What the inputs mean
- Number of lugs: count the studs or bolt holes.
- What you measured: with an even count, measure across to the opposite stud. With 5 lugs there is no opposite stud, so measure to a neighbour, or from the back of one stud to the centre of the farthest one.
- Stud diameter: only needed for the back to centre method, because the back edge sits half a stud further out.
How to measure a bolt pattern
- Take the wheel off or measure the bare hub. Clean the studs.
- Count the lugs.
- Even count: measure from the outer edge of one stud to the inner edge of the stud straight across. That equals centre to centre, because the two half studs cancel out.
- Five lugs: measure from the back of one stud to the centre of the farthest stud, then measure the stud diameter. Or measure centre to centre of two neighbours.
- Enter the numbers. Use calipers if you can: the result needs millimetre accuracy.
The formula
studs k gaps apart: chord = PCD × sin(k × 180° ÷ n)
opposite studs (even n): PCD = measurement
neighbouring studs: PCD = measurement ÷ sin(180° ÷ n)
5 lugs, back to centre of farthest: PCD = (m − stud ÷ 2) ÷ sin 72°
1 in = 25.4 mm, so 5x4.5 = 5x114.3The factor 1 ÷ sin(180° ÷ n) also tells you how much a measuring error grows: 1.70 for 5 lugs, 1.41 for 4 and 2.00 for 6 when you measure neighbours. Measuring across multiplies the error by 1, and the 5-lug back to centre method by 1.05.
Worked example
A 5-lug hub measures 4.53 in from the back of one stud to the centre of the farthest, and the studs are 0.5 in across. PCD = (4.53 − 0.25) ÷ sin 72° = 4.28 ÷ 0.95106 = 4.500 in, or 114.3 mm. That is 5x114.3, written 5x4.5 in inches.
The usual shortcut takes the 4.53 in reading as the bolt circle itself. That gives 115.06 mm, and the nearest common size to 115.06 is 5x115, not 5x114.3. The shortcut only lands exactly when the stud is about 11 mm thick. On a 5x114.3 hub, neighbouring studs sit 67.2 mm (2.645 in) apart centre to centre, a good second check.
Close sizes
Some common bolt circles differ by less than a millimetre, such as 114.3 mm and 115 mm. A wheel with the wrong one may seem to go on, but the studs are pulled sideways. When the calculator lists another size within 2 mm, measure again and confirm from the wheel marking.
Questions
How do you measure a 5 lug bolt pattern?
Measure from the back of one stud to the centre of the farthest stud, and measure the stud diameter. The bolt circle is (measurement − half a stud) ÷ 0.95106. With 4.53 in and 0.5 in studs, that is 4.500 in, or 5x114.3 mm.
Is 5x4.5 the same as 5x114.3?
Yes. 4.5 in × 25.4 = 114.3 mm, so 5x4.5 and 5x114.3 are the same pattern written in inches and millimetres. 5x115 is a different pattern, 0.7 mm larger.
How do you measure a 4, 6 or 8 lug bolt pattern?
Measure from the outer edge of one stud to the inner edge of the stud straight across. That distance equals centre to centre, and it is the bolt circle diameter. For 6x139.7 you should read 139.7 mm, which is 5.5 in.
How accurate does the measurement need to be?
Within about half a millimetre, because some sizes are only 0.7 mm apart. Measuring neighbouring studs on a 5-lug hub multiplies your error by 1.70, so measuring to the farthest stud is more forgiving.