Guides
Articles approfondis sur les systèmes de mesure, les méthodes de conversion et les normes qui les sous-tendent.
Ce contenu n'est actuellement disponible qu'en anglais.
Metric vs Imperial: How the World Ended Up With Two Systems
The 1875 Metre Convention, the US Metric Act of 1866, and the 1893 Mendenhall Order that quietly redefined the American yard and pound in terms of metric standards.
Lire la suite →Temperature Conversion Pitfalls (and the Difference Between a Degree and an Interval)
Why a 10 degree C rise is 18 degrees F and not 50, what ITS-90 actually calibrates, and why engineers still use the Rankine scale.
Lire la suite →How Conversion Factors Are Derived
The SI base-unit chain behind every conversion factor on this site, why the 2019 redefinition inverted which quantities are exact, and a full worked derivation of the watt from a horsepower.
Lire la suite →Why Your 1 TB Drive Shows 931 GB
Decimal versus binary storage prefixes, the IEC standards that named them separately, and the exact arithmetic behind the missing 69 GB.
Lire la suite →What "kg CO2e" Actually Means
CO2 equivalence and GWP100, why methane's figure keeps changing across IPCC assessment reports, and why this site's own 0.445 kg CO2e/kWh grid factor is a snapshot, not a constant.
Lire la suite →Pressure Units in Everyday Life: PSI, Bar, and the Gauge/Absolute Mistake That Actually Matters
Why tyre pressure and weather pressure use different pressure references entirely, the altimeter-setting problem this causes in aviation, and how psi, bar, kPa and mmHg relate exactly.
Lire la suite →The Three Ounces: Why Avoirdupois, Troy and Fluid Ounces Are Not the Same Unit
Avoirdupois ounce (28.349523125 g), troy ounce (31.1034768 g) and fluid ounce (29.5735 mL US, 28.4131 mL Imperial) are three unrelated units that share one English word.
Lire la suite →Unit Errors That Cost Real Money: Mars Climate Orbiter and the Gimli Glider
How a pound-force-second mistaken for a newton-second destroyed a $125 million Mars orbiter in 1999, and how a density figure in the wrong unit nearly downed Air Canada Flight 143 in 1983.
Lire la suite →Lux, Lumens, Foot-Candles and Nits: Four Units That Measure Different Things About Light
Why illuminance, luminance and luminous flux need three different units, the exact foot-candle-to-lux conversion, and a real case of a moonlight-brightness figure that propagated wrong through the literature.
Lire la suite →Knots, Mach, and Flight Levels: The Units Aviation Never Metricated
The nautical mile as exactly 1,852 metres, why a knot is convenient for dead reckoning, Mach as a ratio that changes with temperature, and why most of the world still flies in feet.
Lire la suite →Significant Figures and Rounding: Why a Precise-Looking Number Can Still Be Wrong
How many digits a converted value actually deserves, how error compounds through a chain of conversions, and why an exact-by-definition factor behaves differently from a measured one.
Lire la suite →RMSSD vs SDNN: Why Two Standard HRV Metrics Have No Fixed Conversion Between Them
RMSSD and SDNN are both real, standard HRV measures computed from the same RR-interval series, but no fixed ratio relates them -- why this site removed an HRV conversion category rather than publish one.
Lire la suite →The Gallon Problem: US Liquid, US Dry, and Imperial
Three different gallons share one name -- US liquid (3.785411784 L), US dry (4.4048838 L) and Imperial (4.54609 L) -- and why 1 US gallon holds 128 fluid ounces while 1 Imperial gallon holds 160.
Lire la suite →Grid Carbon Intensity by Country: Why One Number Never Fits All
National grid emission factors span roughly 25-to-1, from Norway's hydro grid near 0.03 kg CO2e/kWh to South Africa's coal-heavy grid near 0.70 -- and why this site's global 0.445 kg CO2e/kWh figure is a poor stand-in for any one country.
Lire la suite →The Energy-Unit Zoo: Joules, Calories, BTUs, kWh and Therms
Seven energy units in everyday use -- the joule, three different calories, the food Calorie, the BTU, the kWh and the therm -- with their exact conversions and which industry uses which.
Lire la suite →Short Ton, Long Ton, Tonne: Three Units, One Word "Ton"
The US short ton (2,000 lb = 907.18 kg), the British long ton (2,240 lb = 1,016.05 kg) and the metric tonne (1,000 kg) are three different masses -- and the long ton sits within about 1.6% of the tonne, which is exactly why the two get mixed up.
Lire la suite →Fuel Economy: MPG, L/100km, and Why the Arithmetic Misleads
Miles per gallon and litres per 100 km are reciprocals of each other, not points on the same linear scale -- which is why going from 10 to 20 mpg saves far more fuel than going from 30 to 50 mpg, and why US and Imperial mpg differ by 20% for an identical car.
Lire la suite →Paper Sizes: A4, Letter, and the Square Root of Two
ISO 216's A-series paper sizes preserve a root-2 aspect ratio every time a sheet is halved, with A0 defined as exactly one square metre -- while US Letter (8.5 x 11 in) follows no such rule and is a genuinely different shape.
Lire la suite →