The universal routine: dimension, factor, cancellation
Every converter on this hub, no matter how specialised, runs the same four-step routine — and skipping any step is how plausible-looking nonsense gets published.
- Name the dimension. Length, mass, time, temperature, electric current, luminance, pressure — each conversion must stay inside one family. A result that "converts" square feet into kilograms or candelas into pascals is not doing physics; no factor can bridge dimensions.
- Write the factor as a fraction equal to one. Put the unit you want on top, the unit you are leaving on the bottom, and let the labels cancel. Prefixes are where this step bites: milli- versus micro- is a 1,000-fold miss, and kilo-versus-raw is the classic spreadsheet error.
- Decide exact or approximate. Defined relationships — 1 in = 25.4 mm, 1 kWh = 3.6 MJ, 1 knot = 1.852 km/h, 1 thermochemical calorie = 4.184 J — carry infinite precision. Measured or conventional ones — horsepower, Btu, many engineering equivalents — arrive rounded and must leave rounded.
- Sanity-check against an anchor. Body-scale references (1.80 m ≈ 5 ft 11 in, 100 km ≈ 62 mi, 1 kg ≈ 2.2 lb) catch direction mistakes that algebra cannot: factors accidentally flipped give absurd magnitudes that still *calculate*.
Half the specialised tools here — lux to foot-candles, poise to pascal-seconds, curie to becquerel — are single-step members of this same routine. The other half need the next section, because their mappings are not multiplication at all.