Field guide
Field guide to weighing and measuring
Practical notes for stallholders, makers and cooks who weigh and measure as part of their work. Each section explains the idea first and the arithmetic second, and each one stands on its own.
Tare, gross and net
Most weighing is a subtraction hidden inside a single reading. The gross weight is everything on the scale: the container and what is in it. The tare is the container alone. The net weight is the part you are buying, selling or mixing, and it is the gross weight minus the tare.
Zeroing the scale with the empty container in place gives the net reading directly on most scales. When a scale cannot be zeroed that way, weigh the empty container once, write that figure down, and subtract it from each gross reading.
Write the gross, tare and net on the same line of your record. A line that does not add up is the quickest sign that a reading was taken with the wrong container on the platform.
Resolution and accuracy
A scale's resolution is the smallest step its display can show, and it is usually the last digit on the screen. Accuracy is how close a reading is to the true value. The two are independent. A display can show many digits and still be wrong, and a coarser display can sit closer to the truth.
To check a scale for everyday use, weigh an object whose weight you already know, several times, in the same place and under the same conditions. If the readings agree with the known weight and with each other, the scale is behaving consistently. Consistency comes first, because a scale that wanders cannot be corrected.
For trade, where a reading decides what a buyer pays, use a certified check by your local weights and measures authority. That is the only route that carries legal weight.
Units that share a name
Several units are called by the same name but describe different quantities. These are the common cases, with their exact sizes.
- Ounce: the avoirdupois ounce is 28.349523125 grams, the troy ounce is 31.1034768 grams, and the fluid ounce measures volume, with the US fluid ounce at 29.5735295625 millilitres.
- Pound: the avoirdupois pound is 0.45359237 kilograms, and the troy pound is twelve troy ounces, which is 373.2417216 grams.
- Ton: the short ton is 2,000 pounds, the long ton is 2,240 pounds, and the metric tonne is 1,000 kilograms.
- Gallon: the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres.
When a recipe, label or price list names one of these units, ask which version is meant. Writing the full unit name beside every figure you record prevents most of the confusion.
Converting volume to weight
Volume and mass are different quantities. A conversion between them needs a density, which links the two through a simple relation.
mass = density × volume
Water has a density close to one gram per millilitre at room temperature, which makes it a handy reference. Oil, syrup and flour do not share that density, so a volume-to-weight conversion for those materials needs its own figure. Weigh a known volume of the material you use, and record the result beside the date.
Scaling batches
Scaling a recipe or a mix means multiplying every quantity by the same factor. The factor is the new batch size divided by the old batch size, and it applies to every ingredient in the same way.
new quantity = original quantity × (new batch ÷ old batch)
Convert every quantity to a single base unit before multiplying. Mixing grams with ounces during the calculation is how proportions drift. Round only at the end, and keep the unrounded figures on your working sheet until then.
Temperature scales
Celsius and Fahrenheit differ in both their zero points and their degree sizes, so converting between them takes a formula. Kelvin uses the same degree size as Celsius but starts at absolute zero.
°F = °C × 9 ÷ 5 + 32
K = °C + 273.15
Keep the offset exact in your working. Rounding 273.15 down to 273 shifts every kelvin figure by 0.15, which matters in any calculation that depends on the difference between two readings.
What this guide does not cover
This guide describes units and measuring practice. It does not set prices, judge the value of a product, forecast markets or say whether a particular measure is legal for trade. It is not financial, legal or medical advice.
For a question about a specific instrument, a legal requirement or a health matter, ask the body or professional responsible for that area. The contact page explains what to include when you write to the desk.