OIML classes explained: scales and weights

OIML is the international organisation of legal metrology. Its recommendations R76 and R111 determine how scales and weights are divided into classes, and those classes appear on every type plate and every certificate.

Scales: Class I, II, III and IIII

OIML R76, adopted in Europe as EN 45501, divides non-automatic weighing instruments into four classes on the basis of the verification scale interval e and the number of verification scale intervals n = Max / e in the range.

Class	Name	Verification scale interval e	Number of intervals n	Typical
I	special	from 1 mg	from 50,000	analytical balance
II	fine	1 mg to 50 mg, or from 0.1 g	100 to 100,000	precision balance, pharmacy, gold
III	medium	from 0.1 g	500 to 10,000	retail, bench, floor, pallet, medical
IIII	ordinary	from 5 g	100 to 1,000	sand, gravel, construction waste

A higher class is not better; it suits different work. A Class III floor scale of 1,500 kg with e = 500 g has 3,000 verification scale intervals and is exactly right for a pallet.

Weights: Class E1 to M3

OIML R111 divides check weights into seven classes, from E1 (the finest) to M3 (the coarsest). Each class may deviate roughly three times as much as the class above it. For a 1 kg weight, the permitted deviation is half a milligram in Class E1, five milligrams in F1 and fifty milligrams in M1.

Class	Use
E1	reference for calibration laboratories
E2	checking analytical balances (Class I)
F1	checking precision balances (Class II) and fine Class III
F2	precision balances and good bench scales
M1	retail, bench and floor scales (Class III)
M2	coarse Class III and Class IIII
M3	heavy industrial scales, weighbridges

We list every class separately: E1, E2, F1, F2, M1, M2 and M3, and together under check weights.

Which weight for which scale

The rule: the check weight must be at least three times more accurate than the scale you check with it. So choose a weight whose permitted deviation is at most a third of the readability of the scale. For a 0.1 mg analytical balance that is E2; for a 1 mg precision balance F1; for a 1 g bench scale F2 or M1; for a 200 g floor scale M1 or M2. If in doubt, take the finer class: it will also be usable if you later buy a better scale. Which calibration weight do you need works it out scale by scale.

With or without certificate

A weight in Class E or F is almost always supplied with a calibration certificate stating the true mass and the uncertainty; for M1 it is optional. For a quality system you need the certificate, because without it traceability cannot be demonstrated. Weights also age: through wear, dirt and corrosion. How to store them is explained in storing calibration weights, and we recalibrate them in our own calibration laboratory, Masslab.

Frequently asked questions

Is Class I better than Class III?

Finer, not better. Class I is for milligrams in a laboratory; Class III for kilos at the counter or on the floor. A Class I balance cannot carry a pallet.

Which weight class for a retail scale?

M1. You check a 15 kg retail scale with e = 5 g using an M1 weight of 5 or 10 kg; that is amply accurate.

What is the difference between a test weight, a calibration weight and a check weight?

In practice, nothing: they are three names for a weight with a known mass in an OIML class. We call them check weights.

Does a check weight itself need to be calibrated?

Yes, periodically. A weight wears and gets dirty. For E and F weights, recalibration within a quality system is usual; we do that in-house.