How to buy bike tires sizing - buying bicycle tyres size

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Notes on buying tyres- the scope of this guide is tyre size (there are various styles of tyres - tread patterns, construction etc. - but nothing at all is as important as getting the right "BSD" - explained below, without that the tyre just won't fit your wheel) 
various way of measuring tyres over different countries - and dating back as much as 100 years or so - has left a confusing historical legacy. 
for eg. there are 5 or 6 different sizes of 26" tyre and both 28 and 29 (-er) inch go on smaller wheels than 27".
Unless you're a real tire geek, you can't rely on the "inches" of a tyre giving you the right size for your wheel (rim)

the ISO method of measuring tyres is the best -  and the only one to really be relied upon, It's expressed as two numbers separated by a dash. The first number relates to the tyre width and the second relates to the size of the 'hole' in the middle of the tyre (known as the BSD or Bead Seat Diameter- which is the diameter of the inside of the tyre where you can feel a wire runing through)
for eg.  40-559  is a tyre of 40mm width and 559mm BSD
although there are ideals of rim (wheel) width to tyre width, in practice the width of a tyre is very forgiving, for eg. the same wheel that takes a 28mm width tyre will probably hold a 50mm width tyre 
BUT it is this 2nd number (the BSD will be a value between 200 and 635 ) which is absolutely critical to the tyre you need to fit your wheel. Don't measure the tyre - just look for that number on the old tyre already fitted to that wheel.

Wheels will often give their BSD size (on the rim) - but in the reverse order - for eg. 559 -17 (here the first number - the larger one between about 200 and 635) gives the BSD (this needs to match the BSD on the tyre you are buying). The second number on the wheel rim relates to the internal width of the rim and is much less important for fit- it normally will be about half the size of the tyre width  - give or take.  

you'll see sizes expressed in the older terminologies on listings and tires too (for eg. a 622 tyre can often be called a 28" a 29" or a 700c). But w hen you are looking for a tyre, it's best to  determine the  BSD  of  your wheel and make sure it matches the tyre you intend to purchase, then you can't go wrong  
559 (26" mountain bike) or 622 (700c/28"/29-er) will be by far the most common for modern adult bikes)

try searching ' tire sizing systems sheldon brown ' for more (much more!) info. and crib sheets which relate all the older designations to ISO designations (i.e translates from old money to new money) - and pics of the BSD i describe above

there's also some advice for ideal tyre rim widths, if you're unsure and don't want to change the width of the tyre (if you don't know why you'd want to go for a different width or how to go about that -probably just don't ) it's advisable to go for the exact same tyre that the old one is according to the two numbers in the ISO 
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