How car boot capacity is calculated and what the numbers really mean

Measuring boot space in litres is not much practical use for the individual motorist or the family. It’s not like the boot will be used as a swimming pool. But measuring luggage capacity in litres at least provides buyers with a means to compare different cars on their consideration list.

As a rough guide, any small hatch with a boot greater than 300 litres is average and more than 400L is very good. Traditional family sedans of the larger kind sit around the 500L mark, and roomier mid-size SUVs range from 400 to 500L or higher.

Kia Australia quotes two different measurements for its K4 sedan: 502 litres and 413 litres. One figure is based on the VDA standard (502L) and the other is calculated using the SAE method (413L).

It’s unusual to quote both figures. Kia itself cites VDA figures alone for most of its model range. The K4 is atypical.

So right about now, you’re probably asking “Well, what’s this ‘VDA’ and ‘SAE’ stuff anyway?”

Here’s some trivia: VDA (Verband der Automobilindustrie) is the German abbreviation of ‘Association of the Automotive Industry’. Later adopted by the International Standards Organisation (ISO) and the German standard DIN (Deutsche Institut für Normung), the VDA method has also been picked up by many other European, Japanese and Korean manufacturers.

Engineers calculate the volume of the boot space by filling it with one-litre blocks. Each block measures 200mm x 50mm x 100mm. The amount of blocks that can fit inside the boot at one time is the VDA number. Since the engineers are paid by the car manufacturer, they take particular care to stack as many blocks in the boot as possible because a higher number always looks good on the spec sheet.

The SAE standard has been around for more than 40 years and was devised by the US-based Society of Automotive Engineers (SAE). So entrenched is the SAE standard after all that time, companies designing and manufacturing cars in the Americas still use this technique to calculate ‘trunk space’.

For closed luggage areas like a sedan boot the SAE method also simulates packing with boxes. However, the boxes used represent differently sized luggage pieces from a 67-litre suitcase to a 5.6-litre shoebox. There’s even a solid equivalent of a golf bag.

For open luggage areas SAE volumes move from the proven loaded volumes to an estimation of volume called a Cargo Volume Index. The luggage space is calculated by multiplying the height, width and length dimensions between specific points in the load space which are dependent on the vehicle body style (hatch, wagon, SUV etc).

But wait... There’s more to loading a boot, of course, than just throwing in stuff until reaching the luggage capacity cited by either standard.

On the face of it, the SAE method may be more realistic, but if you’ve ever moved house you’ll have jammed linen, clothing, cutlery, toys and small trinkets into every last nook and cranny in your car’s boot. And you’ll have likely exceeded the VDA measure in the process – Australians are internationally renowned for overloading vehicles, after all.

Which is why car companies make allowance for that in spec sheets and brochures. There are sometimes different numbers quoted in a single brochure for cars and SUVs with folding seats and luggage space that can extend into the cabin.

Those figures could be the volume up to the top of the rear seat, the volume up to the height with the rear seats folded down and the ‘maximum cargo volume’ which is with seats folded down and loaded to the roof.

Oh, and don’t forget at the other end of the scale, the people movers and seven-seat SUVs which may have as little as 150 litres of luggage space behind the third row of seats when they’re in use.

All of this explains in part the discrepancy between Ford’s now-defunct Territory and BMW’s dimensionally similar X5. The Ford’s brochure listed luggage capacity at 1153 litres, but the BMW’s boot space ranged from 650 litres right up to 1870 in the X5 brochure.

Ford essentially doubled the available cargo volume by measuring for goods stacked all the way up to the headlining behind the second-row seat.

In contrast, the X5’s figures were based on the volume up to the top of the rear seat only (650L), not all the way to the headlining. That’s why it was only about 56 per cent of the Territory’s.

But not to be outmanoeuvred by the chess players at Campbellfield, BMW also published the 1870-litre figure for consumers who wanted to know how much the X5 would carry by volume, if all the rear seats were lowered and the vehicle was filled to its absolute maximum.

So, you want to cart around your portable barbecue in the boot of that Mazda MX-5 you’ve had your eye on. You’re probably going to have to measure up the MX-5 in the showroom before you buy...

If luggage space is the deciding factor in your next car purchase, remember that when it comes to boot volume, things aren’t always as they first seem.

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