holpxay n. [hol.ˈp’aj] number.

A notepad calculator where math just works. Type expressions, see results instantly.

Features

Arithmetic & Numbers

  • Basic math with +, -, *, /, ^, !, mod, and parentheses
  • Alternative operators: ×, ·, ÷
  • Percentages (50%, 200 * 15%), scientific notation, underscore separators
  • Hex (0xFF), binary (0b1010), octal (0o777), arbitrary bases (ABC base 16)
  • Built-in constants (pi, e, phi) and functions (sin, cos, sqrt, log, round, abs, random, …)

Units & Conversions

  • 200+ built-in units across length, mass, time, temperature, data, energy, power, pressure, frequency, angle, and more
  • Automatic cross-unit arithmetic: 5 km + 500 m, 100 ft^2 + 5 m^2
  • Derived units: 60 km/h, 9.8 m/s^2, 50 watts per square meter
  • Composite values: 5 ft 7 in, 2 hr 30 min 15 sec
  • Flexible conversion syntax: to, in, as, ->, →
  • Composite conversion targets: 171 cm to ft in
  • User-defined units: 150 apples / 30 apples/day

Date & Time

  • Date literals, relative dates (today, 3 days ago, 2 weeks from now)
  • Time values in 12h/24h format
  • Date arithmetic: now + 100 days, 2038 Jan 19 - today
  • Timezone conversions: 18:30 Tokyo, now in London, UTC offsets
  • Unix timestamps: 1700000000 unix
  • Property extraction: today to weekday, now to hour

Currency

  • ISO codes, symbols, and names with live exchange rates
  • Arithmetic and conversion: 100 USD + 50 EUR, 100 USD to JPY
  • Derived currency units: 150 USD/person * 4 person

Variables & Conditionals

  • Variable assignment: price = 49.99
  • Boolean expressions: comparisons, &&, ||, !
  • Conditionals: if 5 > 3 then 100 else 200
  • Bitwise operations: &, |, xor, ~, <<, >>

Output Formats

  • Number bases (to hex, to binary), fractions, scientific notation, percentages, ordinals
  • Precision control: pi to 2 decimals, 12345 to 3 sig figs
  • Date/time formats: ISO 8601, RFC 2822, RFC 9557, unix timestamps

Editor

  • Markdown headings and inline comments (#)
  • GFM syntax highlighting alongside live calculations
  • Customizable themes, fonts, number/date formatting, and more

Trivia

This app is 70% written by Claude Opus 4.6 with constant human supervision and intervension.

Initally it was planned to be written 90+% by Claude, but I had to jump in and change the direction of deployment mid-way due to the way it has complicated the architecture.