To size a running armband correctly, measure the circumference of your upper arm (bicep) at its widest point using a soft tape measure or a piece of string, then compare that number to the size range listed on the armband. Most beginner sizing mistakes come from skipping this step and just guessing a size.
It only takes a minute, and it's the single best way to avoid buying an armband that's too loose to stay put or too tight to be comfortable on a long run.
Why does armband sizing matter so much?
A running armband only works if it stays snug against your arm without cutting off circulation. Too loose, and it slides down or twists during your stride — the same problem covered in why a running armband keeps slipping. Too tight, and it can pinch, dig in, or feel uncomfortable well before you finish a run, which is the flip side of the fit question we answer in how tight should a running armband be. Getting the size right from the start solves both problems before they happen.
How do I measure my arm for a running armband?
Here's the simple process:
- Find the widest point of your upper arm. This is usually the middle of your bicep, roughly halfway between your shoulder and elbow.
- Flex slightly, then relax. Measure with your arm in a natural, relaxed position — not fully flexed — since that's how it will sit while you run.
- Wrap a soft measuring tape around that point. If you don't have one, use a piece of string or ribbon, wrap it around your arm, mark where it overlaps, then measure that length against a ruler.
- Write down the number in both inches and centimeters, since brands list sizing differently.
- Check the product's size chart, not just "small/medium/large" labels — look at the actual arm circumference range each size covers.
- When you're between two sizes, sizing up is usually the safer choice, since most armbands have an adjustable strap that can cinch down further.
What if I'm between two sizes?
Go with the larger size if your measurement falls close to the line between two ranges. An armband with a bit of extra strap length is easy to tighten with velcro or an adjustable buckle. An armband that's genuinely too small for your arm, on the other hand, can't be stretched to fit — it will either not close comfortably or sit too tight to wear for a full run.
Does arm size change how the armband feels while running?
Yes. An armband sized correctly for your arm distributes pressure evenly around the strap, which keeps it from becoming a pressure point. That's part of why we designed the LULURUN armband to rotate 360° — even with a good size match, your arm moves differently mile to mile (warming up, swinging your arms harder on hills), and a strap that can rotate keeps the phone facing you instead of twisting toward your elbow or shoulder as your arm shifts.
Does bicep measurement account for muscle growth or swelling?
If you're just starting to run regularly, your arms may change slightly over weeks or months. That's normal and rarely enough to require a different size. If you notice your usual armband suddenly feels tight, it's more often due to a warm day and mild swelling from exercise than a real change in arm size — in that case, loosen the strap by a notch rather than assuming you need a new size.
A quick sizing checklist before you buy
- Measure your bicep at its widest point, arm relaxed, not flexed.
- Record the number in both inches and cm.
- Compare against the specific size chart for the armband, not a generic S/M/L guess.
- If you're between sizes, choose the larger option and tighten the strap.
- Look for an adjustable strap so small day-to-day changes don't matter.
Getting the size right the first time means less fiddling with your armband and more time actually enjoying the run — which is the whole point.
FAQ
What if I don't have a tape measure? A piece of string or a shoelace works fine — wrap it around the widest point of your upper arm, mark the overlap, then measure that length against a ruler or yardstick.
Should I measure over a sleeve or bare skin? Measure on bare skin. If you usually run in long sleeves during colder months, you can round up slightly to leave a little extra room.
Is a bigger armband always safer to buy than a smaller one? Generally yes, since an adjustable strap can tighten a slightly large armband but can't loosen a genuinely too-small one. Still, aim for your true measurement first rather than sizing up by default — an armband that starts too loose has less strap left to cinch down when you actually need it snug.
For more on carrying your phone comfortably on every run, see our complete guide to carrying your phone while running, or check out the LULURUN 360° running armband built to fit a range of arm sizes with a fully adjustable strap.
Run happy, run free.