Almost every tool box that goes back was measured with a tape once, across the wrong two points. Four numbers decide whether a box fits, and none of them is the bed length printed in the brochure.
Which Measurements Do You Need?
Rail to rail width at the front of the bed, wheel arch to wheel arch width, bed floor to rail height and the clear length behind the cab. Take all four before looking at any product, because a box that passes on three and fails on one does not fit.
Rail to Rail Is Not One Number
A crossover box spans the bed rails, so the width that matters is the outside to outside distance across the top of both rails, measured at the point the box will sit. Beds are not perfectly parallel, and the measurement at the bulkhead can differ from the measurement at the tailgate.
- Measure across the top of the rails, not across the inside of the bed.
- Take it at the front of the bed where the box will actually mount.
- Note whether a bed liner adds height or thickness to the rail.
- Check for a rail cap, since it changes what a J-bolt has to hook under.
The Wheel Arch Number
Boxes that sit down in the bed are limited by the wheel arches rather than the rails. Measure between the arches at their widest intrusion and treat that as your ceiling for anything sitting on the floor, including drawer units and chest boxes.
On a full-size truck this is often the number that rules out a box the rails would have accepted, and it is the one most people never take. Take it at the narrowest point of the arch rather than at its end, and take it again at the height the box will actually sit, because the arch tapers.
Height Depends on What Else Is Fitted
If a cover is going on, the height that matters is from the bed floor to the underside of the closed cover, not to the top of the rail. Those are different numbers. If no cover is planned, the constraint becomes the rear window, since a tall box behind the cab blocks it.
Measure the box at its tallest point when you compare, which is usually the latch or the lid seam rather than the lid surface itself.
Then Check the Length
Clear length behind the cab decides whether a box leaves you usable floor. A 5.5 ft bed with a full-depth crossover has very little left, which is why low profile and lo-side boxes exist. Work out what you still need to carry before choosing a depth.
With the four numbers in hand, compare crossover boxes, full-size boxes and mid-size boxes.