Sure, get a helper. Have the helper take a measuring tape and run it in a loop around the widest part of your heel across to where the foot joins the leg. You want to be standing upright and not flexing forward - flexing forward enlarges the measurement - so a helper is pretty much required.
A long time ago we used to have tables like the one I link to below - BUT - don't be surprised if you go under or over depending on the instep volume. Salomon's sizing system was
never intended to map directly to foot lengths. I have a longer and wider foot than
@mdf - but I only measure out to 335 in HIP.
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