Understanding wheel arch width: the measurement that limits most loads
Open the boot of most cars and you'll see two humps on either side of the floor. They're the housings for the rear wheels and suspension, called wheel arches (or wheel wells, or wheelhouses, in the US). The boot may be generously wide elsewhere, but the gap between the arches is usually the narrowest point of the load floor. For anything that sits flat on the floor, that gap is the width that counts.
Why the narrowest point matters
A rigid box can't squeeze past an obstruction. It needs to fit through the tightest point along its whole path. If your boot is 130 cm wide behind the arches but only 100 cm between them, any box sitting on the floor and running past the arches is limited to 100 cm wide, less your clearance.
That's why the car boot fit calculator asks for the narrowest width, and why it treats that figure as the width available for the full length of the item. It's a cautious assumption: it may say "might fit" or "no fit found" for an item that could actually sit above the arches. It won't tell you something fits when the arches would stop it.
How to measure between the arches
- Empty the boot and remove any side panels or organisers that sit between the arches.
- Find the innermost point of each arch's trim. It's usually partway up the hump, not at the floor.
- Hold the tape level and straight across the car, at right angles to the length of the boot.
- Measure between those innermost points. Repeat a few centimetres forward and back to find the true minimum.
- Write down that minimum as your narrowest width (measurement B).
Measuring above the arches
Wheel arches only rise so high. Above them the boot often widens out to the side panels or windows. For thin, flat items, such as a large picture, a sheet of plywood or a table top, that extra width can be usable:
- Measure the height of the arches from the boot floor to their highest point.
- Measure the width just above the arches, side panel to side panel, keeping the tape level.
- Check that width all the way along the boot. Side panels often curve inwards towards the top and towards the tailgate.
A hypothetical example: arches 22 cm high, 100 cm apart, with 125 cm of width just above them. A board 120 × 80 × 2 cm could lie flat on top of the arches (120 cm across, 80 cm along the car) if it can get through the opening and the floor length allows it. But it would need something under the middle to stop it sagging or rattling. Our calculator wouldn't report that as a straight fit, because it uses the 100 cm minimum. Treat a load like this as a manual check.
Got your measurements? Use the boot dimension checker
Open calculatorHow wheel arch width affects common items
| Item | Why arch width matters | Typical workaround |
|---|---|---|
| Flat-pack boxes | Wide boxes lying flat must pass between the arches | Load on edge if height allows, or slide the box over the arches if thin |
| Mattresses (flat) | Most mattresses are wider than the arch gap | On edge if opening height allows, or use a different vehicle |
| Bikes | Handlebars and pedals catch on the arches | Remove the front wheel and turn the bars |
| Pushchairs (strollers) | Wide folded frames can catch | Fold fully and load at an angle |
| Boxes standing on the floor | Two boxes side by side must share the arch gap | Put the wider box behind the arches, nearer the seats, if the boot widens there |
Seats folded
With the rear seats folded, the load floor extends forward into the cabin, where it's often wider than between the arches. But a long item still passes the arches on its way in, and part of it stays beside them. Unless the item sits entirely forward of the arches, the arch gap still limits its width.
Key points
- Measure the narrowest gap, usually between the arch trims, not the widest part of the boot.
- For thin items, also note arch height and the width above them.
- Width higher up can help, but check the boot opening as well. See how to measure a boot opening.
- When in doubt, make a cardboard template at the item's width and test it.