Three measurements decide whether a crate fits: the depth along the floor, the narrowest width, and the height at the lowest point of the opening. Boot volume in litres won't tell you.
Open the fit checker →A tape measure, the rear seats in the upright position you'll actually drive with, and the boot floor in the position you'll use (some cars, like the BYD Atto 3, have a floor board that can sit higher or lower).
Measure along the floor from the base of the rear seat backs to the inside of the closed tailgate or boot lid. Thule's own advice is to measure from the back of the rear seats to the end of the boot floor, and to check where the boot door starts to slope in.
Measure across the floor at its narrowest point. That is usually between the wheel arches, but sometimes it's the opening itself. Use whichever is smaller.
Measure from the boot floor straight up to the lowest edge of the opening: the tailgate seal, trim or latch. Watch for low-hanging edges. A crate can fit inside the boot and still fail to go through the opening.
Many SUVs and hatchbacks have rear seat backs that lean into the boot. A crate sits on the floor, but its top edge is higher up, where the space is shorter. Measure the depth a second time at about the height of the crates you're looking at, and note how high you took it. The fit checker uses both numbers. More on sloping seats.
Boot volume measures the whole space, including corners a rigid crate can't use. Two cars with the same litres can take very different crates: a deep, low boot and a short, tall one give similar figures but suit completely different crates.
Sources: Thule Allax measuring advice; 4pets measuring guide.