How to Calculate the Volume of a Box: Formula, Litres and Worked Examples
To calculate the volume of a box, multiply its length by its width by its height, with all three in the same unit. A box 30 cm long, 20 cm wide and 15 cm high has a volume of 30 x 20 x 15 = 9,000 cm³, which is 9 litres, because a litre is 1,000 cm³.
The answer is only useful if it is the right volume. What a box holds depends on its inside size, while couriers and storage space work from the outside. This guide covers how to calculate the volume of a box both ways, how to convert the result to litres, cubic metres and imperial units, and how UK carriers use it.
| Unit | Equals |
|---|---|
| 1 cubic centimetre (cm³) | 1 millilitre, or 1,000 cubic millimetres |
| 1 litre | 1,000 cm³, a cubic decimetre |
| 1 cubic metre (m³) | 1,000 litres, or 1,000,000 cm³ |
| 1 cubic inch | about 16.39 cm³ |
| 1 cubic foot | about 28.32 litres |
| 1 UK (imperial) gallon | 4.54609 litres |
| 1 US gallon | about 3.79 litres |
The metric and imperial definitions come from Schedule 1 of the Weights and Measures Act 1985. It defines the litre as a cubic decimetre, the inch as one thirty-sixth of a 0.9144 metre yard (25.4 mm) and the gallon as 4.54609 cubic decimetres. The US gallon figure is from NIST’s conversion table. Our guide to M, MM, ML and MIL explains that a millilitre and a cubic centimetre are the same amount.
The formula, step by step
- Measure the length, the width and the height of the box.
- Put all three in the same unit; centimetres are usually the easiest for boxes.
- Multiply them: volume = length x width x height.
- Convert the result: divide cm³ by 1,000 for litres, or by 1,000,000 for cubic metres.
The unit always comes out cubed. Centimetres give cubic centimetres, millimetres give cubic millimetres and inches give cubic inches, so a volume means nothing until you know which unit went in. For where to put the tape and which side is which, see our guide to measuring box dimensions.
Worked examples
| Box | Calculation | Volume | In litres |
|---|---|---|---|
| 30 x 20 x 15 cm | 30 x 20 x 15 | 9,000 cm³ | 9 |
| The same box in millimetres, 300 x 200 x 150 mm | 300 x 200 x 150 | 9,000,000 mm³ | 9 |
| 40 x 30 x 20 cm | 40 x 30 x 20 | 24,000 cm³ | 24 |
| 60 x 40 x 40 cm | 60 x 40 x 40 | 96,000 cm³ | 96 |
| 12 x 10 x 8 inches | 12 x 10 x 8 | 960 cubic inches | about 15.7 |
| A 10 cm cube | 10 x 10 x 10 | 1,000 cm³ | 1 |
The millimetre row shows why the unit matters. The same box comes out as 9,000 or 9,000,000 depending on whether it was measured in centimetres or millimetres. There are 1,000 cubic millimetres in a cubic centimetre, so divide by 1,000 once to reach cm³ and again to reach litres.
For a box measured in inches, multiply the cubic inches by about 16.39 to get cm³. That factor follows from the inch being exactly 25.4 mm.
Inside or outside: which volume do you need?
A box has two volumes. The inside volume is what it can hold. The outside volume is the space it takes up on a shelf, in a van or on a pallet, and it is the one couriers measure. The difference between them is the board.
Take a box with an outside size of 30 x 20 x 15 cm in B-flute corrugated, which published flute tables put at about 3 mm thick, as our comparison of E-flute and B-flute sets out. With about 3 mm of board on each face, the inside is roughly 29.4 x 19.4 x 14.4 cm, or about 8.2 litres, against 9 litres outside. In this example the board takes close to a tenth of the volume.
So when a box size is quoted, check which one it is. Made-to-order boxes are normally specified by their internal dimensions, with a small allowance around the product, which our guide to box clearance and tolerance covers. Thicker board widens the gap between the two volumes, and our guide to single and double wall boxes works through what that does to the outside size.
Converting box volume to litres and cubic metres
To get litres, divide cubic centimetres by 1,000. To get cubic metres, divide cubic centimetres by 1,000,000, or litres by 1,000. Both rules follow from the definitions above: a litre is a cubic decimetre, and a cubic metre holds 1,000 cubic decimetres.
A litre measures space, not weight. A 9 litre box of tissue paper and a 9 litre box of glass jars have the same volume and very different weights, so never read a box’s litres as kilograms.
