Guides

Measuring Tapes

Materials, accuracy classes and temperature length correction, in line with the European Measuring Instruments Directive.

A measuring tape is an essential tool across many fields, from construction and carpentry to tailoring and surveying. Their accuracy is graded into classes, mainly Class I and Class II, under the European Measuring Instruments Directive (MID 2014/32/EU).

The permissible tolerances for length measuring tapes are set by Directive 2004/22/EC, Annex MI-008 (formerly Directive 73/362/EEC).

Reference conditions

When the tolerances apply

The accuracy classes are met at a reference temperature of 20°C and under a defined tensioning force, 20 N for fiberglass tapes and 50 N for steel tapes. Outside these conditions a length correction is required.

Materials and applications

FeMetal (steel) tapes

They stand out for their strength and resistance to stretching. They are common in construction and surveying and come in both Class I and Class II.

GFFiberglass

Lighter and corrosion resistant, which makes them suitable for outdoor use and technical work. However, they are more prone to deformation than steel tapes.

PLPlastic or fabric

Used mainly in tailoring and for measuring curved or soft materials, thanks to their flexibility.

Accuracy classes

Class I

The highest level of accuracy. Over a length of 10 m, the maximum permissible error is ±1.1 mm. Essential for work that demands absolute detail, such as fine carpentry or metal fabrication.

Class II

Slightly less accurate. Over 10 m the permissible error is ±2.3 mm. It is the most common class, for general construction and DIY where extreme accuracy is not required.

Class III covers lower-requirement work, with a permissible error of ±4.6 mm at 10 m.

Tank dipping tapes are made in a special class “S”, with an accuracy of ±1.5 mm at 30 m, under a tension matching the dip weight (senklot).

Tolerance table by class

Permissible tolerances per Directive 2004/22/EC, Annex MI-008.

Length L (m)	Class I	Class II	Class III
Tolerance formula (mm)	0.1 + 0.1·L	0.3 + 0.2·L	0.6 + 0.4·L
1	±0.2	±0.5	±1.0
2	±0.3	±0.7	±1.4
3	±0.4	±0.9	±1.8
4	±0.5	±1.1	±2.2
5	±0.6	±1.3	±2.6
6	±0.7	±1.5	±3.0
7	±0.8	±1.7	±3.4
8	±0.9	±1.9	±3.8
9	±1.0	±2.1	±4.2
10	±1.1	±2.3	±4.6
15	±1.6	±3.3	±6.6
20	±2.1	±4.3	±8.6
25	±2.6	±5.3	±10.6
30	±3.1	±6.3	±12.6
35	±3.6	±7.3	±14.6
40	±4.1	±8.3	±16.6
45	±4.6	±9.3	±18.6
50	±5.1	±10.3	±20.6
55	±5.6	±11.3	±22.6
60	±6.1	±12.3	±24.6
65	±6.6	±13.3	±26.6
70	±7.1	±14.3	±28.6
75	±7.6	±15.3	±30.6
80	±8.1	±16.3	±32.6
85	±8.6	±17.3	±34.6
90	±9.1	±18.3	±36.6
95	±9.6	±19.3	±38.6
100	±10.1	±20.3	±40.6

CE marking and calibration

Measuring tapes with metric graduation are governed by the European Measuring Instruments Directive (MID). Depending on the requirements of the application, a tape may come with different levels of documentation, usually offered as options by suppliers:

CE marking: indicates that the tape has undergone conformity assessment under the MID and meets the legal accuracy class. It is the basic legal requirement for measuring instruments.
Declaration of Conformity: a document in which the manufacturer confirms that the product complies with the relevant Directive, accompanying the CE marking.
Calibration certificate to ISO/IEC 17025: issued by an accredited laboratory, it reports the actual deviations of the specific tape from a reference standard, together with the measurement uncertainty. It provides traceability and documented accuracy where these are required, e.g. in quality management systems.

How to choose the right tape

Required accuracy: for high-accuracy work choose Class I, while for general use Class II is sufficient.

Durability: for harsh environments choose steel tapes, while for lighter work or where there is a risk of corrosion, fiberglass tapes are a good fit.

Length and readability: make sure the length covers your usual needs and that the graduations are clear and easy to read.

Linear expansion

Measuring tapes undergo linear expansion when the temperature changes. When high accuracy is required, length corrections must be made, because the tolerance and error limits apply at a reference temperature of 20°C.

Calculation

Change in length

ΔL = L × ΔT × α

L = length at the measuring point
ΔT = deviation of the actual temperature from the reference temperature
α = coefficient of linear expansion

Carbon steel

0.0117 mm/°C

Stainless steel

0.016 mm/°C

Fiberglass

0.005 mm/°C

The coefficient α is given as the change in length in mm per metre of tape and per degree Celsius.

Example

A carbon steel tape 20 m long at an ambient temperature of 30°C:

ΔL = 20 × 10 × 0.0117 = 2.34 mm

The tape expands by 2.34 mm, so the measured length must be corrected downward by 2.34 mm.

Temperature compensation table

Carbon steel tape, change in length ΔL relative to the 20°C reference temperature.

Tape length	ΔT ±10°C	±20°C	±30°C
1 m	±0.11 mm	±0.23 mm	±0.35 mm
2 m	±0.23 mm	±0.46 mm	±0.70 mm
3 m	±0.35 mm	±0.70 mm	±1.05 mm
4 m	±0.46 mm	±0.93 mm	±1.40 mm
5 m	±0.58 mm	±1.17 mm	±1.75 mm
8 m	±0.93 mm	±1.87 mm	±2.80 mm
10 m	±1.17 mm	±2.34 mm	±3.51 mm
20 m	±2.34 mm	±4.68 mm	±7.02 mm
30 m	±3.51 mm	±7.02 mm	±10.53 mm
50 m	±5.85 mm	±11.70 mm	±17.55 mm
100 m	±11.70 mm	±23.40 mm	±35.10 mm