I want to be able to press a keyboard combination, start typing a mathematical expression that includes units and slightly advanced math (not just a four-function calculator), and get a result immediately, in units that I specify, that I can copy and paste.

  • Currently I open Firefox and press Ctrl+K, type in the search box, and it usually gives me a result in the drop-down from Google Calculator. It doesn't always, though, so I press "=" at the end, wait for a result, remove the equals, wait for a result, realize it doesn't understand the way I typed a unit, open the result in a new tab, etc. it sucks.
  • Wolfram Alpha is smarter, but very much slower, and the output is all images, not text, and I don't have a quick widget for it, if such a thing could even exist.
  • GNU units has a ton of units, which is great, and I can define my own units, which is great, but they have to be written in specific, unintuitive ways, it doesn't handle much advanced math, and I'd need to open a terminal, start units, etc. I hate the command line.
  • I wasted a lot of time trying to make front-ends for units in Deskbar and Launchy, but I'm not a real coder and I don't use either of those anymore.

Any other solutions or enhancements of these?

(Cross-platform is best)

The ideal unit-aware calculator would be able to handle all of these:

  • 7 nV/sqrt(Hz) ⋅ sqrt(20000 Hz - 20 Hz) → μV = 0.99 μV
  • sqrt(4*k*25 °C * 1 kΩ * (20 kHz - 20 Hz)) = 0.57357 μV
  • 3.5 billion kilowatt-hours per year = 399.5 MW
  • 60 billion kWh per year = 6.849 GW
  • 1/sqrt(2 mH * 2 nF) = 500 kHz
  • 1/(2*pi*10 kΩ * 22 µF) = 0.7234 Hz
  • 1/(2·π·100 Hz·30pF) → ohm = 53.05 MΩ
  • −10 dBV + 4 dB + 10 dB − 12 dB → dBu = -5.782 dBu
  • 94 dBSPL → Pa = 1.00237 Pa
  • 54 inches + 2 feet → cm = 198.1 cm
  • 12 V ÷ 141.5 µA = 84.8 kΩ
  • 16 bit * 44.1 kHz * 2 → kbit/s = 1411 kbit/s
  • 1 hundred V / 5 ohms = 20 A
  • furlong per fortnight → cm/minute = 0.9979
  • attoparsec/microfortnight → in/second = 1.004
  • 1 ft / speed of sound = 0.89576 ms
  • 1
    What is the level of complexity of the problems you are trying to solve?
    – spowers
    Apr 26 '10 at 0:33
  • What did you try to script that a combination of bc -l and units couldn't do? Apr 26 '10 at 0:38
  • 1
    Can you give an example of using bc -l and units together? I didn't know that was possible.
    – endolith
    Apr 26 '10 at 15:04
  • @honk: sounds like something you should post in an answer (y'know, with an example). @endolith: use @ for comment notifications so the user gets notified when you reply to them. Apr 26 '10 at 15:29
  • @endolith: Actually my comment was too fast, bc doesn't gain you anything if you do not plan to define your own functions . The trigonometric, exponential and logarithmic function are all already defined in units (only Bessel functions are missing). Apr 26 '10 at 19:48

I'm very impressed with Qalculate!.

Qalculate! screenshot

  • +1 So am I, although my user interface (the Qalculate plasmoid for KDE) doesn't offer a way to express a result in specified units. I work around it by dividing my result by the units I want. I forget whether the full interface, if it even still exists, can do that. (Just so it's clear: even with the UI limitations, it's still my favorite calculator for anything I don't need Mathematica for)
    – David Z
    Apr 26 '10 at 2:26
  • I don't see standard temperature, speed of sound, and I don't understand how it handles dB, but I am also very impressed. I wish the answers appeared after a short delay without pressing Enter, too. But these are all things I can complain about. :)
    – endolith
    Apr 27 '10 at 23:05
  • 1
    @endolith: You can go into Edit>Manage Variables>Physical Constants and Edit>Manage Units and add your own or edit existing entries. I've never seen a calculator complete a calculation after a timeout. One thing I would like to see in this program is history navigation using arrow keys or scroll-and-click with the mouse instead of having to copy-paste from the history. Apr 28 '10 at 4:28
  • Google Calculator in the Firefox search bar completes as I type, though I have to prod it sometimes by typing an equals sign. The thing I wrote for Launchy also displays the result as you type, though it was slow because of the way it accesses Units.
    – endolith
    Apr 30 '10 at 18:54
  • @DavidZ: (meanwhile) Qualculate offers the "to" command to convert units: "2m to cm" will express the result in cm. Note that (lib)qalculate comes with qalc, a command-line version that uses readline and can recall history with the arrow keys or Ctrl-R (and other neat CLI tricks provided by readline, like Tab completion, copy/paste, undo, etc., also found in e.g. Bash).
    – AstroFloyd
    Apr 6 '17 at 17:24

insect has both web- and terminal-based versions. It does support parsing, handling and conversion of physical units, for example:

