Handlebars are spec'd. by the clamp diameter. The common sizes are 1" (25.4mm) common on stems from Japan and Taiwan, 26mm, the currently most common size used on European road bikes, and 31.8mm which is the newest size which was introduced with carbon bars.
Measure your bars right where they emerge from the clamp, or post the brand of the stem here, and someone will advise. Best bet, do both and one will confirm the other.
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