What hlw is trying to say is a standard unwrapped M A or B belt are measured by the inside length
So if you order a 100" belt in M , A & B sections they will measure 101.75" , 102" & 103" on the outside
Fractional horsepower belts as used on garden equipment OTOH are measured on the outside so the same 3 belts will all be 100" around the outside . ( hopefully )
The EU uses effective length which is the circumference approximately 1/3 the depth from the back of the belt
Next problem is that is a "nominal" length so it can be +/- up to 1% depending upon the actual length ( apparently 2% to 5% if they originate in India ).
Following that very few Fractional Horsepower belts are actually true M, A or B sections
Most are L section which is metric
And then there is MTD which use belts unique to MTD
To add to that, unless the belt has a brand & part number burned into the belt they are churned out in the millions , tied in scanes or packed in bales then shipped off to have the brand & size / part number decal / stencel applied to the belt and this can go horribly wrong
The reject ones are the ones you see on Amazon / Ebay as "unbranded "
Belts are bulk shipped then put in the sleeves either at the warehouse or a sheltered workshop so another chance for mislabeling .
Now Gates make Lawn & Garden belts and they have a web site that lists most belts crossed by the mowers part number and ( sometimes ) the Gates number will link to their catalogue specification section that lists the dimensions & section of that belt
The Mower belts will have a BR ( Blade Runner ) prefix or suffix to the 4 digit number
Rotary resells a lot of Gates BR belts in their premium OEM replacement range
Gates catalogue
So if you use the link above you click on the red "Vehicle Parts Search" button which brings up another search window
You click on the "Interchange" box which brings up another window where you type in the mower makers part number
If the computer recognises the part number it will bring up a result window with one of the BR numbers underlined and that is the link to the specifications section.
For me where everything is secret or I am geobanned the Gates site has been a Godsend , when it works .
If the belt is not listed then I use a length of 1/2" or 5/8" rope around all of the pulleys with the tension pulley at about 1/2 of its travel then measure the rope
Worked most of the time .