Boots are sized on foot length, in centimetres
Sidi give one instruction and it is the right one: “to choose the correct number, measure the length of the foot in cm, from the heel to the big toe”.
Not your usual shoe size. Motorcycle boots are built on lasts that vary between manufacturers far more than street shoes do, and the only input that means the same thing everywhere is the length of your foot.
- Stand on a sheet of paper, heel against a wall, weight on the foot. Measuring a foot that is not bearing weight gives a reading several millimetres short.
- Mark the longest toe. On many people that is not the big toe.
- Measure both feet. They differ, and you fit boots to the larger one.
- Do it at the end of the day, when your feet are at their largest.
- Measure in the socks you ride in. A thick sock is easily half a size.
Sidi’s published chart
| EU | US | UK | Foot length (cm) |
|---|---|---|---|
| 36 | 4 | 3.5 | 22.5 |
| 37 | 4.5 | 4 | 23.2 |
| 38 | 5.5 | 5 | 23.8 |
| 39 | 6 | 5.5 | 24.5 |
| 40 | 7 | 6.5 | 25.2 |
| 41 | 8 | 7.5 | 25.8 |
| 42 | 8.5 | 8 | 26.5 |
| 43 | 9.5 | 8.5 | 27.2 |
| 44 | 10 | 9.5 | 27.8 |
| 45 | 11 | 10.5 | 28.5 |
| 46 | 11.5 | 11 | 29.2 |
| 47 | 12.5 | 12 | 29.8 |
| 48 | 13.5 | 12.5 | 30.5 |
| 49 | 14 | 13.5 | 31.2 |
| 50 | 15 | 14.5 | 31.8 |
Look at the US column: eight sizes are missing
Run down it and the sequence stutters. It goes 4, 4.5, 5.5, 6, 7, 8, 8.5, 9.5, 10, 11, 11.5, 12.5, 13.5, 14, 15.
There is no US 5. No 6.5, no 7.5, no 9, no 10.5, no 12, no 13, no 14.5. Eight US sizes have no row.
This is not sloppiness on Sidi’s part. It is arithmetic, and it is the single most useful thing to understand about boot sizing.
The two scales step in different units
The European and Anglo-American shoe scales were not designed to interoperate, and they still do not.
The European scale counts in Paris points. One Paris point is two thirds of a centimetre, so each European size is 0.667 cm longer than the last.
The US and UK scales count in barleycorns. One barleycorn is a third of an inch, which is 0.847 cm, so each full US size is 0.847 cm longer than the last, and each half size is 0.423 cm.
You can verify both figures on Sidi’s own numbers rather than taking them on trust. From EU 36 to EU 50 is fourteen European sizes, and the foot length goes from 22.5 cm to 31.8 cm:
(31.8 − 22.5) ÷ 14 = 0.664 cm per European size
Over the same rows the US column runs from 4 to 15, which is eleven US sizes:
(31.8 − 22.5) ÷ 11 = 0.845 cm per US size
0.664 against the theoretical 0.667, and 0.845 against the theoretical 0.847. The measured steps match the two conventions almost exactly.
And because 0.667 and 0.847 have no common step, the two ladders cannot be lined up. Fourteen European rungs have to be mapped onto eleven American ones, so roughly one in four US sizes gets skipped. Any brand publishing a conversion table has to make the same compromise, and different brands round it differently.
Which is the actual answer to the question everyone asks about boots: a 9.5 in one brand and a 10 in another really can be the same boot, and neither table is wrong. They are rounding an impossible conversion in different directions.
What to do about it
The consequence is practical and simple.
Carry the centimetre figure, not your shoe size. Foot length in centimetres is the only value in that table that is a measurement rather than a convention. Every brand publishes it, and it is directly comparable across all of them.
Treat the US column as a hint. It is the brand’s rounding of a conversion that does not divide, not a statement about your foot.
When you fall between rows, go up. A boot that is fractionally long can be managed with a thicker sock or an insole. A boot that is fractionally short cannot be managed at all, and it will hurt on the first long ride and every one after.
What foot length still does not tell you
Length gets you to one row. It says nothing about the two things that most often make a boot wrong:
- Width and instep volume. Motorcycle boots, especially racing and off-road ones, are built on narrow lasts with a fixed volume over the instep. A high instep can make the correct length impossible to close, and no boot chart has a column for it.
- The shaft. Tall boots have to clear your calf, and calf circumference is unrelated to foot length. Some touring and adventure boots offer expansion panels for exactly this reason; most sport boots do not.
Two more things worth knowing before ordering:
- Boots do not stretch in length. Leather relaxes across the width and over the instep with use. It does not get longer, so length is the one dimension you cannot break in.
- Riding boots are worn over riding socks. Fit them with what you will actually wear, and if you use heated socks in winter, fit them with those.
The rest of the kit
None of the four sizing systems talks to any of the others, and each has a different quirk worth knowing:
- Jackets go on chest circumference, and their numeric sizes do have exact formulas — the converter and the workings are in the motorcycle jacket size chart, with fit covered in how a motorcycle jacket should fit.
- Gloves go on palm circumference, and the published charts have gaps in them — see the motorcycle glove size chart.
- Helmets go on head circumference, and no chart is universal — the motorcycle helmet size chart and how tight a helmet should be.
Our boot picks, once you know your length, are in best motorcycle riding boots, and the trousers that go over them in best motorcycle riding pants for men.
Frequently asked questions
How do I measure my foot for motorcycle boots?
Stand on paper with your heel against a wall and your weight on the foot, mark the tip of your longest toe, and measure the distance in centimetres. Do both feet, fit to the larger one, and measure at the end of the day in the socks you ride in.
Why don’t EU and US boot sizes convert exactly?
Because the two scales step in different units. A European size is one Paris point, or 0.667 cm; a US size is one barleycorn, or 0.847 cm. The steps have no common multiple, so any conversion table has to round, which is why Sidi’s chart skips eight US sizes between 4 and 15.
Should motorcycle boots be a size bigger?
Not automatically, but if your foot length falls between two rows, take the larger. Boots relax across the width and over the instep but never get longer, and a slightly long boot can be adjusted with socks or an insole while a short one cannot be fixed.
What size motorcycle boot is a 27 cm foot?
On Sidi’s chart 27 cm falls between EU 42 at 26.5 cm and EU 43 at 27.2 cm, so it is an EU 43, which they list as a US 9.5. Another brand may map the same length to a US 10, since the EU to US conversion has to be rounded.
Can I use my normal shoe size for motorcycle boots?
Only as a starting point. Motorcycle boots are built on narrower lasts with fixed instep volume, and the brands round the EU to US conversion differently, so the same nominal size varies between makers. Measure your foot in centimetres and use that instead.