The diameters come from Wheels Manufacturing’s compatibility guide and Sheldon Brown’s measurement crib sheet; the headset sizes, from Park Tool’s technical articles; and the handlebar widths, from the Ribble and bike-components guides (checked in October 2026). The stem is the part that connects the handlebar to the fork: it has two clamps, and both have to match what they hold. Bars and stems are sold in millimeters in the US too.
Handlebar clamp diameter
| Diameter | Name | Where it’s used |
|---|---|---|
| 31.8 mm | Oversize | Current standard on road and mountain bikes |
| 35 mm | — | Mountain bike bars, since about 2020 |
| 26.0 mm | Italian | Older high-end road bars |
| 25.4 mm | ISO | Flat bars from before 2000 and some road bars |
| 26.4 mm | Old Cinelli | Cinelli switched to 26.0 mm in 1998 |
| 25.8 mm | In-between | Some Italian brands, to fit both 25.4 and 26.0 mm stems |
The diameter is measured at the center of the bar, where the stem clamps, not at the ends. A bar thinner than the stem can be fitted with a shim; a thicker one, never.
Steerer tube diameter
| Steerer | Top (where the stem goes) | Bottom (at the crown) | Typical SHIS code |
|---|---|---|---|
| 1 1/8” straight | 28.6 mm | 28.6 mm (30 mm crown race seat) | ZS44/28.6 top and ZS44/30 bottom |
| Tapered 1 1/8”-1.5” | 28.6 mm | 1.5” (40 mm crown race seat) | ZS44/28.6 top and ZS56/40 bottom |
| 1.5” straight | 38.1 mm | 38.1 mm | EC49 (OnePointFive) |
| 1” (older bikes) | 25.4 mm | 25.4 mm | — |
The SHIS code Park Tool uses describes the headset: the letters give the cup type, the first number is the diameter of the bore in the frame and the number after the slash is the steerer diameter. The top is written first. A tapered head tube on the frame doesn’t mean the fork is tapered: Park Tool mentions frames with a tapered head tube and a straight fork.
Road handlebar width by shoulder width
| Shoulder width (acromion to acromion) | Bar width center to center (bike-components) | Bar width center to center (Ribble, +2 cm) | Bar width outside to outside (Ribble, +4 cm) |
|---|---|---|---|
| 34 cm (13.4 in) | 34 cm | 36 cm | 38 cm |
| 36 cm (14.2 in) | 36 cm | 38 cm | 40 cm |
| 38 cm (15.0 in) | 38 cm | 40 cm | 42 cm |
| 40 cm (15.7 in) | 40 cm | 42 cm | 44 cm |
| 42 cm (16.5 in) | 42 cm | 44 cm | 46 cm |
| 44 cm (17.3 in) | 44 cm | 46 cm | 48 cm |
It’s a starting point, not a rule. bike-components suggests going slightly narrower if you’re after aerodynamics and slightly wider for comfort. Check how each brand measures width: most give it center to center at the hoods, but some, like Zipp, measure outside to outside, and the difference can be more than 2 cm. In UCI road racing, since January 1, 2026, handlebars must measure at least 400 mm outside to outside.
How to measure your shoulders: stand relaxed and have someone measure, with large calipers or a tape measure, the distance between the two bony points at the tips of your shoulders.
Your bike size is worked out with the bike size calculator and explained in our guide on how to measure bike size; to compare reach and handlebar height between two frames, use the geometry comparison tool. How to decide whether you need a shorter stem or more spacers is covered in the DIY bike fit. Stem and handlebar torque specs are in the bike torque specs chart, and the names of every part in six languages are in the components glossary.
Frequently asked questions
What diameter is a modern handlebar?
Almost every current road and mountain bike handlebar measures 31.8 mm where the stem clamps. Since about 2020 there are 35 mm mountain bike bars, and 25.4 and 26.0 mm bars show up mostly on older bikes.
Does a 1 1/8" stem fit a tapered fork?
Yes. A tapered steerer measures 1 1/8" (28.6 mm) at the top, where the stem goes, and 1.5" at the bottom. The stem always clamps the 28.6 mm section.
What width road handlebar do I need?
Measure the distance between the two bony points at the tip of your shoulders (the acromion). Ribble recommends a bar that width plus 2 cm if measured center to center, or plus 4 cm if measured outside to outside; other brands suggest matching your shoulder width.
Can I put a 25.4 mm handlebar in a 31.8 mm stem?
With a shim, yes. Not the other way around: a bar thicker than the stem clamp won't fit.
Sources
- Wheels Manufacturing: Will it fit? (handlebar diameters)
- Sheldon Brown: metric and inch measurement crib sheet
- Park Tool: Standardized Headset Identification System (SHIS)
- Park Tool: Headset Standards
- Ribble Cycles: How to choose the right handlebar width
- bike-components: Road handlebars, your personal cockpit
- UCI: changes to the equipment regulations (minimum handlebar width)