What accuracy and capacity do I need?
Capacity and readability are the two numbers that define a scale, and they work against each other. This is how to choose both well.
Readability is not accuracy
The readability d is the smallest step the display shows: 0.1 mg, 1 g, 500 g. Accuracy is how close the reading is to the true weight, and it is always coarser than the readability: a scale with d = 1 g may well deviate by 2 or 3 grams. The actual deviation is stated in a calibration certificate; the maximum it may be on a verified instrument is determined by the class.
Start with the smallest weighing
Determine the smallest weight or the smallest difference that still matters to you. A weighing is only usable if that smallest weight covers a good number of readability steps: with a difference of ten steps, an error of one step is already ten percent. Rule of thumb: the smallest thing you weigh should be at least a hundred steps, and for accurate weighing-in a thousand. If you want to weigh out 5 grams with an error below one percent, you therefore need a readability of 0.01 g at most. On a verified scale there is also a hard limit: below Min, which in Class III is twenty times the verification scale interval, the weighing is not valid.
Then the heaviest
The heaviest thing you weigh, including container, crate or pallet, plus ten to twenty percent, is your capacity. Do not go further, because capacity costs readability: in a given price bracket, a manufacturer builds a scale with a fixed number of steps, usually between 3,000 and 30,000. A 6 kg bench scale then has 1 gram, the same model at 30 kg has 5 grams. If you cannot fit the smallest and the heaviest into one scale, choose a multi-range scale that reads more finely in the lower range, or two scales.
A table to start with
| You weigh | Smallest | Heaviest | Readability | Category |
|---|---|---|---|---|
| samples in the lab | 10 mg | 200 g | 0.1 mg | analytical balance |
| raw materials, recipes | 1 g | 3 kg | 0.01 g | precision balance |
| portions, sales at the counter | 50 g | 15 kg | 2 to 5 g | retail scale |
| counting parts | 2 g per piece | 6 kg | 0.1 g | counting scale |
| boxes, parcels | 500 g | 60 kg | 10 to 20 g | parcel scale |
| pallets | 20 kg | 1,500 kg | 200 to 500 g | pallet scale |
| big bags, crates, drums | 50 kg | 3,000 kg | 500 g to 1 kg | floor scale |
What else counts
Stability: a balance standing on a vibrating table is never as good as its readability. Temperature: a laboratory balance needs half an hour after switching on to reach temperature. Tare: a heavy container eats into capacity. Draught: an analytical balance without a draught shield never settles. So choose not just a number but also a location, and let us help: call or email us with what you weigh, how small and how heavy.
Frequently asked questions
How many kg should my scale be able to weigh?
The heaviest thing you will ever weigh, with container or pallet, plus ten to twenty percent. More capacity than that costs readability without giving you anything in return.
What does d = 0.1 g mean?
That the display reads in steps of 0.1 gram. It does not mean the scale is accurate to 0.1 gram; that deviation is stated in the calibration certificate.
What is a multi-range scale?
A scale that reads more finely in the lower part of its range than in the upper part, for example 1 g up to 6 kg and 2 g up to 15 kg. Useful if you weigh small and large items on the same scale.
Can I weigh a 2 kg box on a 3 tonne floor scale?
It will show a reading, but in steps of 500 g or 1 kg; that says virtually nothing about 2 kg. For boxes you need a parcel scale alongside it.