NapkinCalc

engineering calc / unit-aware / live worksheet

Hand calcs that lose the units
carry every unit.

NapkinCalc is a live worksheet for engineers and students. Write the math the way you would on paper — with real units — and it solves as you type, in dependency order, every time you change a number. Free, in your browser. No install.

Open NapkinCalc — it's free Try the live demo

opens .mcdx & .xmcd / no account to start / 200+ units built in

app.napkincalc.com / cantilever-deflection
The NapkinCalc editor solving a cantilever beam deflection check — unit-carried inputs, a live-evaluated δ_max of 7.1639 mm, and a passing design check
Close-up of the worksheet: δ_max = w·L⁴ / (8·E·I) evaluates live to 7.1639 mm and the design check reads pass
computed by the engine — not a mock-up
  • 200+ units & constants
  • live recalculation
  • 0 syntax to memorize
  • .mcdx Mathcad import
  • PDF export to hand in

the problem

A wrong unit ships as a real-looking number.

Spreadsheets and scratch paper don't know a newton from a pascal. The dimensions live in your head — until one slips, and there's nothing to catch it.

the old way

  • Units live in your head, not in the math
  • A unit slip looks just like a right answer
  • Change one input → re-check the whole sheet by hand
  • The "deliverable" is a screenshot of a spreadsheet

with napkincalc

  • Every quantity carries its dimensions
  • A slip comes back as an error you see
  • Change a number, the whole sheet re-derives live
  • One click → a page-exact PDF a reviewer trusts

units are the feature

Units aren't a comment. They're the math.

A length is metres, a force is newtons, a stress is pascals — and NapkinCalc treats them that way. Quantities carry their dimensions through every operation, so a unit slip becomes an error you see, not a number you ship.

12 kN/m×(3 m)⁴÷8·E·I=7.1639 mm//guess the unit

kN/m times m⁴ over GPa·mm⁴ is a length — so the answer arrives as one. Ask for it in mm, inches or light-seconds; a slip comes back as an error, not a guess.

no demo-reel math

This worksheet is real.

Not a screenshot — a worksheet computing in your browser right now: a steel cantilever, a uniform load, and a serviceability check to L/250.

  • Inputs carry their units — 12 kN/m, 84.8×10⁶ mm⁴ — straight into the algebra.
  • The tip deflection δ_max = 7.1639 mm falls out in the unit you asked for.
  • The design check δ_max < δ_allow reads ✓ pass — and flips the moment an input makes it untrue.

Try it: change w from 12 to 36 kN/m in the card and watch the sheet re-derive — δ_max becomes 21.5 mm, the check turns to ✗ fail, and the scoreboard in the corner tells on you. That's the point.

Try it on your own beam
cantilever-beam.ncalc ✓ 1/1 checks

Cantilever Beam — Deflection Check

Steel W310×39 cantilever carrying a uniform load. Serviceability check to L/250.

// this is live — edit the blue inputs

Inputs
L=L = m span
w=w = kN/m uniform load
Es=E_s = GPa modulus
Ix=I_x = ×10⁶ mm⁴ second moment
Deflection
δmax=w⋅L48⋅Es⋅Ix\delta_{max} = \dfrac{w \cdot L^4}{8 \cdot E_s \cdot I_x} = 7.1639 mm
δallow=L250\delta_{allow} = \dfrac{L}{250} = 12 mm
δmax<δallow\delta_{max} < \delta_{allow} = ✓ pass
client-side micro-engine — the real one does this for every cell
The full cantilever worksheet: headed sections, unit-carried inputs, fraction-rendered deflection formulas with live results, and a passing design check

one tool, all of it

From homework to design review.

Algebra, calculus, matrices, statistics — and the unit-carrying physics and engineering work they feed. Same worksheet, same keystrokes.

x=−b+b2−4ac2ax = \dfrac{-b + \sqrt{b^2 - 4ac}}{2a}

Algebra

Roots, exponents, logs and trig — typed the way the textbook prints them.

ddx ⁣(x3e2x),∑i=0ni2\dfrac{d}{dx}\!\left(x^3 e^{2x}\right), \quad \sum_{i=0}^{n} i^2

Calculus

Symbolic derivatives, finite sums and products — evaluated live in the sheet.

K−1,det⁡(K),eig(K)K^{-1}, \quad \det(K), \quad \mathrm{eig}(K)

Matrices

Determinants, inverses, eigenvalues and transposes straight on matrix cells.

f(x) = e⁻²ˣ·sin(5πx)

Plots

Plot any expression in x — range, sampling and the curve redraw live as inputs change.

Ek=12mv2=450 JE_k = \tfrac{1}{2} m v^2 = 450\ \mathrm{J}

Physics

Kinematics to thermo with units carried through every intermediate step.

σ=M⋅cI=86.2 MPa\sigma = \dfrac{M \cdot c}{I} = 86.2\ \mathrm{MPa}

Engineering

Sizing, stress and flow calcs with design checks that gate the result.

xˉ,σx,median(v)\bar{x}, \quad \sigma_x, \quad \mathrm{median}(v)

Statistics

Mean, deviation, variance and friends over whole data vectors at once.

z=3+4i,∣z∣=5,arg⁡(z)z = 3 + 4i, \quad |z| = 5, \quad \arg(z)

Complex numbers

Full complex arithmetic — magnitude, argument, conjugates, real and imaginary parts.

