hold both of your belts side by side in one hand
Draw a line across both of them ( I use a white paint pen )
Now while holding them tightly together with one had pull then through the other say 1 foot
Without letting the belts loose move your hands together then pull another foot through.
Continue shuffleing the belts through your hands till you get back to the line you drew
It should be a straitish line across both belts if they are the same length .
I allow about 1/4" of difference ( old slightly longer ) .
In theory V belts do not stretch, they get thinner so sit deeper in the grooves that become loose and people assume they have stretched .
It takes a little bit of practice because the old belt will be loose & very flexiable while the new belt will be quite stiff with some hard bends where it was folded for storeage .
My go to for belt dimensions is always the Gates Catalogue as it shows the real length, the outside length & the effective length plus the actual belt width & angle plus depth .
Gates do make exact OEM spec belts in their Lawn & garden range in this case it is a
6458BR and of course the size look up is currently not working ( Murphy strikes again)
Now UK ( & OZ ) measure belts by their inner length & the USA measure them by their out side length and on a 1/2 " belt that is ≈ 2" longer than the inside length
Getting my head around belts was by far the hardest part of becoming a mower tech .