napkin.
Menu

+ A LIVING ENGINEERING SKETCHBOOK

Sketch.
Calculate.
Question.
Build.

The sketchbook for things that have to work.

A drawing, a few equations, and a question worth answering. Keep the dimensions, assumptions, and checks on the same page. Work through the engineering with your agent.

Get the plugin ↗Try it here ↓
cantilever.napkinLIVE STUDY / 001
F = 20 Nδ = 0.72 mmL = 300 mmDEFLECTION SHOWN ×30
TIP DEFLECTION 0.72 mm✓ Within 2 mm limit
— Deflected╌ UnloadedEdit the model ↗

TRY THIS WITH YOUR AGENTUse Napkin to study an aluminium cantilever. Compare a 300 mm and 500 mm reach under a 20 N end load. Check tip deflection against a 2 mm limit and show the assumptions.

Get Napkin Codex · Claude Code · MCP +

Use the local plugin in Codex or Claude Code, or connect a compatible MCP host to the hosted server.

Installation & connection guide ↗Instructions for agents ↗Source on GitHub ↗

ONE QUESTION. THREE SCENARIOS.

How much
will it bend?

An aluminium beam. Fixed at one end. A load at the other. Enough physics to make the next decision.

Open the working study ↓
δ = FL³ / 3EIEvery symbol has a unit.
Every result has a source.
01 / SKETCH

Start with
the question.

Will this bracket hold? How far can it reach? Describe the parts, loads, and limits to your agent. Build just enough model to test the idea.

THE QUESTION“Can a 300 mm aluminium cantilever carry 20 N with less than 2 mm of tip deflection?”
02 / CALCULATE

Make the
reasoning visible.

A readable .napkin file connects quantities, geometry, equations, and checks. Change an input and the study recalculates. Inspect the source whenever you want to understand a number.

length = 300 mm in 100..500
load = 20 N

check tip deflection: deflection <= 2 mm
Meet the Napkin language ↗
03 / COMPARE

See the change.
Keep the context.

More reach means more bending. A deeper section brings the deflection back down. Compare alternatives against the original, with arrows, callouts, and the unloaded shape still in view.

300 mm reach · 12 mm depth0.72 mm ✓
500 mm reach · 12 mm depth3.35 mm ×
500 mm reach · 20 mm depth0.72 mm ✓

Calculated from the same example source. Fixed support, end load, small deflections; shear and self-weight omitted.

A FEW LINES OF ENGINEERING

The model, written down.

Dimensions, equations, and a limit.
One readable .napkin file.

cantilever.napkin / editable
✓ checks pass
Cantilever: 0.72 mm tip deflection, displacement shown at 30 times actual size.CantileverF = 20 Nδ = 0.72 mmL = 300 mm
Current, unloadedDeflected ×30
XZ / DEFLECTION EXAGGERATED ×30FIXED END ALIGNED

Cantilever selected · related inputs are marked.

tip deflection0.72 mm✓ passes

BASELINE INPUTS

CALCULATED

inertia3600 mm^4
deflection0.72464 mm

Edits change the baseline source.

Calculated from your source · not saved
Baseline · 300 mm reach · 12 mm depth
tip deflection0.72 mm ✓ passes
more reach · 500 mm reach · 12 mm depth
tip deflection3.35 mm × fails
deeper section · 500 mm reach · 20 mm depth
tip deflection0.72 mm ✓ passes

Edit a number, equation, or check. Results update automatically. Fixed support, end load, small deflections; shear and self-weight omitted.

Read the language reference ↗

04 / EXPLORE

A good question
is a starting point.

527 editable calculation examples across 51 engineering topics. Each page includes a scoped .napkin model with inputs, equations, checks, and a comparison scenario.

GEARS

Planetary gear system

sun, ring and planet teeth, held member, input member → ratio, member speeds, geometry feasibility

Open the calculator ↗STRUCTURES

Beam deflection

section stiffness, span, supports, load pattern → maximum deflection, slope, bending stress

Open the calculator ↗MOTORS

Servo sizing

load motion, inertia, gearing, duty → peak and RMS torque, speed demand

Open the calculator ↗
All engineering calculations ↗Historical sketches & examples ↗Guides & workflows ↗

05 / INSIDE NAPKIN

One study.
Connected pieces.

The same engineering model behind the source, the drawing, and the conversation.

01 / SOURCE

Engineering you can read.

Inputs with units, equations, parts, checks, and scenarios in a small text file. Edit it directly and get errors where they occur.

Read the language reference ↗02 / STUDY

A model you can question.

The source becomes a Study: linked quantities and parts, evaluated scenarios, and explicit checks. Keep the assumptions with the result.

Explore the working example ↗03 / AGENT TOOLS

A conversation with consequences.

The local plugin bundles an MCP server, a design workflow skill, and an interactive widget. Your agent can create, inspect, and revise a study.

Connect your agent ↗04 / VIEWS

Drawings that answer back.

Linked drawings, dimensions, and calculated overlays. Use the MCP App in compatible hosts, or the core and React APIs in your own application.

Explore the source & APIs ↗

06 / A FEW GOOD QUESTIONS

Before you
start sketching.

What is Napkin for?

Early engineering decisions: work out a section depth, check a load, compare a motor, or estimate an energy budget. Sketch models keep the dominant effects and state the simplifications. Read why we’re building it ↗

Can I try it without connecting an agent?

Yes. The live study above runs on this page. Change inputs or edit the source, compare scenarios, and inspect the checks. Page edits are not saved.

Where do my studies live?

The local plugin saves studies on your computer, outside the plugin cache. The hosted MCP connection uses the account you sign in with. The landing page example is a temporary study.

Does this replace CAD or simulation?

Napkin focuses on first-order analytical models and linked drawings. Use the results to inform CAD, detailed analysis, and physical testing. General parametric CAD import, shape recognition from images, and dynamic simulation are not currently shipped.

Can I edit every calculation example?

Every calculator page includes an editable .napkin worked example. Each states the particular question it answers; the broader topic may require more data and equations. Start with the cantilever here or the planetary gear calculator.

How do I use Napkin with my agent?

Install the local plugin for Codex or Claude Code, or use the hosted MCP connection in a compatible host. Follow the setup guide ↗

Every good design starts somewhere.

Start on a napkin.

Get the plugin ↗

Sketch the model. Run the study. Get the size right.