What is the Box Size Calculator?
Choosing the right box protects the product and keeps shipping costs down. The box size calculator adds padding (bubble wrap, foam or void fill) on every side of your product to give the inside dimensions, then adds the board thickness to give the outside dimensions that couriers measure.
Box makers quote boxes by inside size, while couriers charge by outside size. Once you have the outside size, check the courier's billing weight with the Volumetric Weight Calculator. All calculators accept centimetres, inches or millimetres and kilograms or pounds, so you can use the units on your supplier's spec sheet.
How to use the Box Size Calculator
- Enter the product length, width and height.
- Enter the padding per side.
- Choose the board type or thickness (3-ply ≈ 3–4 mm, 5-ply ≈ 6–7 mm).
- Read the inside and outside box dimensions.
Formula
Inside = product + 2 × padding · Outside = inside + 2 × board thickness
Example
A 30 × 20 × 10 cm product with 2 cm padding needs a box of 34 × 24 × 14 cm inside. With 5 mm 5-ply board, the outside is about 35 × 25 × 15 cm.
How much padding do you need?
| Product | Padding per side |
|---|---|
| Clothes, books | 0–1 cm |
| Cosmetics, small electronics | 2–3 cm |
| Glass, ceramics | 5 cm+ |
| Heavy machinery parts | Custom foam or wooden crate |
Related packaging and shipping tools
Packaging decisions are connected: the box size sets the volumetric weight, which sets the courier bill, and the carton size decides how many fit on a pallet or in a container. These related calculators help with the next step:
- Box volume calculator: volume in cm³, litres, CBM and ft³.
- Box weight calculator: empty carton weight from GSM.
- Corrugated box calculator: GSM and BCT strength.
- Cardboard sheet calculator: RSC blank size and sheet yield.
- Box dieline generator: printable cut-and-fold template.
Frequently asked questions
What is the difference between inside and outside dimensions?
Inside dimensions are the usable space; outside dimensions add the board thickness on each side.
Which dimensions do couriers use?
Outside dimensions, for volumetric weight.
How thick is corrugated board?
3-ply (single wall) is about 3–4 mm; 5-ply (double wall) about 6–7 mm.
How do I order a box from a manufacturer?
Give the inside dimensions in L × W × H order and the board type.
Should I use a bigger box to be safe?
Only as much as the padding needs; bigger boxes cost more to ship.
Is the Box Size Calculator free to use?
Yes. The Box Size Calculator on Custom Converter is completely free, with no sign-up, no daily limit and no watermark.
Does the Box Size Calculator work on mobile phones?
Yes. It is built mobile-first and works on Android phones, iPhones, tablets and computers in any modern browser, with no app to install.