Working out the inside capacity of a cardboard box is simple once you know which measurements to take. It helps when you are moving house, storing stock, choosing postal packaging, or checking whether a product will fit safely inside a box before shipping.
The key is to measure the internal dimensions, not just the outside of the box. The outside size includes the board thickness, which can make a difference, especially with double wall boxes or heavy-duty corrugated packaging.
Most cardboard boxes are measured using three dimensions: length, width and height. These measurements are often written as L x W x H.
The length is usually the longest side of the opening. The width is the shorter side across the opening. The height is the depth from the base of the box to the top edge.
For shipping cartons, these dimensions are usually shown in millimetres in the UK. For example, a box listed as 305 x 229 x 102mm would usually mean:
When checking capacity, measure from the inside faces of the box. This gives you the usable internal space. In packaging terms, this is often called the internal dimension, which means the clear space available for the product, void fill and any inserts.
A few millimetres can make a real difference in packaging. If the box is too small, the product may not fit or may press against the sides. If it is too large, you may need extra void fill, which means more material, higher cost and a less tidy unboxing experience.
Accurate measurements help you:
This is especially useful for e-commerce businesses, 3PL fulfilment teams and retailers sending repeat product sizes. A small improvement in box fit can save time and material across hundreds or thousands of parcels.
You do not need specialist equipment for most cardboard boxes. A few simple measuring tools will do the job.
A tape measure is the easiest option for medium and large boxes. Use a rigid or locking tape if possible, as it will be easier to hold straight across the inside of the box.
A ruler works well for small postal boxes, book wraps, cartons and samples. It can also help when checking the board thickness or smaller internal features.
Digital callipers are useful when you need a more accurate measurement, especially for small cartons, die-cut packaging, inserts or product fit checks. They are not needed for every job, but they can help if your tolerances are tight.
A laser measure can be useful for larger boxes, storage areas or pallet spaces. For normal e-commerce boxes, a tape measure is usually more practical.
Follow these steps to measure the internal volume of a box.
Remove any products, inserts, loose packaging or void fill. The box needs to be empty so you can measure the clear internal space.
Put the box on a table, bench or floor. Make sure it sits squarely and is not twisted, crushed or bowed. Damaged packaging can give you an inaccurate reading.
Measure from one inside wall to the opposite inside wall along the longest side of the box opening. Keep the tape straight and flat against the base where possible.
Now measure the shorter side of the opening, again from inside wall to inside wall. This measurement should run at a right angle to the length.
Measure from the inside base of the box up to the top edge. If the box has flaps, do not include the flaps in the height. You only want the usable depth inside the box.
To find the internal volume, multiply the length by the width by the height.
Volume = Length x Width x Height
For example, if the internal measurements are 300 x 200 x 150mm, the calculation is:
300 x 200 x 150 = 9,000,000 cubic millimetres
To convert this into litres, divide by 1,000,000.
9,000,000 ÷ 1,000,000 = 9 litres
So, a box with internal dimensions of 300 x 200 x 150mm has an internal capacity of around 9 litres.
For packaging, storage and shipping, litres can be easier to understand than cubic millimetres.
Use this simple formula:
Length x Width x Height in millimetres ÷ 1,000,000 = capacity in litres
For centimetres, the formula is slightly different:
Length x Width x Height in centimetres ÷ 1,000 = capacity in litres
For example, a box measuring 30 x 20 x 15cm has a volume of:
30 x 20 x 15 = 9,000 cubic centimetres
9,000 ÷ 1,000 = 9 litres
Measuring a box is straightforward, but a few small mistakes can throw the numbers off.
External dimensions include the thickness of the corrugated board. With single wall boxes, the difference may be small. With double wall board, the gap between external and internal size can be more noticeable.
If you are checking whether a product will fit, always use internal measurements.
The flaps are not part of the usable internal height. Measure from the inside base to the top crease or top edge of the box body.
A crushed or distorted box may not give you a reliable measurement. If possible, use a clean, properly formed sample.
The product may fit inside the box, but you still need space for protection. Fragile items may need paper cushioning, bubble wrap, corrugated inserts or other protective packaging. This space should be allowed for before choosing the final box size.
Rounding every measurement down can leave you short on space. For product fit checks, measure carefully and allow a small tolerance, which means a little extra space to account for board movement, packing method and product variation.
Most cardboard shipping boxes are rectangular, so the length x width x height formula works well. Some packaging styles, such as book wraps, triangular tubes, tapered cartons or die-cut mailers, may have a less regular shape.
For irregular packaging, it is best to measure the main usable space and check product fit with a physical sample. Volume alone may not tell the full story. For example, a product may technically fit by volume but still be too long, too wide or too tall for the pack.
Box capacity is only one part of choosing the right packaging. You also need to think about product weight, transit risk, packing speed and courier handling.
For lighter products, a single wall corrugated box may be enough. Single wall means the board has one fluted layer between two liners. For heavier or more fragile items, a double wall box may be better. Double wall means the board has two fluted layers, giving extra rigidity and stacking strength.
If you are shipping e-commerce orders, aim for a snug fit with enough room for protection. Too much empty space can let products move around in transit, increasing the risk of damage.
Measuring the inside capacity of a cardboard box is a useful skill, whether you are packing one parcel or reviewing a full e-commerce packaging range. Once you know the internal length, width and height, you can calculate capacity, compare box sizes and make better decisions about product fit.
If you need a standard box size, take a look at our cardboard boxes. If you need a custom size, printed finish or a packaging format built around your products, our bespoke packaging team can help.
We manufacture and source packaging for e-commerce brands, retailers and 3PL fulfilment teams across the UK, with options for custom printed boxes, postal packaging, paper tape and product-specific packaging designs.
Start your bespoke packaging project or contact our packaging team to talk through the best box size for your products.
