Size-based weight, often called volumetric weight, is a weight worked out from how much space an item takes up. On most door-to-door services it is length x width x height in centimetres, divided by 5,000. That figure is compared with what the item really weighs, and you pay for whichever is higher.
So a large suitcase holding 23 kg of clothes can be charged as 25.5 kg, and a golf bag that weighs 15 kg can be charged as 31 kg. Below we work through the sums for ten everyday items, show how wheels and handles push the number up, and explain what you can do about it.
Why size counts as well as weight
A van or an aircraft hold usually runs out of space before it reaches its weight limit. A big, light item takes up room that could have carried something heavier, so transport companies charge for the space as if it were weight. Size-based weight is simply how they turn space into kilograms, so that everything can be priced on one scale.
It is not a penalty aimed at luggage. It applies to parcels of every kind; luggage is just big for its weight more often than most.
How the sum works, step by step
- Measure the outside of the item at its widest points, in centimetres, including wheels, handles and pockets.
- Multiply length x width x height. This gives the volume in cubic centimetres.
- Divide by 5,000. The answer is the size-based weight in kilograms.
- Compare it with the real weight on the scales. The higher of the two counts.
- Round up to the next 0.5 kg. This is the chargeable weight, and it is what the price is built on.
Here it is for a large suitcase of 78 x 52 x 31 cm holding 23 kg:
- 78 x 52 x 31 = 125,736 cubic centimetres
- 125,736 divided by 5,000 = 25.15 kg
- 25.15 kg is more than the real 23 kg, so size wins
- 25.15 kg rounded up to the next 0.5 kg = 25.5 kg chargeable
A shortcut: 1 kg for every 5 litres
5,000 cubic centimetres is 5 litres. So the rule boils down to this: an item is charged at least 1 kg for every 5 litres of space it fills. A 100-litre box counts as at least 20 kg.
If your item weighs less than 1 kg per 5 litres, you are paying for its size. If it weighs more, you are paying for its weight. Clothes, bedding and sports kit are usually light for their size. Books, tools and kitchenware are usually heavy for theirs.
Ten common items, worked out
These are the ready-made sizes and typical weights on our price page. Your own items will differ, so measure them, but the pattern holds.
| Item | Size (cm) | Volume (cm³) | Size-based weight | Real weight | Charged on | Chargeable weight |
|---|
| Small suitcase (cabin) | 55 x 40 x 23 | 50,600 | 10.12 kg | 10 kg | Size | 10.5 kg |
| Medium suitcase | 68 x 46 x 28 | 87,584 | 17.52 kg | 18 kg | Real weight | 18 kg |
| Large suitcase | 78 x 52 x 31 | 125,736 | 25.15 kg | 23 kg | Size | 25.5 kg |
| Holdall or rucksack | 70 x 35 x 35 | 85,750 | 17.15 kg | 15 kg | Size | 17.5 kg |
| Small box | 45 x 35 x 30 | 47,250 | 9.45 kg | 10 kg | Real weight | 10 kg |
| Medium box | 55 x 45 x 40 | 99,000 | 19.80 kg | 15 kg | Size | 20 kg |
| Large box | 65 x 50 x 45 | 146,250 | 29.25 kg | 20 kg | Size | 29.5 kg |
| Golf bag | 125 x 35 x 35 | 153,125 | 30.63 kg | 15 kg | Size | 31 kg |
| Skis or snowboard in a bag | 180 x 30 x 20 | 108,000 | 21.60 kg | 12 kg | Size | 22 kg |
| Bike in a bike box | 140 x 80 x 25 | 280,000 | 56.00 kg | 18 kg | Size | 56 kg |
What the table tells you
- Eight of the ten are charged on size. Only the medium suitcase and the small box, packed to their typical weights, are charged on what they weigh.
- Suitcases and holdalls are close. A packed case is fairly dense, so the gap here is never more than 2.5 kg.
- Part-filled boxes and sports kit are not. A large box with 20 kg in it is charged as 29.5 kg. The golf bag is charged at about twice its weight, and the bike box at just over three times. Our guide to sending bikes, golf clubs and skis covers ways to cut those numbers.
How wheels and handles add centimetres
Suitcase makers usually quote the size of the body, the part that holds your things. Wheels, the pull-handle housing, carry handles and feet often sit outside that. When Consumer Reports measured 11 carry-on cases, 9 were bigger than their makers said, and the makers explained that their figures described packing space, not the full outside size (Consumer Reports, December 2015).
