# Why do the ISO/ETRTO numbers not match the English measure on my Tires?

Given:

• A flat tire
• Tire size 47-507, 24 x 1.75

I want to replace both the tire and the tube. But in the shop it's only available 44-507, 24 x 1.75.

The tire size of 24 x 1.75 is common to both my current tire and the potential replacement, so that matches up.

What is the difference between the marking on the tires of 47-507 vs 44-507 and does it have any bearing on the compatibility beyond the actual size of the tire?

• Does this answer your question? What is the maximum tire or minimum tire width I can fit on my bicycle Mar 18, 2023 at 12:52
• Not really. I've reread it 2 times and still can't figure it out. I see my sizes and see the table and the answer and can't match it. I only approximately understand what all the numbers mean "47-507, 24 x 1.75". I see that the answer that you posted is well written. I believe that if I spend time reading Wikipedia and watching some YouTube videos, do more research I can actually get an answer from the answer. Mar 18, 2023 at 17:25
• Still a useful experience because now I better understand newbies on Stack Overflow and why they can't get an answer from well written articles Mar 18, 2023 at 17:29
• @Evmorov please don't be put off - bike tyre sizing is a horrible mess, and has had hundreds of years to get even more complex. Since you have accurate ETRTO measurements then use those - the old inch-based sizing is often chock-full of rounding errors. To the point 1.75" is not equal to 1 3/4" Yeah - it's crazy.
– Criggie
Mar 18, 2023 at 22:53
• Tire sizing makes much more sense when you accept that it is completely nonsensical Mar 19, 2023 at 17:41

The 44-507 and 47-507 are "ISO/ETRTO" tire sizes. The 44 and 47 are widths in millimeters, and the 507 just relates to the 24 inch diameter.

So the 44 "should" be a little narrower than the 47. The reason for the same tire width (1.75) in the English measurement of is due to rounding (or laziness?) by the tire company. The English measurement for the "44" would actually be closer to a 24 x 1.625 instead of a 24 x 1.75. So expect it to be a touch narrower even though it is labeled as a 1.75.

A difference that small will not be a problem in 99 percent of cases, so you can proceed with using the "44" as long as you can accept that it "may" be about a tenth of an inch (thereabouts) narrower.

• Additionally, the same tyre can vary in width by up to 10% when mounted on different rims. Its definitely a "nominal width" whereas the Bead Seat Diameter has to be exactly right
– Criggie
Mar 18, 2023 at 22:55
• 44 instead of 47 will not likely be a problem. The other way around it will depend a lot of the space around the tire, and a few mm difference in size can be a problem. Mar 19, 2023 at 13:20
• One thing to keep in mind is that the larger number in the ETRTO system, which is the bead seat diameter of the tire MUST be the same for any potential replacements. The rim ETRTO bead seat diameter must be identical to the tire's as well. Width, of course matters in regards to frame clearance (both laterally and radially because wider tires generally have a larger outside diameter) and there is a finite range of tire widths that should be used with a given internal width of a rim. But the bead seat is primary importance because if it differs, it won't work.
– Jeff
Mar 20, 2023 at 14:12