instrument that measures the area under a curve by tracing its outline. It is a calculating device from before calculators: you move it by hand and read the answer off a dial. That is roughly what these tools are, and the name is a reminder that the reading is only as good as the instrument.

What these are

Toys for thinking with. Each one runs a simple model and draws what it says. A hobby, mostly — I make them because I want to see the curve myself, and putting them online costs nothing extra. There is no company behind it and no plan beyond finishing the next one.

If you are going to act on a number from one of these, check it somewhere else too.

How I check the numbers

Each tool cites its formula to something you can open. Where I could not verify a source, the page says so instead of linking to a good guess.

Where I can, I check a formula against a second version I wrote separately, because running the same model twice is not a check.

Every page also lists what its model leaves out. That is usually the most useful part.

Getting things wrong

I will. If you find an error I would genuinely like to know — the contact page has my email. I fix them and note what changed on the page, with the date. Typos I just fix quietly.

Copying

The writing, the diagrams and the code are mine. Read them, link to them, quote a bit with credit, use the tools for whatever you like. Please don't republish the articles wholesale or lift the calculators onto another site. If you want to do something bigger with them, just ask.

The maths belongs to nobody. Formulas come from published sources and are cited on the page that uses them.

Where to start

Finished so far: Monty Hall with N doors. The rest of the sections: Scale, Motion and Energy.

Everything here is provided as is, with no promise that it's accurate or working. It's a hobby site.