Most measuring mistakes come from technique rather than from the tool. A good tape measure is accurate to about a millimetre over several metres, yet a reading can easily be several millimetres out if the tape sags, the hook is pressed the wrong way or you read the scale at an angle. Learning a few habits makes a bigger difference than buying a more expensive tool.

The guides in this section cover choosing, reading and checking measuring tools, and they support every other part of the site, from measuring for blinds, curtains and carpets to identifying pipe and screw sizes. For the units themselves, see the units of measurement guide.

Which measuring tool should you use?

Use the simplest tool that reaches the accuracy you need. The table compares the tools most people own or can borrow. Resolutions are typical for each type of tool; check the markings or the manual of your own model.

Tool	Typical range	Typical resolution	Best for
Steel tape measure	3 m to 10 m	1 mm (1/16 in on imperial scales)	Rooms, furniture, doors, windows, DIY
Soft tape (tailor’s tape)	Usually 150 cm (60 in)	1 mm or 1/8 in	Body measurements, curved objects, pets
Steel rule	150 mm to 1 m	0.5 mm or 1 mm	Small parts, marking out, checking straightness
Vernier or dial caliper	Usually 150 mm	0.02 mm to 0.05 mm	Pipe diameters, screw sizes, thicknesses
Digital caliper	Usually 150 mm	0.01 mm display	The same jobs, with an easier readout
Micrometer	0 to 25 mm per tool	0.01 mm	Very precise thickness or diameter
Laser distance measure	About 20 m to over 100 m by model	1 mm display	Room sizes and heights, long distances indoors
Measuring wheel	Unlimited	Coarser than a tape	Gardens, driveways, fences, paths

How accurate is a tape measure?

Tape measures made to the Measuring Instruments Regulations 2016 carry an accuracy class, I, II or III, and the class sets the maximum permitted error. The error limit is a + bL millimetres, where L is the length measured rounded up to the next whole metre. Class I allows 0.1 + 0.1L mm, Class II allows 0.3 + 0.2L mm and Class III allows 0.6 + 0.4L mm. Most good-quality DIY tapes are Class II, and the class is usually printed near the start of the blade as a Roman numeral in an oval.

Length measured	Class I	Class II	Class III
2 m	±0.3 mm	±0.7 mm	±1.4 mm
3 m	±0.4 mm	±0.9 mm	±1.8 mm
5 m	±0.6 mm	±1.3 mm	±2.6 mm
8 m	±0.9 mm	±1.9 mm	±3.8 mm
10 m	±1.1 mm	±2.3 mm	±4.6 mm

For hanging a picture or measuring a room, Class II is more than good enough. Class I tapes are used where small errors matter, such as setting out joinery or engineering work. The guide to reading a tape measure explains the metric and imperial markings, including the sixteenth-inch divisions on dual-scale tapes.

Why does the hook on a tape measure move?

The hook slides by its own thickness so that inside and outside measurements both start from a true zero. When you hook the tape over an edge, the hook pulls out; when you push it against a wall, it pushes in. A loose-looking hook is therefore correct, and you should not bend it or tighten its rivets.

For an inside measurement, such as a window recess, many tapes have the case length printed on the back. Push the case into the corner and add that figure to the reading on the blade, or measure from wall to wall with a laser measure instead.

How do you use a caliper?

Close the jaws, set the zero, then measure with the outside jaws for diameters, the inside jaws for holes and the rod at the end for depths. Calipers are the right tool for identifying pipe, screw and bolt sizes, where a millimetre makes the difference between two standard sizes.

1. Clean the jaws. Wipe away dust or swarf, because a speck of grit adds to every reading.
2. Check the zero. Close the jaws fully. A digital caliper should read 0.00; press the zero button if it does not.
3. Hold the part square. Close the jaws gently on the widest point, keeping them at right angles to the part.
4. Read without forcing. Squeezing the thumbwheel tilts the jaw and makes the reading too small.
5. Check against a known size. A UK £1 coin has a diameter of 23.43 mm, which makes a handy everyday check.

The screw size guide and the plumbing and hardware sizes hub show how caliper readings translate into standard sizes.

Are laser distance measures accurate?

A laser distance measure is typically accurate to a few millimetres and is often quicker and more reliable than a tape over long distances. It measures from the back edge of the device by default, so place that edge firmly against the starting surface. Many models also calculate area and volume from two or three readings.

Lasers need a solid surface to bounce off. Glass, mirrors and very dark or shiny surfaces can give false readings, and bright sunlight reduces the range outdoors. For measurements under a metre, a tape or rule is usually just as quick and easier to check.

How do you take a reliable measurement?

1. Choose a tool with a suitable resolution. A tape for a room, a caliper for a pipe.
2. Start from a clear zero. Hook the tape properly, butt the rule against an edge or zero the caliper.
3. Keep the tool straight and taut. A sagging tape over 3 m reads long; support it in the middle if needed.
4. Read the scale square-on. Looking at an angle (parallax) shifts the reading by a millimetre or more.
5. Measure in more than one place. Windows, doors and recesses are rarely square, so measure the width at the top, middle and bottom and use the smallest figure.
6. Write it down with the unit. Record 1,245 mm or 124.5 cm, never just 1245, and measure twice before you cut or order.

What mistakes should you avoid with measuring tools?

• Mixing units. Measuring in inches and ordering in centimetres without converting; use the inches to cm converter if you need to switch.
• Using a soft tape for building work. Cloth tapes stretch over time and are meant for bodies, not rooms.
• Trusting a damaged blade. A kinked or bent tape, or one with a damaged hook, no longer reads true.
• Ignoring flat batteries. Digital calipers and lasers can give erratic readings when the battery is low.

Measuring tool guides in this section

• How to read a tape measure: metric and imperial markings, fractions and the sliding hook.
• Length converter: convert any reading between millimetres, centimetres, metres, inches, feet and yards.
• Cm to inches converter and inches to cm converter: for dual-scale tapes and imperial plans.
• How to measure for blinds: a practical example of measuring a recess at several points.

Latest measuring tools guides

Related sections

Put these tools to work with the home and DIY measuring guides and the plumbing and hardware sizes guides, or return to the units of measurement guide for the background to metric and imperial units.

Frequently asked questions

What does Class II on a tape measure mean?

It is the accuracy class under the Measuring Instruments Regulations 2016. A Class II tape must be accurate to within 0.3 + 0.2L mm, where L is the length in metres rounded up, so within 1.3 mm over 5 m.

Why is the end of my tape measure loose?

The hook is designed to slide by its own thickness, so that it gives a true zero whether you hook it over an edge or push it against a surface. It is not broken.

How can I check a digital caliper is accurate?

Close the jaws and check it reads zero, then measure an object of known size. A UK £1 coin has a diameter of 23.43 mm, so a reading within a few hundredths of a millimetre shows the caliper is working.

Is a laser measure more accurate than a tape?

Over long distances a laser measure is usually as accurate as a good tape and easier to use alone. For short or awkward measurements, a tape or rule is often more practical.

What tape should I use to measure my body?

Use a soft tailor’s tape, usually 150 cm or 60 inches long. A steel tape is too stiff to follow the body and gives inaccurate results.

Sources

• The Measuring Instruments Regulations 2016, legislation.gov.uk: accuracy classes for material measures of length
• The Royal Mint: specification of the 12-sided £1 coin
• National Physical Laboratory: the UK’s national measurement institute
• Office for Product Safety and Standards, GOV.UK: weights and measures regulation