The sum uses the full outside size, so those extra centimetres count. Take a large case sold as 74 x 52 x 29 cm, packed to 23 kg:
| How it was measured | Size (cm) | Size-based weight | Chargeable weight |
|---|
| Body only, the maker's figure | 74 x 52 x 29 | 22.32 kg | 23 kg (real weight wins) |
| With wheels (4 cm) and handle housing (2 cm) | 78 x 52 x 31 | 25.15 kg | 25.5 kg (size wins) |
Six centimetres of plastic add 2.5 kg to the chargeable weight, and switch the case from being charged on weight to being charged on size. That is why the price page asks you to measure at the widest points, including wheels and handles, and why our guide to measuring and weighing luggage at home starts with the wheels.
Each item is worked out on its own
The sum is done item by item, and each item is rounded up on its own. You cannot add two items' volumes together, or let a heavy item cancel out a light one.
Take a large suitcase (23 kg real, 25.5 kg chargeable) and a holdall (15 kg real, 17.5 kg chargeable). The real weights add up to 38 kg, but the chargeable weights add up to 43 kg. If you send several items, it pays to look at each one.
Why 0.5 kg can matter
Transport prices usually rise in steps of weight rather than by the gram. If an item's chargeable weight lands just over the top of a step, it is priced on the next one up. One extra centimetre on a side, or a pair of shoes squeezed into a case that is already charged on its weight, can be enough. Change a size or a weight on the price page and you will see straight away whether the price moves.
Inches and pounds
The price page works in centimetres and kilograms. If your tape measure or the label on the case uses inches, multiply each figure by 2.54 and round up to the next whole centimetre before you do the sum.
A case of 30 x 20 x 12 inches becomes 77 x 51 x 31 cm (76.2, 50.8 and 30.48, each rounded up). That is 121,737 cubic centimetres, a size-based weight of 24.35 kg, so 24.5 kg chargeable. For weight, divide pounds by 2.2046 to get kilograms. Never mix the two systems in one sum: inches multiplied together and divided by 5,000 give a meaningless answer.
How to bring the chargeable weight down
- Use the smallest box that fits. Eight kilograms of things in a medium box (55 x 45 x 40 cm) are charged as 20 kg. The same 8 kg in a small box (45 x 35 x 30 cm) are charged as 9.5 kg.
- Fill the box you have. A large box is charged as 29.5 kg whether it holds 12 kg or 20 kg. Until the real weight passes the size-based weight, extra things in the same box add nothing to the chargeable weight.
- Mix heavy and light. Books on their own are heavy for their size; a duvet is light for its size. Packed together, with the heavy things low, a box gets closer to 1 kg per 5 litres and you pay for less empty space. Our guide to packing a box for shipping shows how.
- Think before boxing a suitcase. A box around a case adds centimetres on every side. We work through the numbers in box or suitcase.
- Make long items shorter. Pedals off, handlebars turned and the front wheel out can shorten a bike box. Every 10 cm off the length of a 140 x 80 x 25 cm box takes 4 kg off its chargeable weight.
Why sizes and weights get checked
The price you see is built from the sizes and weights you enter. Our transport partners measure and weigh items as they travel and charge on what they find, so if an item turns out bigger or heavier than entered, the price can change. Careful measuring at home is the simple way to avoid that.
Questions people ask
Is volumetric weight the same as size-based weight?
Yes. Volumetric weight, dimensional weight and size-based weight are three names for the same idea: a weight worked out from an item's size.
Do all services divide by 5,000?
Most door-to-door services do. Some use a different figure, such as 4,000 or 6,000, and a smaller divisor gives a higher size-based weight. You do not need to work this out for each one: the price page applies each service's own rule. Sea freight is different again, usually priced on the space your things take up in cubic metres. Our guide to courier, air freight, sea freight or excess baggage compares them.
Does packing my case tighter make it cheaper?
Not on its own. Squeezing more into the same case raises the real weight but leaves the size the same, so the size-based weight does not change. It only saves money if it lets you use a smaller case or box, or send one item instead of two.
Why is my small box charged on its real weight?
Because it is dense. A small box of 45 x 35 x 30 cm has a size-based weight of 9.45 kg, so once its contents weigh more than that, the real weight takes over. That is normal for books, shoes and kitchen things, and it means you are not paying for empty space.
What to do next
Measure your items at their widest points, weigh them, and enter the figures on the price page. It works out the chargeable weight of each item and compares every service side by side, with an all-in price for our charges.
Figures in this guide were checked on 3 October 2026 against the ready-made item sizes on our price page. If those sizes change, we update this guide.
