OIML R111 weight classes E1 to M3 explained

OIML R111 weight classes E1 to M3: tolerances and use

MASSLAB calibration laboratory · 14 September 2026 · updated 18 October 2026 · Reading time: approx. 4 minutes

The seven OIML R111 classes determine how much a weight may deviate. Read what each class means, what you use it for and find the complete tolerance table.

Every calibration weight and every certificate states a class: E1, E2, F1, F2, M1, M2 or M3. That class comes from OIML R111, the international recommendation for weights issued by the Organisation Internationale de Métrologie Légale. It tells you how much a weight may deviate from its nominal value at most, and therefore which scales it is suitable for.

In this article we explain how the classes are structured, which requirements apply in addition to the tolerance and how to use the classes in practice. At the bottom you will find the complete tolerance table. If you want to know how a calibration according to R111 is carried out, read about weight calibration in our laboratory.

Seven classes, each roughly three times wider

The classes are ranked from fine to coarse. E1 is the most accurate class, M3 the coarsest. Between two consecutive classes there is always a factor of roughly three. A 1 kg weight may deviate by no more than 0.5 mg in class E1, 1.6 mg in E2, 5 mg in F1, 16 mg in F2, 50 mg in M1, 160 mg in M2 and 500 mg in M3.

The letters stand for the application the classes were originally intended for. The E classes are the reference classes for laboratories, the F classes the fine classes for precision weighing, and the M classes the medium classes for trade and industry. That classification is still a good rule of thumb.

More than just a tolerance

A weight only conforms to a class if it also meets the other requirements of R111. These become stricter the finer the class:

Material and density: in the fine classes, the density must lie within narrow limits, because otherwise air buoyancy causes a measurable error. That is why E and F weights are made of stainless steel. Cast iron is found only in the M classes.
Surface: fine weights are polished or finely turned, without pores in which dirt and moisture can accumulate.
Magnetism: E and F weights may be barely magnetic, otherwise they affect electronic balances.
Shape and adjustment cavity: E1 and E2 weights are usually made from a single piece. Weights in the lower classes often have an adjustment cavity, which is closed after adjustment.

A cast-iron M1 weight therefore does not become an F2 weight, even if the measured error happens to be small. Read more about the differences per material in cast iron or stainless steel weights.

Which class do you use for what?

Class	Typical use
E1	Reference in calibration laboratories, for calibrating E2 weights
E2	Analytical balances and semi-micro balances; reference for F1 weights
F1	Precision balances and finer class III scales; reference for M weights
F2	Precision balances with coarser readability and good bench scales
M1	Class III scales: retail, bench, floor and pallet scales, many weighbridges
M2	Coarser class III and class IIII scales
M3	Coarse industrial applications and test loads

What is decisive is not the name of the scale, but the ratio between the error of the weight and the error the scale is permitted to make. How to calculate this is explained in which calibration weight do you need, including a selection guide.

Tolerance and measurement uncertainty

The tolerance in the table is the maximum permissible error of the conventional mass relative to the nominal value. During a calibration, that error is measured, and every measurement has a measurement uncertainty. R111 therefore sets an additional requirement: the expanded measurement uncertainty may not exceed one third of the tolerance.

For a 100 g F1 weight, that means a tolerance of 0.5 mg and a measurement uncertainty of at most approximately 0.17 mg. The finer the class, the higher the requirements for the reference weights, the mass comparator and the environment. That is why the certificate always shows both the error and the measurement uncertainty. An explanation of how to read these can be found in measurement uncertainty on a calibration certificate.

When a weight falls outside its class

Weights change through wear, dirt and corrosion. If a weight falls outside the tolerance at recalibration, there are three options. You have the weight adjusted, if its design allows this. You continue to use it in a lower class, if it meets all the requirements there. Or you use it with the measured value from the certificate, if your procedure permits this. Which choice is sensible depends on what you use the weight for. Read more in adjusting weights.

Which classes does MASSLAB calibrate?

MASSLAB calibrates weights in classes F1, F2, M1, M2 and M3 in our own laboratory in Geldermalsen, from 1 mg to 2,000 kg. We clean, adjust and calibrate according to R111 and provide a traceable certificate with measurement uncertainty. For E1 and E2 weights, we advise you on the right route. New weights in all classes can be found in our web shop for calibration weights, including the KERN weights range.

