How deep should a planter box be?

For most vegetables and flowers, 300-450mm deep is ideal. Root vegetables like carrots need at least 300mm. Shallow-rooted herbs and salad can grow in 200mm depth. For small trees or shrubs, aim for 450-600mm deep.

Do I need to line a wooden planter?

Yes, lining with pond liner or heavy-duty polythene protects the timber from constant moisture and extends its life significantly. Make drainage holes through the liner at the bottom. Do not use standard black polythene as it degrades quickly.

What timber is best for planter boxes?

Pressure-treated softwood (UC4 rated) is the most affordable option. For a more natural approach, use untreated larch, cedar, or oak which are naturally rot-resistant. Avoid using old railway sleepers near food crops due to creosote.

How much compost do I need to fill a planter?

This calculator gives the volume in litres. A standard 50-litre bag of compost fills about 0.05 cubic metres. For a 1200 x 400 x 450mm planter, you need approximately 216 litres or about 4-5 bags of standard multi-purpose compost.

WoodCalcs

Planter Box Calculator — Timber & Liner Quantities

Calculate timber and liner quantities for a raised planter box construction.

Timber Size

Standard UK timber dimensions

Planter Length

Longest dimension of the area

Planter Width

Shortest dimension of the area

Planter Height

Measure floor to ceiling or top of area

Include liner

Protects timber from moisture

Timber Price per Metre (optional)

Enter your supplier price per metre

How We Calculate This

This calculator determines the timber needed for a rectangular planter box with corner posts and horizontal boards.

Board count

Boards per side = ceil(Planter height / Board height)

Total side boards = Boards per side x 4 sides

Soil volume (litres) = Length x Width x Height (all in mm) / 1,000,000

The liner area covers the internal surfaces: 2 long sides + 2 short sides + base.

Frequently Asked Questions

Related Calculators

Decking Board Calculator

Calculate the number of decking boards, screws, and joist hangers needed.

Deck Joist Calculator

Work out joist sizes, spacing, and quantities for a deck subframe.

Pergola Calculator

Calculate timber quantities for pergola posts, beams, and rafters.

Fence Panel Calculator

Work out fence panel, post, and gravel board quantities for a boundary.

Garden Bench Calculator

Calculate timber quantities and dimensions for a garden bench build.

Last updated: February 2026

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