Guide · Mounts
How to measure your handlebar diameter for a light or phone mount
Measure the bar where the clamp will sit, then pick a mount whose maker states a range that includes it. Quad Lock, SP Connect, CatEye, Knog and Wahoo ranges.
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Short answer
Measure the bar exactly where the clamp will sit, then pick a mount whose maker states a range that includes your number. A bar's diameter can change along its length, so measure at the clamp's spot, not anywhere else.
Measure where the clamp will sit
Measure at the exact spot your light, phone or computer clamp will go, not where it is easiest to reach. A bar's diameter can differ along its length, so a reading taken elsewhere may not match the clamp. We give no typical diameters by bar position, because no source we checked states them.
Two ways to measure:
- Calipers. Close the jaws gently on the bar at the clamp position and read the number in millimeters. Take two readings at right angles; if they differ, the bar is not round.
- A paper strip. Wrap a thin strip of paper once around the bar at the clamp position and mark where it overlaps. Lay it flat, measure the length in millimeters and divide by pi (about 3.14). A 100 mm strip means a bar of about 31.8 mm.
Write down the number and where you took it. If your bar has a sleeve, tape or a grip at that spot, measure what the clamp will actually close around.
Our size pages cover the common diameters: 22.2 mm bars, 25.4 mm bars, 31.8 mm bars, 35 mm bars and non-round bars and stems. Or use the handlebar size mount finder.
Compare with the maker's stated range
A mount fits the range its maker states, and only that. These are the ranges the makers publish:
| Mount | Maker's stated range | Notes |
|---|---|---|
| Quad Lock Handlebar/Stem Mount | 25-40 mm | Zip ties for most tubes outside the range; Flat Bar Adaptor sold separately for flat stems |
| SP Connect Universal Bike Mount | About 20-44 mm | Tool-free; one flat and one round adapter included |
| CatEye H-34N headlight bracket | 22-35 mm | Not for AMPP models with rubber strap mounts |
| Knog Blinder 1400 bar mount | 22.2-35 mm | Comes with the light |
| Wahoo BOLT 3 / ROAM 3 out-front mount | 31.8 mm round bars | Over-torquing can strip its threads |
Sources: Quad Lock,[1] SP Connect,[2] CatEye,[3] Knog[4] and Wahoo.[5]
Some details from those makers:
- Quad Lock secures its Handlebar/Stem Mount with supplied rubber O-rings and nylon zip ties, and says the zip ties can be used to fit most tubes outside the 25-40 mm range.[1] "Most tubes" is not a promise for your bar shape.
- CatEye's H-34N attaches without tools and says AMPP models with rubber strap mounts are not compatible.[3]
- Wahoo's out-front mount is for 31.8 mm round bars, and Wahoo warns that over-torquing can strip its threads.[5]
Peak Design does not publish a millimeter range we could confirm for its Out Front Bike Mount V2, so we list none.
Picks that fit3 products
Phone, widest stated range: SP Connect Universal Bike Mount, mount only (SKU 53342), about 20-44 mm per SP Connect; it needs an SP Connect case or adapter, sold separately.
More on choosing
CatEye lights: the H-34N FlexTight bracket, 22-35 mm per CatEye, not for AMPP models with rubber strap mounts. Phone, no special case: the ROCKBROS aluminum holder, handlebar clamp version; check the seller's sizing against your number, as we have not verified its range. On Amazon, check that 'Ships from / Sold by' shows Amazon.com or the brand.
Phone mount with the widest stated range
SP Connect Universal Bike Mount (phone, 20‑44 mm bars and stems)
Light bracket for CatEye headlights
Clamp phone holder without a special case
ROCKBROS aluminium bike phone holder, handlebar clamp (4.7‑7.1 in phones)
Stems, flat surfaces and non-round bars
A diameter range describes round tubes. For anything else, look for an adapter from the mount's own maker:
- Quad Lock sells a Flat Bar Adaptor separately for mounting the Handlebar/Stem Mount on flat stems.[1]
- SP Connect includes one flat and one round diameter adapter with its Universal Bike Mount.[2]
If your bar is aero-shaped, or the clamp only closes with packing material, stop and ask a bike shop. Do not drill, file or modify a bar or stem to fit a mount, and do not improvise packing under a clamp.
Next
Questions
What size is my handlebar?
Measure it at the exact spot the clamp will sit, with calipers or a paper strip wrapped once around the bar. Divide the strip's length by pi (about 3.14) to get the diameter in millimeters.
Will a phone mount fit my handlebar?
Only if your measurement falls inside the range the maker states for that mount. Quad Lock states 25-40 mm for its Handlebar/Stem Mount, and SP Connect lists about 20-44 mm for its Universal Bike Mount.[1][2]