OIML R111 tolerance table

Maximum permissible error (±) per nominal value and class. Filter by range or highlight a class. MASSLAB calibrates classes F1 to M3 in its own laboratory.

Range Highlight class

Nominal value	E1	E2	F1own lab	F2own lab	M1own lab	M2own lab	M3own lab
1 mg	0.003 mg	0.006 mg	0.02 mg	0.06 mg	0.2 mg	–	–
2 mg	0.003 mg	0.006 mg	0.02 mg	0.06 mg	0.2 mg	–	–
5 mg	0.003 mg	0.006 mg	0.02 mg	0.06 mg	0.2 mg	–	–
10 mg	0.003 mg	0.008 mg	0.025 mg	0.08 mg	0.25 mg	–	–
20 mg	0.003 mg	0.01 mg	0.03 mg	0.1 mg	0.3 mg	–	–
50 mg	0.004 mg	0.012 mg	0.04 mg	0.12 mg	0.4 mg	–	–
100 mg	0.005 mg	0.016 mg	0.05 mg	0.16 mg	0.5 mg	1.6 mg	–
200 mg	0.006 mg	0.02 mg	0.06 mg	0.2 mg	0.6 mg	2 mg	–
500 mg	0.008 mg	0.025 mg	0.08 mg	0.25 mg	0.8 mg	2.5 mg	–
1 g	0.01 mg	0.03 mg	0.1 mg	0.3 mg	1 mg	3 mg	10 mg
2 g	0.012 mg	0.04 mg	0.12 mg	0.4 mg	1.2 mg	4 mg	12 mg
5 g	0.016 mg	0.05 mg	0.16 mg	0.5 mg	1.6 mg	5 mg	16 mg
10 g	0.02 mg	0.06 mg	0.2 mg	0.6 mg	2 mg	6 mg	20 mg
20 g	0.025 mg	0.08 mg	0.25 mg	0.8 mg	2.5 mg	8 mg	25 mg
50 g	0.03 mg	0.1 mg	0.3 mg	1 mg	3 mg	10 mg	30 mg
100 g	0.05 mg	0.16 mg	0.5 mg	1.6 mg	5 mg	16 mg	50 mg
200 g	0.1 mg	0.3 mg	1 mg	3 mg	10 mg	30 mg	100 mg
500 g	0.25 mg	0.8 mg	2.5 mg	8 mg	25 mg	80 mg	250 mg
1 kg	0.5 mg	1.6 mg	5 mg	16 mg	50 mg	160 mg	500 mg
2 kg	1 mg	3 mg	10 mg	30 mg	100 mg	300 mg	1 g
5 kg	2.5 mg	8 mg	25 mg	80 mg	250 mg	800 mg	2.5 g
10 kg	5 mg	16 mg	50 mg	160 mg	500 mg	1.6 g	5 g
20 kg	10 mg	30 mg	100 mg	300 mg	1 g	3 g	10 g
50 kg	25 mg	80 mg	250 mg	800 mg	2.5 g	8 g	25 g
100 kg	–	–	500 mg	1.6 g	5 g	16 g	50 g
200 kg	–	–	1 g	3 g	10 g	30 g	100 g
500 kg	–	–	2.5 g	8 g	25 g	80 g	250 g
1,000 kg	–	–	5 g	16 g	50 g	160 g	500 g
2,000 kg	–	–	10 g	30 g	100 g	300 g	1 kg

Source: OIML R111-1. The expanded measurement uncertainty of a calibration may be no more than one third of this value. Buying weights yourself? See calibration weights by class.

Frequently asked questions

What is the most accurate OIML class?

Class E1. A 1 kg E1 weight may deviate by no more than 0.5 mg. E1 weights are mainly used as references in calibration laboratories.

Is an M1 weight suitable for an analytical balance?

No. For analytical balances, E2 weights are customary. An M1 weight is well over a hundred times less accurate than an E1 weight and is suited to class III scales.

May a weight be used in a higher class if the error is small?

Only if it also meets the requirements for material, density, surface and magnetism of that class. A cast-iron weight remains an M weight.

Up to what nominal value do the E classes exist?

The E classes exist up to 50 kg. Heavier weights, up to several tonnes, are available in classes F1 to M3.

Which classes does MASSLAB calibrate itself?

Classes F1, F2, M1, M2 and M3, from 1 mg to 2,000 kg, in our own laboratory. Read more at weight calibration.