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 after deducting the gravel drainage layer. A standard 50-litre bag of compost fills about 0.05 cubic metres. A 1200 x 400 x 450mm planter holds about 216 litres in total, but once the default 50mm gravel drainage layer is deducted you need roughly 192 litres of compost — about 4 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 SizeStandard UK timber dimensions

Planter Length

Longest side of the planter box

Planter Width

Shortest side of the planter box

Planter Height

Overall height of the planter sides (300-450mm suits most vegetables)

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 - Drainage layer) (all in mm) / 1,000,000

A gravel drainage layer (default 50mm) is deducted from the height, so a 1200 x 400 x 450mm planter gives 1200 x 400 x (450 - 50) / 1,000,000 = 192 litres of compost.

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

Frequently Asked Questions

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Last updated: February 2026

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