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I've spotted differences in the ways sites handle ERD
There are also differences in how rim manufacturers handle ERD. Some measure to the face of the spoke bed, some give the dimension to where the spoke should end if you use the right nipples (true ERD). Obviously calculators will give different results depending on which assumption they make. A rim with a spoke bed of 600mm (which may well be what the manufacturer lists as "ERD") will have a true ERD of about 604mm.
gbj_tester
I've spotted differences in the ways sites handle ERD. Some sites treat you like a clever person that has managed to measure from rim-bed to rim-bed (i.e. nipple surface), whilst other sites assume that you've only measured the inside diameter of your rim so they helpfully add 2, 3 or 4mm to your ERD measurement and then spit out spoke lengths 1-2mm too long.
There may also be some other tom-foolery happening beside this particular tom-foolery.