What you need
A five-metre tape, a straight edge or spirit level, and a flat, level surface. A second person makes it faster but is not essential.
Do not use manufacturer brochure figures alone. They usually exclude mirrors, aftermarket spoilers, tow bars and roof bars — all of which change how a cover sits.
Length
Measure horizontally from the furthest forward point to the furthest rearward point, including a tow bar if one is fitted and cannot be removed. Do not follow the curve of the bodywork; a tape draped over the bonnet adds a false 100 mm or more.
Drop a plumb line or hold a straight edge vertically at each end and measure between those two points along the ground.
Width and height
Width is measured across the widest point of the body excluding mirrors, because mirror pockets take those separately. On vehicles with pronounced arches, that widest point is usually at the rear arch rather than the door.
Height is measured from the ground to the highest fixed point — the roof, or roof bars if they stay on the car. Aerials that fold or unscrew are ignored; fixed shark-fin aerials are included.
The three common mistakes
Measuring over the curve rather than point to point. Forgetting a permanently fitted tow bar. Measuring a loaded car with roof bars fitted, then removing the bars before covering it — the cover is then too tall and sits loose across the roof.