Planter Box Calculator — Timber & Liner Quantities

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.

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.

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.

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.