Cars, bikes and sports equipment are some of the last places where British, metric and international conventions sit side by side on the same product. A road bike might have a 56 cm frame and 700c wheels, a mountain bike a “large” frame with 29-inch wheels, and the pump that fills both may read in psi and bar. Fuel economy is quoted in miles per gallon, but the fuel is sold in litres.

The aim of this section is to make those markings readable and to show you how to take the measurements yourself, whether you are checking a tyre before an MOT, sizing a child’s first bike or buying a hockey stick. For the units behind all of it, see the units of measurement guide.

How do you read a car tyre size?

A tyre size such as 205/55 R16 91V gives the width in millimetres, the sidewall height as a percentage of the width, the construction, the rim diameter in inches, then the load index and speed rating. The code is moulded into the sidewall and is also listed in the vehicle handbook. When replacing tyres, match the size and at least the load and speed ratings the manufacturer specifies.

The profile figure is a ratio, not a measurement. In 205/55 the sidewall is 55% of 205 mm, which is about 113 mm, so a lower profile number means a shallower sidewall. Adding twice the sidewall height to the rim diameter gives the approximate overall diameter: 406.4 mm + 225.5 mm, or roughly 632 mm.

What is the legal minimum tyre tread depth in the UK?

For cars, light vans and light trailers the legal minimum is 1.6 mm across the central three-quarters of the tread, around the entire circumference. The Highway Code, Annex 6 sets a 1 mm minimum for motorcycles, large vehicles and passenger-carrying vehicles, and requires visible tread on mopeds.

A tread depth gauge gives a reading in millimetres. Take readings in several main grooves at a few points around each tyre, because wear is rarely even. Most car tyres also have small raised tread wear indicators in the main grooves; when the tread is level with them, the tyre is at the legal limit. The tyre tread measuring guide shows the full method.

How do you convert tyre pressure from psi to bar?

Multiply psi by 0.0689 to get bar, or multiply bar by 14.5 to get psi. A typical car pressure of 32 psi is 2.21 bar. The correct pressure for your vehicle is printed in the handbook and usually on a label inside the driver’s door frame or fuel filler flap, and it should be checked when the tyres are cold.

psi	bar	Typical use
28 psi	1.93 bar	Small car (check handbook)
30 psi	2.07 bar	Car
32 psi	2.21 bar	Car
35 psi	2.41 bar	Car or loaded car
40 psi	2.76 bar	Loaded car, light van
50 psi	3.45 bar	Van, hybrid bike
60 psi	4.14 bar	Hybrid or touring bike
80 psi	5.52 bar	Road bike
100 psi	6.89 bar	Road bike, narrow tyres

The “typical use” column is a rough guide only. Always follow the vehicle maker’s figure, and for bikes stay within the minimum and maximum pressures printed on the tyre sidewall. The pressure converter handles kPa as well as psi and bar.

How are bikes sized?

Bike frames are sized by seat-tube length, usually in centimetres for road bikes and in inches or S/M/L for mountain bikes, while wheels are sized by their bead seat diameter in millimetres. Brands measure frames slightly differently, so compare the geometry chart rather than the size label alone.

Wheel names are even less consistent. “700c” and “29-inch” wheels share the same 622 mm bead seat diameter, which is the ISO (ETRTO) number printed on the tyre as, for example, 28-622. That number is what decides whether a tyre fits a rim. The bike wheel size guide and bike frame size guide explain how to measure each.

Common wheel name	ISO bead seat diameter	Typical bikes
16 in	305 mm	Young children’s bikes
20 in	406 mm	BMX, children’s and folding bikes
24 in	507 mm	Older children’s mountain bikes
26 in	559 mm	Older and budget mountain bikes
27.5 in / 650b	584 mm	Mountain and gravel bikes
700c / 29 in	622 mm	Road, hybrid, gravel and 29er mountain bikes

How do UK and US miles per gallon compare?

UK mpg figures use the 4.546-litre imperial gallon, so they are about 20% higher than US mpg figures for the same car. A car returning 50 mpg in UK terms manages about 41.6 mpg in US gallons. To convert UK mpg to litres per 100 km, divide 282.48 by the mpg figure: 50 mpg is 5.65 L/100 km.

How is sports equipment sized?

