Vernier is a unit converter modelled after the precision instrument that gave it its name — the auxiliary scale invented by Pierre Vernier in 1631 that turned ordinary rulers and calipers into tools accurate to a tenth of a unit. Calipers, micrometers, sextants, and theodolites have used vernier scales for nearly four hundred years. This app keeps that idea on a phone: type a value, choose a unit, and the converted result appears with the precision you've asked for, no surprises and no rounding shortcuts.

Pick a category from the row at the top, type a value into the FROM card, tap the unit you want to convert into, and the answer lands in the TO card formatted exactly to your chosen number of decimal places. The whole interface is built like a drafting page — deep blueprint cyan with warm cream "ink", drafting orange accents, dimension marks, and the right-angle corner brackets you'd find on architectural plans.

Ten measurement categories cover the conversions you actually reach for:
• Length — meter, kilometer, centimeter, millimeter, inch, foot, yard, mile, nautical mile
• Mass — kilogram, gram, milligram, metric ton, pound, ounce, stone
• Temperature — Celsius, Fahrenheit, Kelvin
• Volume — liter, milliliter, US/UK gallon, US fluid ounce, cup, pint, quart, cubic meter, cubic foot
• Area — square meter, square kilometer, square mile, square foot, square inch, hectare, acre
• Speed — m/s, km/h, mph, knots, ft/s
• Time — millisecond, second, minute, hour, day, week, year
• Data — byte, KB, MB, GB, TB, PB (binary, base-1024)
• Energy — joule, kilojoule, calorie, kilocalorie, watt-hour, kilowatt-hour, BTU
• Pressure — pascal, kilopascal, bar, atmosphere, PSI, mmHg, torr

Saved conversions plot into the Archive — grouped by day, every entry shown with the source value, converted result, units, and category, all framed by the same architectural corner marks. Two summary cards across the top track your conversion count and the category you reach for most. Long-press any row to remove it.

Settings give you proper engineering control:
• Decimal places, 0 to 8 — choose your precision
• Group thousands as 1,234,567 or run them flat
• Scientific notation for very large or very small results (1.23e+8) — useful at any scale
• Default category to open the app in
• Haptic feedback on chip and button presses
• Two themes: a deep blueprint mode with warm cream ink, and a vellum drafting paper mode with deep navy ink — both keep the same instrument character

No accounts, no analytics, no syncing of any kind. Vernier runs entirely on your phone — your conversions and settings never leave the device.

The original vernier turned a coarse scale into a precise one. Vernier the app does the same for everyday measurements.