One cubic metre is a cube 1 m on each side, but any box whose sides multiply to 1,000,000 cm³ holds the same. A box 125 x 100 x 80 cm is one example.
For imperial units, a cubic foot is about 28.32 litres and a cubic metre is about 35.31 cubic feet. Gallons need care: the UK gallon is 4.54609 litres, but some conversion tables online, including one of the ranking box guides, use the smaller US gallon. The same 9 litre box is about 1.98 UK gallons or about 2.4 US gallons.
How UK carriers use box volume
Carriers turn volume into rules in different ways. Royal Mail’s size and weight guide sets each UK band by its largest length, width, depth and weight. Its Small Parcel band tops out at 45 x 35 x 16 cm, about 25.2 litres of outside volume, and its Medium Parcel band at 61 x 46 x 46 cm, about 129 litres.
Parcelforce uses litres directly for account customers. Its guide to measuring a parcel for UK delivery says parcels must be within a maximum length and litre volume, worked out as length x width x height in centimetres divided by 1,000. Its own example is a 100 x 30 x 20 cm parcel of 60 litres.
Some services charge on volumetric weight instead. For international parcels to zones 4 to 12, Parcelforce’s guide to measuring a parcel for international delivery divides length x width x height in centimetres by 5,000. It then charges on that figure when it is higher than the actual weight, so a 40 x 30 x 50 cm parcel counts as 12 kg. Divisors can differ between carriers and services, so check the one you book. Volumetric pricing is covered in more depth in standard box sizes.
Both Parcelforce guides also measure an irregular item as the smallest box it would fit into, which matters for tubes. For the full set of bands, see our comparison of Royal Mail and Parcelforce parcel sizes.
Tubes and round boxes
A tube’s volume is the area of its circular end multiplied by its length: π x radius x radius x length, the method BBC Bitesize sets out for the volume of a cylinder. Halve the diameter to get the radius. A tube 8 cm across and 30 cm long holds π x 4 x 4 x 30, or about 1,508 cm³, which is about 1.5 litres.
With Parcelforce, the box around it counts: measured as the smallest box it fits, that tube is 8 x 8 x 30 cm, or 1,920 cm³, about a quarter more than its own volume. Royal Mail’s size guide has a separate rule for tubes and rolls instead: the length plus twice the diameter must not exceed 104 cm, and the longest side cannot be over 90 cm. Our paper tube packaging and round boxes are made to the diameter and length you give us.
Common mistakes
- Mixing units, such as one side in millimetres and two in centimetres.
- Using the outside size to work out what a box holds.
- Reading litres as kilograms.
- Rounding each side before multiplying, which compounds the error.
- Pricing a tube on its own volume when the carrier measures the box around it.
- Treating volume as a count of products. Products rarely fill a box completely, so count units by layout; our guide to how many units fit in a box shows how.
Frequently asked questions
What is the volume of my box?
Multiply its length, width and height in the same unit. For what it holds, use the inside measurements; for postage and storage, use the outside.
How do I convert box dimensions to litres?
Measure in centimetres, multiply the three figures and divide by 1,000. A 40 x 30 x 20 cm box is 24,000 cm³, or 24 litres.
What size box is 1 cubic metre?
Any box whose sides multiply to 1,000,000 cm³, such as a 100 cm cube or a box 125 x 100 x 80 cm. One cubic metre is 1,000 litres.
What is the volume of a 3 x 3 x 3 cube?
27 cubic units: 27 cm³ if the sides are in centimetres, or 27 cubic feet if they are in feet.
Is volume measured in squared or cubed units?
Cubed. Area has two dimensions and uses squared units; volume has three, so it uses cubic units such as cm³ or m³.
How many litres is a 60 x 40 x 40 cm box?
96 litres from those measurements. If they are outside measurements, the inside holds a little less, by the thickness of the board on each face.
Does Royal Mail charge by volume?
Not by volume as such. Its UK size and weight guide sets each band by maximum length, width, depth and weight, so a box has to fit every limit of the band.
What is volumetric weight?
A charging weight worked out from a parcel’s size. For international zones 4 to 12, Parcelforce divides length x width x height in centimetres by 5,000 and charges on that figure or the actual weight, whichever is higher.
Need a box built around a volume? We make custom shipping boxes and cardboard boxes to your internal dimensions, in single or double wall. Orders start from 100 units. Design support is free. A digital 3D mock-up is free. Tell us the product size or the inside volume you need. For the quote, send the artwork, the quantity, the delivery postcode and the date the order is needed by.