Most sports kit is sized from one or two body measurements: height, inside leg, head circumference, hand size or chest. The tape measure method matters more than the size label, because brands use different charts. Measure in light clothing, keep the tape level and snug but not tight, and round to the nearest centimetre or half inch.

• Hockey sticks: sold by length in inches. The hockey stick sizing guide explains how to match length to height and playing position.
• Helmets: sized by head circumference in centimetres, measured just above the eyebrows and around the widest part of the back of the head.
• Wetsuits: sized by height, chest and weight together, so take all three before checking a brand’s chart.
• Golf clubs and cricket bats: sized by length or by named sizes linked to the player’s height; measure the player standing in their usual footwear.

Fixed playing dimensions are different: they are set by each sport’s governing body and many still reflect their imperial origins, which is why the metric figures look oddly precise.

Sport measurement	Metric	Imperial
Cricket pitch (stumps to stumps)	20.12 m	22 yd
Football goal (inside width × height)	7.32 m × 2.44 m	8 yd × 8 ft
Football penalty mark from goal line	11 m	12 yd
Tennis court (doubles)	23.77 m × 10.97 m	78 ft × 36 ft
Dartboard: floor to centre of bull	1.73 m	5 ft 8 in
Darts throwing distance (oche to board face)	2.37 m	7 ft 9¼ in

Vehicle and sports quick-reference chart

These are the conversions most often needed for driving, cycling and sport in the UK. Factors are rounded for everyday use; the mile and inch definitions are exact.

From	To	Factor or example
Miles	Kilometres	× 1.609344 (exact)
30 mph	km/h	48.3 km/h
70 mph	km/h	112.7 km/h
psi	bar	× 0.0689
bar	psi	× 14.5
Rim inches	mm	× 25.4 (16 in = 406.4 mm)
UK mpg	L/100 km	282.48 ÷ mpg
UK mpg	US mpg	× 0.833
Tread depth legal minimum (car)	mm	1.6 mm

Guides in this section

Vehicles and cycling

• How to measure tyre tread (UK): check depth against the 1.6 mm legal minimum with a gauge.
• How to measure bike frame size: seat-tube length and how it relates to your height and inside leg.
• How to measure bike wheel size: read ISO tyre markings and measure a rim.

Sport

• How to measure for a hockey stick: choose the right stick length in inches.

Useful converters

• Speed converter: mph, km/h, knots and metres per second.
• Miles to km and km to miles: for route distances and race lengths.
• Pressure converter: psi, bar and kPa for tyres and pumps.

Latest vehicle, cycling and sports guides

For sizing that starts with body measurements, such as helmets, gloves and footwear, see the body and clothing measurement hub. The measuring tools hub covers gauges, callipers and tape measures in more depth.

Frequently asked questions

What does 205/55 R16 mean on a tyre?

205 is the tyre width in millimetres, 55 means the sidewall height is 55% of that width, R means radial construction and 16 is the rim diameter in inches. The numbers and letter that follow, such as 91V, are the load index and speed rating.

Is 2 bar the same as 30 psi?

Close, but not exactly. 2 bar is 29.0 psi, and 30 psi is 2.07 bar. For tyres, use the exact figure from the vehicle handbook or door-frame label rather than a rounded conversion.

What is the minimum tread depth for a motorbike in the UK?

The Highway Code gives a minimum of 1 mm across three-quarters of the breadth of the tread and in a continuous band around the entire circumference for motorcycles. Mopeds must have visible tread. Cars, light vans and light trailers need at least 1.6 mm.

Are 700c and 29-inch wheels the same size?

Yes, both have a 622 mm bead seat diameter, so tyres are interchangeable as long as the width suits the rim and frame clearance. 29-inch mountain bike tyres are simply much wider than typical 700c road tyres.

How high should a dartboard be?

The centre of the bull should be 1.73 m (5 ft 8 in) from the floor, and the throwing distance from the oche to the face of the board is 2.37 m (7 ft 9¼ in) measured along the floor.

Sources

• The Highway Code, Annex 6: vehicle maintenance, safety and security (tread depth)
• The Road Vehicles (Construction and Use) Regulations 1986, regulation 27
• ETRTO (European Tyre and Rim Technical Organisation): tyre and rim size standards
• IFAB Laws of the Game (football), MCC Laws of Cricket and ITF Rules of Tennis
• Sports dimensions guide: darts (board height and throwing distance)