GNU Units

this page seriously undersells the versatility and utility of the units program

how long will my laptop take to charge at its current rate of charging?

    You have: (22.8 Wh - 16.8 Wh)/7.4W
    You want: time
            48 min + 38.918919 sec

how long will a 2000mAh 18650 cell take to discharge at 2.5 watts, using a nominal voltage of 3.7 volts?

    You have: 3.7 V 2 amp hour / 2.5 watt
    You want: time
            2 hr + 57 min + 36 sec

what energy density is that, so i can compare it to the volume needed for other forms of energy storage?

    You have: 3.7 V 2 amp hour / circlearea(half 18 mm) 65 mm
    You want: MJ/ℓ
            * 1.6105936
            / 0.62088909

what's the specific energy of stoichiometrically mixed oxyhydrogen fuel?

    You have: 44000 J/mol / ((2 hydrogen + oxygen)g/mol)
    You want: MJ/kg
            * 2.4423711
            / 0.40943818

okay but how much volume? say at atmospheric pressure?

    You have: 3 mol gasconstant tempC(20) / 1 atm
    You want: l
            * 72.165351
            / 0.013857066

so that's how much energy density?

    You have: 44kJ/_
    You want: J/l
            * 609.71089
            / 0.0016401216

(i may be off by a factor of 2 here)

how much energy can this capacitor hold?

    You have: half (10V)**2 47 uF
    You want: mJ
            * 2.35
            / 0.42553191

how much energy density is that?

    You have: half (10V)**2 47 μF / 15mm circlearea(3mm)
    You want: J/ℓ
            * 5.5409499
            / 0.18047447

how thick of a cable do i need to support me in a lightweight fabric-sling chair (or, from a different point of view, to pose a risk of accidental strangulation)? suppose its tensile strength is 2.7 gigapascals

    You have: 120kg gravity / 2.7 GPa
    You want: mm2
            * 0.43585111
            / 2.2943615
    You have: _
    You want: circlearea
            0.00037247244 m
    You have: _
    You want: mm
            * 0.37247244
            / 2.6847624

note that this is the radius of the cable, not its diameter!

the datasheet says this 400×240 display uses 175 μW if all the pixels flip once per second and 60 μW for a static display. how much energy is that per pixel flip?

    You have: (175 uW - 50 uW) / 400 240 1 Hz
    You want: nJ
            * 1.3020833
            / 0.768

if i overclock it to 60 fps how much power will it use?

    You have: 60 Hz 400 240 1.3nJ
    You want: μW
            * 7488
            / 0.00013354701

and how many pixels is its diagonal?

    You have: 400**2+240**2
    You want: 
            Definition: 217600
    You have: _**.5
    You want: 
            Definition: 466.47615

what is the visual angle subtended by the sun as seen from earth?

    You have: 2 sunradius/sundist
    You want: milliradians
            * 9.3049358
            / 0.10746984
    You have: _
    You want: dms
            31 arcmin + 59.280781 arcsec

okay, how does that compare to the moon?

    You have: moonradius 2 / moondist
    You want: 
            Definition: 0.0090426639

on average the moon looks a little smaller, which is why annular eclipses are so common, but we can also calculate that total eclipses are possible because sometimes the moon looks bigger

    You have: moonradius 2 / moondist_min
    You want: 
            Definition: 0.0097530864

what percentage of this copper sulfate is actual copper?

    You have: copper / (copper + (sulfur + 4 oxygen))
    You want: %
            * 39.813395
            / 0.025117175

how fast can i write to this slc flash chip without wearing it out in 53 years, assuming perfect wear leveling and no write amplification?

    You have: 100 thousand 128 MiB/53 years
    You want: bytes/second
            * 8024.8943
            / 0.00012461223

how much fuel will this truck need to get across the country?

    You have: 4000 km / (6.5 miles/gallon)
    You want: l
            * 1447.4744
            / 0.00069085852

how much is that per kilogram of lettuce or sodium lauryl sulfate or whatever?

    You have: _/28 tonnes
    You want: ml/kg
            * 51.695513
            / 0.019344039

okay, but how much energy is 52 mℓ of diesel per kg of lettuce?

    You have: _ 38.6 kJ/l
    You want: kJ/kg
            * 1.9954468
            / 0.5011409

how much data can i transfer overnight during unmetered hours on a 2400-baud modem?

    You have: 8 hours 2400 bps
    You want: MB
            * 8.64
            / 0.11574074

how much power does the earth receive from the sun, assuming a solar constant of 1400 W/m²?

