Note 01
Two ounces, two substances
The ounce causes more confusion on a market stall than any other unit. The avoirdupois ounce, used for everyday weighing where the pound is common, is exactly 28.349523125 grams. The troy ounce, used for precious metals, is 31.1034768 grams. The fluid ounce is a measure of volume rather than weight, and the US fluid ounce is exactly 29.5735295625 millilitres.
Water links weight and volume only roughly. Before you convert anything, check whether a label says by weight or by volume, and write the unit's full name next to the figure.
Note 02
Tare, gross and net
Tare is the weight of the container on its own. Gross weight is the container plus what is in it. Net weight is the part you are actually buying, selling or mixing, and it is found by subtracting the tare from the gross: net equals gross minus tare.
Zero the scale with the empty container in place whenever the scale offers that control. If it does not, weigh the empty container once, write that figure down, and subtract it from each gross reading.
Note 03
Units that share a name
Several units share a name without sharing a size. A pound in everyday use is exactly 0.45359237 kilograms, while a troy pound is twelve troy ounces, or 373.2417216 grams. A short ton is 2,000 pounds, a long ton is 2,240 pounds and a metric tonne is 1,000 kilograms. A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres.
When a recipe, label or price list names one of these units, ask which version is meant. The fastest protection is to write the full unit name beside every figure you record.
Note 04
Resolution is not accuracy
The last digit a scale shows reflects its resolution, the smallest step the display can show. Accuracy is how close a reading lies to the true value. The two are independent: a display can show many digits and still be wrong, and a plainer display can be closer to the truth.
To test a scale for everyday use, weigh an object whose weight you already know, several times, in the same place and under the same conditions. Consistent readings that match the known weight are the sign of a scale you can trust for a bench. For trade, use a certified check from your local weights and measures authority.
Note 05
Scaling a batch without drift
To scale a recipe or a mix, multiply every quantity by the same factor. The factor is the new batch size divided by the old batch size. Convert each quantity to one base unit before you multiply, so that grams are multiplied as grams and never as a blend of grams and ounces.
Round only at the final step. Rounding each ingredient along the way is the usual reason a scaled batch drifts away from the original proportions, and keeping unrounded working figures costs nothing.
Note 06
Where the temperature scales start
Celsius is set so that water freezes at 0 degrees and boils at 100 degrees at standard atmospheric pressure. Fahrenheit places the same freezing point at 32 degrees. Kelvin starts at absolute zero, and its degree is the same size as a Celsius degree, so kelvin equals degrees Celsius plus 273.15 exactly.
Use the formulas on the converter page whenever a figure must move from one scale to another. Keep the offset exact in your working, because a rounded offset shifts every result and hides small differences between readings.