How much boot do you need: turning a vague worry into a number

A quoted boot volume is the result of filling the load bay with small blocks and counting them. The German VDA method uses blocks of 200 by 100 by 50 mm — one litre each — packed in until no more will go. Repeatability is its virtue. Measuring a shape nobody ever loads is its limit.

Two conventions matter more than the number itself. Seats up is normally quoted to the top of the parcel shelf, and that is the honest figure, because it is the space you can fill and still see out of the back window. Seats down is normally quoted loaded to the roof, which nobody does. So a seats-down figure rewards tall boxes over long ones, and it is the number most likely to mislead you across two brands that did not use the same method.

Experts disagree about how comparable these figures are between makers, and the disagreement is real rather than pedantic. The decision rule that survives it: compare seats-up figures, compare within a brand wherever you can, and treat a gap of a few tens of litres between two different brands as noise rather than fact.

Across the 101 cars here, boot volume runs from 130 litres to 1,844 litres, with a median of 451. Those three numbers are worth holding on to, because they set the scale for everything below.

130 litres is a small car with room for a rucksack and a shop. 451 litres, the middle of the list, takes a weekly supermarket run for a family or a weekend away for two without argument. 1,844 litres is the largest figure here: a load bay that replaces a van trip, in a car you will be aware of every time you park it.

Seats sit alongside that figure and change it. The catalogue runs from 2 to 7 seats, median 5, and a volume quoted with every seat in place is a different measurement from one quoted with a row folded. In a seven-seater, the gap between those two states is usually the gap between luggage for two people and luggage for seven.

1. 1

   Load your worst normal week

   Not your worst day ever. Put the real load in the boot you have now — the shop, the bags, the pushchair, the dog crate, the work kit — and photograph it.

2. 2

   Measure the things that do not bend

   Rigid objects set the requirement: a folded pushchair, a dog crate, golf clubs, a cello, a wheelchair, a box of tools. Measure each one in three dimensions and keep the numbers on your phone. Soft bags fill whatever is left over.

3. 3

   Convert to litres

   Multiply length by width by height in centimetres and divide by 1,000. A box 60 by 40 by 30 cm is 72 litres. Add the rigid items together, then add about half as much again for the soft luggage that has to pack around them — a rule of thumb rather than a law, and one that errs on the safe side.

4. 4

   Take the tape to the car, not the brochure

   On any car you are serious about, measure the width between the wheel arches, the floor length from the seat backs to the tailgate, and the height of the load lip off the ground. Those three numbers beat any single litre figure ever printed.

5. 5

   Fit the awkward item before you pay

   Carry the pushchair or the crate to the test drive and put it in. Two minutes settles what the brochure cannot. How to test-drive a car covers what else to do with the rest of the appointment.

Two boots quoting the same litres can hold quite different luggage. Six things decide which one you get.

• Shape. Suitcases are rigid rectangles. They want floor length and width, not headroom, so a tall narrow boot and a long flat one of the same volume are not interchangeable.
• The load lip. How high you lift to, and whether there is a step down on the far side. A low lip with no step is what lets you slide a crate in rather than lift it over.
• The opening. A wide, square tailgate aperture beats a larger boot you have to post things into at an angle past the light clusters.
• Under the floor. A full-size spare wheel takes a real bite out of the space. A repair kit gives the space back and leaves you with a repair kit. Decide which you want before you find out which you have.
• The parcel shelf. If it does not stow somewhere in the car, it lives in your hallway on exactly the day you needed the height.
• Whether the seats fold flat. A folded seat that leaves a slope or a step turns a big number into an awkward shape.

Body style is where most of this is decided, and the trade is not the one the litre figures suggest. SUV, hatchback or estate covers which shape puts the volume where you can reach it.

Who travels changes the boot you need twice over. More people means more luggage, and the seats those people sit in occupy the space the luggage was going to use.

Third rows are the clearest case. Seven seats up leaves a load bay for shopping rather than suitcases. Seven seats folded means carrying the weight and the mechanism every day of the year for the handful of days it is needed. Neither is wrong; both are a cost that the seat count on a spec sheet does not show you.

If six or seven seats are a weekly fact, that is a real requirement and the finder asks about it directly, along with how much boot you need. If they are a twice-a-year fact, a five-seat car with the right boot and a second trip is usually cheaper to buy and cheaper to keep — what a car costs to run sets out how much cheaper. Where the load is already the binding constraint, several of the prepared answers start from a big boot rather than ending at one.