WoodCalcs.uk
Box Volume Calculator — Internal & External Volumes
Calculate internal and external volumes for storage box design.
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
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.
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.
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.
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.
Related Calculators
Box Dimension Calculator
Calculate material dimensions for box sides, base, and lid from desired internal size.
Lid Fit Calculator
Calculate lid dimensions and clearance for tight-fitting box lids.
Veneer Calculator
Calculate veneer sheet quantities for box and panel veneering.
Mitre Box Calculator
Work out mitre joint dimensions and material lengths for box corners.
Hinge Placement Calculator
Determine hinge positions and mortise dimensions for box lids.
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Last updated: February 2026
All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.