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I don't know my bike's stock handlebar diameter. I am going to upgrade some parts, for handlebar I tried to read any information written on it but there is only it's brand name. Can you help me please?

EDIT: Sorry for my poor English. I am trying to mean the "diameter" by "width", like the cylinder here http://www.nordenexpress.com/images/packaging.gif (For example I thought- the drop bar I want to buy has a "length" of 44cm, not width. Sorry again)

My bike has a flat handlebar and I am willing to replace it with a drop bar. Drop bars I've found on online bike stores are mostly 31.8mm. I don't know how to measure the diameter of my flat bar, I don't have calipers. Even if I had calipers, I don't know where to measure; the middle of the bar is thicker, ends are thinner.

I am not sure about where is 31.8mm. Just thought maybe it is the diameter of the bar, am I right?

If a handlebar (like mine) is said to be 25.4, can you tell how we get the result 25.4? Measure the diameter? Radius? Periphery?

enter image description here

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    The answers provided so far are excellent and cover all the options. If they don't answer your question, please provide some more info as to what you're asking.
    – Criggie ♦
    Apr 9, 2016 at 0:57
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    Oh, I'm terribly sorry. The mistake is I am trying to measure the diameter I think. I'll edit the question.
    – count_campeador
    Apr 9, 2016 at 3:04
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    Your english is excellent. Diameter is the correct word. I'd recommend you consider buying some cheap plastic calipers for your toolbox. They're quite handy.
    – Criggie ♦
    Apr 10, 2016 at 1:58
  • Orange is diameter, cyan and red are circumference. You can use a string to measure the circumference (you wrap the string once around the bar, then you stretch it and compare the length with a ruler). Once you have the circumference, you divide it by 3.14159 (or π) and compare with some standard bar diameters to eliminate the measurement error.
    – Mike
    Sep 11, 2018 at 11:53

4 Answers 4

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That looks like a 31.8mm clamp diameter, but the only way to be sure is to measure.

Consider buying calipers. They're a useful tool, and tools are good to have. Otherwise, you can do this: take a strip of paper, fold it around the bar (at the thickest part, in the center), and mark the place on the paper where the two ends meet. Then put the paper flat, and measure from the end to the mark you just made. That will give you the circumference. Divide that by π (~3.14159) and you'll have the diameter. This isn't too accurate, but there's only a small set of sizes to choose from.

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  • It's around somewhere between 10 and 10.5 cm, that means my handlebar is 31.8, right?
    – count_campeador
    Apr 9, 2016 at 21:57
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    Most bars are either 25.4, 26.0 or 31.8 mm. 100 / π ~= 31.8, so that must be it.
    – Mike Baranczak
    Apr 9, 2016 at 23:59
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Is it a road dropbar or a MTB riser style bar? These two images should prove useful. If it is a mountain bike style riser or straight bar it's quite simple.

If it's a road bar most people (zipp) measure center to center of the drops.

enter image description here

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Also if it is a Mountain bike were speaking of, most bars will be sold as 730mm or more wide and have pre-printed lines to cut them shorter. There is a lot of preference involved in bar length. I prefer a shorter length than a lot of people my size, some downhill guys prefer 800mm bars, too long for me, I'm good at finding trees! :P

A good reference if its a road bar is about shoulder width in the drops. This can vary with preference and riding style slightly but is a solid method for the best on bike ergonomics and comfort.

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    Excellent answer. In addition to trees, wide MTB bars are good at finding wing mirrors, pillars/bollards, pedestrians, and other cyclists in close quarters.
    – Criggie ♦
    Apr 9, 2016 at 0:56
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Calipers. A tape measure.

Usually the stem clamp width is 25.4mm MTB and road. Rarely it could be 26mm. At the ends it goes down to 22mm.

You might have an oversize bar in which case it is 31.8mm.

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    He said width :) but good advice to double check clamp diameter as well.
    – Nate W
    Apr 8, 2016 at 22:15
  • Is Stem clamp the middle thick area? I don't have calipers, how can I measure it?
    – count_campeador
    Apr 9, 2016 at 12:23
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    You can use a scrap piece of paper. Use scissors to cut the paper into a narrow strip 1/2" x 6" long [15mm x 150mm]. Wrap the strip around the bar but let the strip spiral a little bit. Use a ball point pen to leave a mark on the paper where the wrap overlaps. You will be marking the paper in two places with a single stroke of the pen. When you have removed the paper you have marked the circumference of the bars. Measure the distance with a ruler. Diameter = circumference / pi (3.14).
    – zipzit
    Apr 9, 2016 at 13:51
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If you don't have a pair of calipers a crescent wrench will accomplish much the same thing.

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    You're totally right, but this answer is really short and doesn't say where to measure. Probably would have been better as a comment on the original question.
    – Criggie ♦
    Sep 6, 2018 at 10:45
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    We're looking for answers with more detail. Please consider expanding your answer to further elaborate on how and where you would use the wrench to measure. A short answer like this is likely to get downvoted, flagged for moderator intervention, and possibly deleted. Welcome to the site and please consider reviewing the tour page for more information on how StackExchange sites work.
    – Gary.Ray ♦
    Sep 19, 2018 at 18:11

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