Re=ρ⋅v⋅Dμ,Ix, Sx, N ⁣PV\mathrm{Re} = \dfrac{\rho \cdot v \cdot D}{\mu}, \quad I_x,\ S_x,\ N\!PV

Function plugins coming soon

Domain packs you enable per worksheet — beams & sections, pipe flow, statistics, engineering economics.

grid view

Pages, not an endless scroll.

Flip the worksheet into grid view and it becomes real pages on a canvas — drag any cell anywhere, lay calculations out side by side, and see exactly where each page ends before you ever print.

  • Free 2-D placement — put the sketch next to the math that derives it.
  • Real page boundaries at the paper size you picked, with rulers and a minimap.
  • What you lay out is what exports — the PDF below is these pages, one for one.
The full NapkinCalc editor in grid view: outline panel, spatial page canvas showing the FRP vessel's hydrostatic stress page — formula, live plot of hoop stress vs fill height, passing checks — and the variables panel with every quantity's value
Cantilever Beam — Deflection Check
L=3 mL = 3\ \mathrm{m} = 3 m span
w=12 kNmw = 12\ \tfrac{\mathrm{kN}}{\mathrm{m}} = 12 kN/m uniform load
Es=200 GPaE_s = 200\ \mathrm{GPa} = 200 GPa modulus
Ix=84.8×106 mm4I_x = 84.8 \times 10^{6}\ \mathrm{mm}^4 = 8.48 × 10⁷ mm⁴ second moment
δmax=w⋅L48⋅Es⋅Ix\delta_{max} = \dfrac{w \cdot L^4}{8 \cdot E_s \cdot I_x} = 7.1639 mm tip deflection
δallow=L250\delta_{allow} = \dfrac{L}{250} = 12 mm serviceability limit
✓ PASS δmax<δallow\delta_{max} < \delta_{allow} deflection within L/250
Cantilever Beam — Deflection Check NapkinCalc

the deliverable

The PDF is the deliverable.

One click exports the worksheet exactly as you laid it out — page for page, fractions and all. No retyping into a report, no screenshots of a spreadsheet.

  • Page-exact: every worksheet page becomes exactly one PDF page, at the paper size you picked.
  • Real notation on paper — the same fractions, subscripts and units a reviewer would write by hand.
  • Checks print with their verdicts, so the document argues for itself in a design review.

coming from mathcad?

Your .mcdx files open here.

Drag a Mathcad worksheet — .mcdx or legacy .xmcd — into NapkinCalc and it converts right in your browser: definitions, units, headings and all. No upload, no licence server, no "export wizard".

Import a worksheet

why I built this

Hey, I'm Paco 👋 I'm an engineer, and I was tired of two bad options: a $700-a-year desktop tool with a licence server, or a spreadsheet that has no idea a force isn't a length. So I built NapkinCalc — calcs you can read like a worked solution, with the units carried through every step, that open in a tab and export to a PDF you'd hand to a reviewer.

There's a free tier to try it for real, and Pro is $49.99/yr during the beta — a price that locks in for as long as you stay. If it saves you one re-checked spreadsheet, it did its job, and I'd genuinely love to hear what's missing.

— Paco, founder · hello@napkincalc.com

simple pricing

Engineering math without the engineering-software bill.

Legacy Mathcad-style tools cost hundreds a year and want an install and a licence server. NapkinCalc opens in a tab.

Mathcad & the old desktop tools

$700/yr

Per-seat subscriptions, desktop installs and licence servers for math you do every day.

napkincalc

$49.99/yr

Free to start in the browser. Pro at beta pricing — units, live recalculation and PDF export, all in a tab.

Free

$0forever

Everything the math engine does, to try it for real.

  • 5 worksheets
  • Units, symbolic solving, matrices & plots
  • Mathcad .mcdx import
  • PDF export of the full solution
Start free
beta price

Pro

$129 $49.99/yr

or $8/mo · the beta rate locks in for as long as you stay.

  • Everything in Free, plus:
  • 100 worksheets
  • 1,000 symbols per worksheet
  • 30 images per worksheet, up to 500 KB each
Get Pro — $49.99/yr

Student with a school email? $29.99/yr.

Team

$3/seat / mo

Billed annually · 2 seats or more · one bill for the whole crew.

  • Pro for everyone on the team
  • Invite teammates by email
  • One central subscription & invoice
  • Add seats as the team grows
Start a team

Secure payment by Stripe · cancel anytime · no per-install licence server, ever.

before you ask

Questions, answered.

Do I have to learn a syntax?

No. You type math the way you would write it — assignments with :=, fractions, exponents, subscripts and units. The worksheet renders it as proper notation as you go.

How do units work?

Units are attached to values, not bolted on as text. Add a length in mm to one in m and it just works; multiply a force by a distance and you get a moment. Ask for the result in any compatible unit.

Can I bring my Mathcad worksheets?

Yes. Drop a .mcdx or .xmcd file into the editor and it converts in your browser — definitions, units and structure included. Nothing is uploaded anywhere to do it.

Who is it for?

Practising engineers who need auditable, unit-safe hand calcs, and students who want their homework to read like a worked solution instead of spreadsheet cells.

Is it free?

There's a free tier — 5 worksheets with the full math engine, units, import and PDF export — and you can start with no account. Pro lifts the limits at $49.99/yr (beta pricing, locks in), $8/mo, or $29.99/yr for students. Teams are $3/seat/mo billed annually.

do the math

Stop guessing the units.

Open a worksheet, define your first variable, and watch the answer fall out — carried units and all.

Open NapkinCalc — free Try the live demo