How do you measure a truck bed for a tonneau cover?

Measure inside the bed, from the inside face of the bulkhead (the wall behind the cab) to the inside face of the closed tailgate, at the top rail. That inside length is the number cover makers build to. Then measure the inside width at the top, and the width between the wheelhouses at the floor.

Do you measure a truck bed inside or outside?

Inside. Fitment dimensions are always inside dimensions: bulkhead to tailgate, rail to rail, and between the wheelhouses.

How to measure a truck bed

You need one tape measure and two minutes. Every measurement is an inside measurement — that is the only kind accessory makers build to.

1. Length: bulkhead to tailgate

Stand in the bed or lean over the side. Hook the tape on the inside face of the bulkhead — the wall directly behind the cab — and run it to the inside face of the closed tailgate, along the top of the bed floor or just under the rail. Write down the figure to the nearest tenth of an inch. This is the number that decides your tonneau cover class, and it is the number published on every truck page on this site.

2. Width: rail to rail

Measure across the bed at its widest point, inside face to inside face, usually just behind the cab. This is the bed's true width.

3. Between the wheelhouses

Measure the narrow gap between the two wheelhouse humps at floor level. This is the number that decides whether plywood, a mattress, or a wide toolbox actually fits flat — on most full-size trucks it is close to 50 inches, but close is not a measurement.

Things that change the numbers

Drop-in bed liners sit on top of the steel and can eat measurable space at the bulkhead and tailgate. Measure the liner's inside faces, not the steel, if the liner stays.
RamBox and in-bed storage systems narrow the usable width dramatically. Measure the opening you actually have.
Fifth-wheel and gooseneck hitches do not change the dimensions but do change which covers fold clear — check the cover's folded position against your hitch.

Then check it against the chart

Compare your three numbers with your truck's page in the bed size chart. If your tape disagrees with the published figure by more than a small tolerance, trust your tape for your truck — and tell us, so we can verify the record.