Box Size Calculator

About this calculator

The box you choose is a shipping cost you pay on every parcel. Buying a size up because it was in stock adds volumetric weight to every order that ships in it, and nobody ever goes back and checks.

This works out the internal size a given number of identical items needs, the external size once the board is allowed for, and what that box will be billed at. The last figure is the one worth looking at before you place a packaging order.

How to use this calculator

1. Enter the product's three dimensions and how many go in one box.
2. Say how they sit: stacked, in a row, or laid out in a single layer.
3. Set the clearance you want around them for padding.
4. Enter the board thickness if you want the external size as well.
5. Check the volumetric weight before buying anything.

The formula

Internal size = arranged size + 2 x clearance on each axis
External size = internal size + 2 x wall thickness on each axis
Box volume    = external L x W x H
Volumetric    = box volume / divisor

For a single layer, the items are arranged in the grid that wastes least. Where two grids waste the same, the squarer box wins: squarer boxes are the sizes actually stocked, they are stronger, and they have a smaller girth, which matters for oversize rules.

Clearance is per side, so one centimetre of clearance adds two centimetres to each dimension. This is the step most people get wrong by half.

Worked example

Six phone cases, each 16 × 8 × 2 cm, stacked, with 1 cm clearance and 0.3 cm board:

Stacked     = 16 x 8 x 12 cm
+ clearance = 18 x 10 x 14 cm             (internal)
+ board     = 18.6 x 10.6 x 14.6 cm      (external)
Volume      = 2,879 cm³
Volumetric  = 2,879 / 5000 = 0.58 kg

Raising the clearance from 1 cm to 2 cm takes the box to 20.6 × 12.6 × 16.6 cm and the volumetric weight to 0.86 kg — half a slab more, on every parcel.

Notes and limits

• These are the dimensions the arrangement needs. Real boxes come in stocked sizes, so you will round up to whatever your supplier sells.
• A product that is not a rectangular block needs more room than its bounding dimensions suggest. Measure the largest cross-section.
• Mixed items and anything that has to sit a particular way up are outside what this solves.
• Empty space is air you pay to ship. Where the figure is above about 40%, a different box or arrangement is usually available.
• Clearance is not waste - a product that arrives broken costs far more than the board did. The point is to buy it deliberately.

What this calculation assumes

• Items are identical and packed in the arrangement chosen.
• Clearance is applied to every side, so twice on each axis.
• The product is treated as a rectangular block. An irregular shape needs more room.
• The result is a size to buy against, not a box that exists - stocked sizes are coarser.