    You have: 1400 W/m**2 * circlearea(earthradius)
    You want: petawatts
            * 178.52313
            / 0.0056015152

what would the equilibrium temperature of an object be if it were illuminated at that brightness and had a flat emission spectrum?

    You have: (1400 W/m**2 / stefanboltzmann)**(1/4)
    You want: tempC
            123.24583

how about here in buenos aires at the winter solstice? first, what angle is the sun at anyway? we're at 34°36’ south, and the sun's latitude at the solstice is 23°26’

    You have: 34° + 36' + 23° + 26'
    You want: dms
            58 deg + 2 arcmin

so that reduces the peak insolation to how much? here underneath the atmosphere we only get 1kW/m²

    You have: cos(_) 1000 W/m^2
    You want: W/m^2
            * 529.4258
            / 0.0018888388

and that would be what temperature in equilibrium?

    You have: (_/stefanboltzmann)**(1/4)
    You want: tempC
            37.698189

(integrating the sun's angle over the course of the day as the earth rotates is sadly beyond its capacities)

how much money could a sensible heat storage reservoir of 15 kg of water save me over 16 years? say power rates go down to only 2.5¢/kWh because of solar

    You have: 1500 kcal/day * 16 years * 2.5 cents/kWh
    You want: 
            Definition: 254.69556 US$

what's the surface area of a 300mm × 400mm × 150mm backpack? like how much cloth?

    You have: 2 (300mm 400mm + 400mm 150mm + 150mm 300mm)
    You want: 
            Definition: 0.45 m^2

okay but in cm²

    You have: _
    You want: cm2
            * 4500
            / 0.00022222222

what's the electrical impedance of a 1000 μF cap at an audio highpass frequency of 20Hz?

    You have: 1/(2 pi 20 Hz 1000 uF)
    You want: ohms
            * 7.9577472
            / 0.12566371

what's the time constant of 100 pF (roughly the smallest capacitance you can get in a macroscopic circuit with any degree of precision) and 1 MΩ?

    You have: 100 pF 1 megohm
    You want: ms
            * 0.1
            / 10

okay. so how long will an 0.1μF cap take to discharge through a 100kΩ resistor from 5 volts down to a 1.3 volt threshold?

    You have: ln(5V/1.3V) .1 uF 100kilohm
    You want: ms
            * 13.470736
            / 0.074234991

how many bits of precision does a linear adc need to be able to measure a difference of 1.8 millivolts if 1.5 volts is full-scale?

    You have: log(1.8mV/1.5V)/log(2)
    You want: 
            Definition: -9.7027499

if this oxygen absorber contains 7 grams of iron which oxidizes to Fe₂O₃, how much air can it remove all the oxygen from? air is 21% oxygen by volume (and roughly by mass) and weighs 1.2 grams per liter

    You have: 3 oxygen / 2 iron * 7 g
    You want: g
            * 3.0082138
            / 0.33242318
    You have: _/21%/(1.2g/ℓ)
    You want: ℓ
            * 11.937356
            / 0.083770641

i've lost 7 kg over the last two months; how much of a caloric deficit does that represent in my diet?

    You have: 7 kg 3500kcal/pound / 2 months
    You want: kcal/day
            * 887.30034
            / 0.0011270141

if you were to spread the moon evenly over russia, how deep would it be?

    You have: spherevol(moonradius) / area_russia
    You want: 
            Definition: 1286134.4 m

how big is nigeria compared to massachusetts?

    You have: area_nigeria/area_massachusetts
    You want: 
            Definition: 33.793093

how many ounces of platinum is a ton of oil worth at 40 dollars per megawatt hour?

    You have: tonoil 40 dollars/MWh
    You want: platinumounce
            * 0.58368883
            / 1.7132416

or in grams?

    You have: tonoil 40 dollars/MWh / platinumprice
    You want: g
            * 18.154752
            / 0.055081997

Worth looking at: https://github.com/aurora-opensource/au It's a C++ library for handling physical quantities (and subsequent unit conversions)

Is there a spreadsheet that supports units in its cells and GNU Units in formulas? Seems like that would be an amazing open source tool.

I get it that some suggest the graph is a complicated/over-engineer while other suggest just stick to SI. Well, my rationale is that not everything has a SI conversion, and simple SI/hub/spoke would need you to maintain a unit -> SI (or base unit like USD in currency conversion).

What I like about the graph solution is that you don’t have to use SI or base unit at all! If your graph has km -> m, now you add (updating your graph) for m -> feet, you could traverse from km -> feet. Later on, you can add nautical mile -> feet, using this graph you could basically get km -> nautical mile if you need to.

F# also has units integrated into the type system. Seems like a great feature though I've never used a language with it.