If you spend half your week translating a vendor drawing from inches into millimeters, or a US datasheet from psi into bar, an engineering unit converter app that lives in a notepad saves the back-and-forth between a calculator, a search bar, and a sticky note. This page is for mechanical, electrical, and process engineers who want one place to write the conversion, label it, and have the math follow you when the numbers change.
Why an Engineering Unit Converter App Beats a Tab Full of Search Queries
A search-engine conversion answers one question at a time. You type 12 in to mm, you get 304.8, and you copy it into the spec. Five minutes later the drawing changes, and you start the loop again: only this time you cannot remember whether the previous figure was for the plate length or the hole spacing. The cycle eats the morning.
A notepad calculator flips the work order. You write the parts of the problem you actually know (Plate length: 18 in to mm, Hole spacing: 1.5 in to mm, and Bolt circle: 4.25 in to mm), and the millimeter figures appear beside the lines. Change one input and its result updates without you touching another line. The labels stay attached to the numbers, so when the project lead asks why the bolt circle moved by 3 mm, the answer is on screen, not buried in your call history.
Two honest competitors do parts of this well. Soulver is a polished notepad calculator on Apple hardware and worth considering if you stay on Apple devices. Calca is the markdown veteran for engineers who want symbolic algebra in the same document. Competitor platforms were checked against official sources in October 2026. Notes Calculator is the option that runs on Mac, Windows, Linux, Android, iOS, and the web with cross-device sync on the Lifetime plan. That matters once the laptop on your desk is not the device you take to the test bench. Try it free at app.notescalculator.com, and bookmark the unit-conversion docs for the full list of supported units and aliases.
Reading a US Drawing on a Metric Shop Floor
Open a new note and type the drawing the way you would dictate it.
| Note | Result |
|---|---|
| // dimensions arrived in inches, the shop runs in mm | |
| Plate length: 18 in to mm | 457.20 mm |
| Plate width: 12 in to mm | 304.80 mm |
| Hole spacing: 1.5 in to mm | 38.10 mm |
| Fillet: 0.25 in to mm | 6.35 mm |
Each line resolves to the metric figure beside the input, so nobody on the shop floor re-keys a number. When the customer revises the plate length to 20 in, you change one number on one line. The comment at the top with // is just a note for the next-day version of you; comments do not affect the math.
If the conversion direction flips next week for a metric drawing going to a US partner, replace to mm with to in on each line. The notepad does not care which way the math runs.
Temperature, Mass, and Process Specs Without Switching Tools
Process engineers and materials people end up reading specs across °C and °F all day long. Lithium-ion shipping windows, solder reflow profiles, kiln cure temperatures: they all show up in whichever units the original lab used.
| Note | Result |
|---|---|
| // SAC305 reflow profile shared with a US partner | |
| Preheat low: 150 °C to °F | 302 °F |
| Preheat high: 200 °C to °F | 392 °F |
| Peak: 245 °C to °F | 473 °F |
The converted figures land next to the source line, so the operator setting up the oven sees both columns side by side. The same approach works for mass (5.2 kg to lb), volume (2 US gallons to liters), and time (720 hours to days). See the units overview in the docs for the full grammar.
For pressure, datasheets are the worst offender: the same valve appears in psi on the US site and bar on the EU site, and you spend an embarrassing minute working out whether 150 psi clears a 10 bar regulator.
| Note | Result |
|---|---|
| // tank rating versus a metric pressure transmitter range | |
| rated = 150 psi | 150 psi |
| sensor max = 16 bar | 16 bar |
| rated to bar | 10.34 bar |
| sensor max to psi | 232.06 psi |
Now the comparison is one glance instead of one browser tab. If upgrade-only features such as unlimited notes and multi-device sync matter for your team, the pricing page lists the lifetime upgrade in one place.
Tolerance Stack-Ups and Mixed-Unit Material Math
A stack-up calculation lives in your head until somebody asks for the worst case. Putting it in a labeled notepad makes the worst case obvious, and lets the next engineer audit it without opening your CAD model.
| Note | Result |
|---|---|
| base length = 250 mm | 250 mm |
| spacer = 12.5 mm | 12.50 mm |
| washer thickness = 1.6 mm | 1.60 mm |
| nominal stack = base length + spacer + washer thickness | 264.10 mm |
| nominal stack to in | 10.40 in |
Two small things worth knowing. First, variable names can contain spaces: washer thickness is one variable, not washer * thickness. Second, you can mix raw figures with conversions on the same line; nominal stack to in reads back the way an engineer would speak it. The Totals and Subtotals docs cover the related total, average, and previous keywords if you want to stack several sub-assemblies on one page.
For procurement math the same notepad covers vendor lead times, even when the quote mixes weeks, hours, and minutes:
| Note | Result |
|---|---|
| // converting a quote that mixes weeks, hours, and minutes | |
| Prototype lead: 6 weeks to days | 42 days |
| Qualification: 720 hours to days | 30 days |
| Travel allowance: 180 minutes to hours | 3 hours |
Roll several of these up at the bottom of the page with the total keyword and you have a single figure to drop into the project plan, with the source figures still visible above.
To combine unit conversions with stress, electrical power and thermal expansion, see the calculator for engineers.
Related Use Cases
If your work crosses into invoicing or international travel as well as the bench, two sibling notepads cover the adjacent jobs:
- The freelance invoice calculator workflow for billable hours, VAT, and discounted retainers.
- The multi currency calculator for travelers for trip budgets that mix USD, EUR, JPY, and crypto.
Both use the same syntax: variables with spaces, to for conversions, and total at the bottom. The muscle memory carries across.
That is the whole pitch. An engineering unit converter app, built into a notepad, that survives the messy reality of real engineering work: drawings in inches, specs in psi, vendors in weeks, and a partner on the other side of the world who insists on °F. Try Notes Calculator free at app.notescalculator.com and upgrade to Lifetime to keep one note per project for the rest of the quarter.