>>> 2min + 30s

   = 2.5min

>>> 40000km / speedOfLight -> ms

   = 133.426ms

>>> 6Mbit/s * 1.5h -> GB

   = 4.05GB

>>> 2J·s + 3W

  Unification error:

    Cannot unify unit W (base units: kg·m²·s⁻³)
            with unit J·s (base units: kg·m²·s⁻¹)

For a easily scriptable solution you might want to have a look a the sympy python module.

$ cat conv.py

#!/usr/bin/env python
import sys
from sympy.physics.units import *
from sympy.printing.pretty.pretty import pprint
from sympy.abc import x, y, z
from sympy import *

# adding extra units is easy
parsec = 3.26163626*ly

if __name__ == '__main__':
  s_input = sys.argv[1]
  s_unit  = sys.argv[2]
  input = eval(s_input)  # input string
  unit  = eval(s_unit)   # output unit

  print 'Converting:'
  print str((input/unit).evalf()) +' '+ s_unit

which gives e.g. for some moderately ugly expression converted to mm:

$./conv.py 'tanh(3*m/(2*m))*sinh(60*deg)*1*parsec' 'mm'

                           \3 /

3.48955431541888e+19 mm

Of course this is really studpid code that does no checking thet unit compatibility of the input and output, so you might end up extra units in the result.

$./conv.py 'c' 'parsec'


9.71539598165644e-9/s parsec
  • Also see launchpad.net/python-quantities
    – endolith
    Apr 27 '10 at 11:56
  • @endolith: It all really only depends on what kind of math you want. The basics are in units, but obviously it doesn't have symbolic algebra or higher math. Apr 27 '10 at 13:41

I mostly share your ideas about an ideal calculator and the disadvantages of using GNU Units and Wolfram and rolled my own online calculator which I put online at PhyCalc.com. It does its own unit algebra for SI units and uses GNU Units for conversion to other units, or as a fallback when the expression itself contains non-SI units.

Its aims to interpret Unicode input where it makes sense. And tries to be smart about guessing the exact quantity and corresponding unit. It automatically inserts SI-prefixes. (much like my trusty old Casio pocket calculator does in "ENG" mode)

Some comments about your "ideal calculator" inputs and responses:

sqrt(4*k*25 °C * 1 kΩ * (20 kHz - 20 Hz)) = 0.57357 μV

Its not exactly accurate to automatically assume °C to be an absolute temperature. Also k is not exactly an unambiguous symbol for the Boltzmann constant. However this should give you the required answer:

sqrt(4*1.38064852e-23J/K*(273.15K+25 °C) * 1 kΩ * (20 kHz - 20 Hz)) in μV

These fail in GNU Units:

3.5 billion kilowatt-hours per year

1 ft / speed of sound

GNU Units alternatives could be:

3.5 billion kWh per year
1 ft / mach in ms

1/(2·π·100 Hz·30pF) → ohm

You can leave out the "→ ohm" part. That is what I meant with "being smart about guessing the quantity" :-).

−10 dBV + 4 dB + 10 dB − 12 dB → dBu

GNU Units has some dB conversion, but it is defined as 10^(x/10) which is a scalar that it will not add to a voltage, where you assume that the calculator should recognize that it should be scaling Voltage instead of Power and should apply scaling of 10^(x/20) instead. That would be very cool, but is a bridge too far for gnu units I'm afraid. This will at least give you the correct answer using GNU Units:

 1V*dB(-10)^0.5*dB(4)^0.5*dB(10)^0.5*dB(-12)^0.5 → dBu

But I can see why you may find that un-intuitive.

Otherwise I think PhyCalc.com may come pretty close to your ideal calculator.

To have fast access to it, I map my dedicated calculator keyboard key to it by adding to the following string (REG_SZ) name/value pair in the Windows registry:


Wolfram Alpha is capable of crunching math and doesn't require you to download or install anything.

Check here for how to use its math functions.

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