Belts
The largest flat project most people attempt. Simple in principle and long in practice.
Length and width
Waist measurement plus the buckle, plus about fifteen centimetres of tail. Width follows the buckle: a forty millimetre buckle needs a forty millimetre weave, and adjusting the weave later is not possible.
Cord
A belt uses far more cord than any other project on this site. Value packs in thirty metre lengths exist for exactly this reason: value packs (https://www.123paracord.co.uk/paracord/paracord-550-type-iii/value-pack/paracord-550-30-mtr/).
Keeping it even
Width drifts over a long piece. Checking every ten centimetres against a marked strip of card catches it while it can still be corrected.
Buckles
Belt buckles are listed separately from the side release range: belt buckles (https://www.123paracord.co.uk/paracord-buckles/belt-buckle/).