Type an expression and press Enter (or the play button) to evaluate it and save it to the history. As you type you will see a live preview of the result.

Press the Functions button to open the catalogue: search by name and click a function to insert its call with the cursor ready for the arguments.

If the expression has a variable (e.g. x), the Plot (draws y = f(x)), Differentiate and Simplify buttons appear. With two variables you also get Plot 3D. From the plot's export menu you can download the image or the table of values as CSV.

A multi-line calculation, with each line's result beside it. Variables carry downwards (width = 3.5 m and then area = width * height), units travel with the value —adding metres and kilos is flagged on that line and does not break the rest— and # starts a comment. ans is the previous result. An error stays on its own line. Exports to Markdown or CSV so someone else can read it.

In the Sheet you can declare currencies with a comment: # @moneda EUR and # @moneda USD = 1.08 EUR. From then on (20 EUR + 10 USD) to EUR works, and mixing money with metres is an error. You set the rate and it stays written in the document: nothing goes to the Internet and the calculation can be reproduced tomorrow. The format button chooses how results are written (fixed decimals, significant figures, engineering, exact fraction) without changing the calculation.

Equations (x^2 - 5x + 6 = 0), systems (one equation per line) and derivatives, with the step by step and the reason for each step. The result always says whether it is exact or approximate, and in that case over which interval it searched. When it cannot explain a case it says so instead of making up the reasoning: you get the result plus a note with what it can explain.

With m, g, t, h and 16 other letters there is ambiguity: they are units and ordinary variable names. The calculator decides by position —next to a number it is a unit (100 km), on its own it is a variable (g*t^2/2)— and tells you which it chose, with a button to switch. In the Sheet, if you assign to one of those letters (m = 5) it warns you that from then on 3 m is no longer three metres.

The link button copies an address with your expression inside, to pass on to someone. And the save/load button in the top bar stores the whole document under a name, with version history: you can go back to how it was. Everything stays on your machine.