Expr
Expr app icon

Beautiful mathematical notes.

Write a line, get a result, typeset as you go. Expr is a notebook for the math in your day — from a quick conversion to a back-of-the-envelope model.

Coming soon to the App Store For Mac, iPad and iPhone
Expr on the Mac: a note estimating what it takes to train Llama 3 8B, with typeset formulas and results

Every line is live

Change a value. Watch the note follow.

Lines are evaluated top to bottom and redrawn as real math, fractions and exponents included. Edit anything and every line below it recalculates. Give a number a range and it becomes a slider.

Rotating a point

θ ∈ [0°, 360°]
R = [cos θ, −sin θ; sin θ, cos θ]
R @ [2, 1]

A ball of radius r

radius ∈ [1 cm, 20 cm]
surface = 4π · radius²
volume = 4/3 π · radius³ → L
Filled with water, that's about 4.9 cups.

What it understands

Math the way you'd write it down.

Vectors, matrices, numbers with units, money, plain words. Expr reads what you mean and shows its work.

Typeset, not printed

Every result is drawn as proper math, so a note reads like a page, not a terminal.

decay(t) = (1 + cos(π·t/T)) / 2

Vectors, matrices, colors

Swizzle, index and multiply. Colors convert between color spaces.

v.yxm[1][0]#ff8800

Sliders and plots

Turn a value into a slider and drag it. Plot a function and see its shape.

warmup = 3400 in 0..5000 plot lr on [0, steps]

Notes that build on notes

Keep your constants in one note and use them from any other. Change one, and the rest follow.

use c, h from Physical Constants

Units and currencies

Convert as you calculate, with exchange rates kept up to date.

180 cm in feet and inches5 ft 10.87 in 5 km at 10 km/h0.5 h

Math in words

Write it the way you'd say it. Rates, areas and percentages just work.

40 h at $20/h$800 10 ft x 12 ft120 ft²

Works with your agent

Ask for the math. Get a note you can check.

On the Mac, Expr comes with an MCP server. Your agent can write worked examples into your notes, read them back and move a slider to explore — and every line it adds stays marked until you've reviewed it.

Claude Desktop Claude Code Any MCP client

Set up from Settings ▸ Agents in one click.

The Expr note after the agent added a section, its lines marked as added by Claude A conversation asking Claude to extend an Expr note

Mac, iPad and iPhone

Wherever you do your thinking.

Expr on iPad: rotating a point with a matrix, and attention in miniature Expr on iPad: counting GPT-2's parameters, with the inspector open Expr on iPad: where a Gaussian splat's bytes go, with plots Expr on iPad: is a 2-point benchmark gain real? Expr on iPad: 143,199 tweets a second
Expr on iPhone: a learning-rate schedule with a slider and a plot Expr on iPhone: a damped sine explored in the graph viewer Expr on iPhone: typing a fine-tune budget, with suggestions above the keyboard Expr on iPhone: the sinc function, with its graph in the inspector

Synced through your iCloud

Turn on sync and your notes follow you. Edits never wait for the network.

Shared with the people you choose

Invite someone to a note and you both edit the same page.

Exported when you need it

Send a note as PDF, HTML or text from the Mac, with every value formatted.

Free

Everything, for everyone.

No ads, no subscription, nothing locked. If Expr earns a place in your day, there's an optional tip jar in Settings.

Private

Your notes stay yours.

No account, no analytics, no tracking. Notes live on your devices and, if you turn on sync, in your own iCloud. Gofio never sees them.

Read the privacy notice →

It all adds up.

Coming soon to the App Store

Made by Gofio.