When is a 25 inch inner tube not a 25 inch inner tube? Answer: when it's a 26 inch inner tube! Yep, you're probably totally baffled reading that, however let me explain. Without going deep into the history of cycle tyres, standardisation and decimalisation, what you need to know is when it comes to the 25 inch size of tyre and tube, not is always what it seems.
A common wheelchair tyre size is 25 x 1 (expressed in imperial sizing / inches). When expressed in millimetres it's 25-559 (this is as per ETRTO sizing).
All straightforward so far? Hopefully! So if you need a new tube for your tyre, you're looking for:
25 x 1 (25-559)
However, when it comes to buying this size, it can be a little tricky.
Because an inner tube dialties to fit a variety of sizes, many inner tubes can not only fit a variety of widths but in some cases they are fit a variety of diameters to.
So what can can see are multple sizes on the front of an inner tube box. With one of the best inner tube brands in the business, Schwalbe, this is what you'll see.
This is where the confusion of imperial sizing and cycling's long history of strange sizing kicks in. A common tube used to fit the 25 x 1 (25-559) tube is the Schwalbe No.12A. However, if you look at the box front below, it's marked up exclusively as 26"! Because 559 is also 26" in the inner tube world which is baffling as 26" can also be 590 in ETRTO and on it goes.
When it comes to replacing an inner tube, ALWAYS check the ETRTO size on the tyre matches what you are replacing it with. Because the ETRTO size is not open to interpretation in the same way the old imperial sizing system can be given that ETRTO is a member of ISO, which is the International Standards Organisation. Next time you're looking for a 25 inch wheelchair tube, make life simple and just check the ETRTO size is on the box cover and you'll be good to go!