How do I maximise internal volume in a box?

Use thinner walls where structural strength allows. 6mm plywood gives good results for small boxes; 12mm for medium boxes. Use a thinner base (4-6mm set in a groove) rather than a thick solid base. Rebated or mitre joints use material more efficiently than butt joints.

What is a good ratio of internal to external volume?

For small decorative boxes, 40-60% usable space is typical due to the proportionally thicker walls. For larger storage boxes, aim for 70-80% usable space. Very thin-walled boxes (6mm plywood) can achieve over 80% but may lack rigidity.

How do I calculate the volume of an irregular box?

For tapered or irregular shapes, divide the box into simpler rectangular sections and sum the volumes. For curved shapes, use the average dimensions. This calculator handles standard rectangular boxes where both the external and internal shapes are rectangular.

How many litres does a storage box hold?

One litre equals 1,000 cm³ or 1,000,000 mm³. A 300 x 200 x 150mm internal box holds about 9 litres. This calculator shows the internal volume in both cm³ and litres for easy reference.

WoodCalcs

Box Volume Calculator — Internal & External Volumes

Calculate internal and external volumes for storage box design.

External Length

Outer front-to-back / longest side

External Width

Outer side-to-side dimension

External Height

Total height including lid

Wall Thickness

Side-panel stock thickness (e.g. 12mm ply)

Base Thickness

Bottom-panel stock thickness

Lid Thickness

Top / lid-panel stock thickness

How We Calculate This

This calculator determines the internal and external volumes of a rectangular box, accounting for wall, base, and lid thickness.

Formulas

Internal length = External length - (2 x Wall thickness) - (2 x Joint allowance)

Internal width = External width - (2 x Wall thickness) - (2 x Joint allowance)

Internal height = External height - Base thickness - Lid thickness - Joint allowance

Volume = Length x Width x Height

Joint allowance is an optional advanced adjustment (default 0mm) for material lost to rebates or grooves on each face — leave it at 0 for plain butt joints. Volumes convert as 1 cm³ = 1,000 mm³ and 1 litre = 1,000,000 mm³.

The usable space percentage shows how much of the total external volume is available for storage. The difference is the volume of wood used in construction; multiplying that by the timber density gives an estimated empty-box weight.

Frequently Asked Questions

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Work out mitre joint dimensions and material lengths for box corners.

Lid Fit Calculator

Calculate lid dimensions and clearance for tight-fitting box lids.

Hinge Placement Calculator

Determine hinge positions and mortise dimensions for box lids.

Divider Calculator

Calculate divider dimensions and slot positions for compartmented boxes.

Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.