MetraNestMeasure the inside of the planter
Advertised dimensions may be external. Wall thickness reduces the available volume. Measure the length and width inside, then the planned compost layer height. If you leave a gap below the rim, subtract it from the fill height.
Count only the layer concerned
Usable height is the internal height minus the top gap and any other bottom layer. The bottom-layer field describes your project; it does not recommend a drainage method. Root balls also take up space, without automatic subtraction. A tapered pot does not match the cylindrical model exactly.
From litres to bags
Geometric volume describes the available space. Compost may settle; any reserve should remain an explicit assumption suited to the product. Bags are rounded up. Check the resulting surplus before adding an extra reserve. This calculation covers quantity without choosing a suitable growing medium for the plants.
Rectangular planter: litres = length × width × usable height ÷ 1,000, using cm. Cylinder: litres = π × (diameter ÷ 2)² × usable height ÷ 1,000. Bags = volume ÷ litres per bag, rounded up.
A balcony planter
A planter measuring 80 × 30 × 30 cm internally, with 5 cm left clear at the top and no other layer, holds 60 usable litres. With 20 L bags, you need three bags. If it already contains a root ball, that volume reduces the material actually added, without automatic subtraction.
Details to check
- Keep external and internal dimensions separate.
- Enter the planned compost fill height, which is not necessarily the full height of the planter.
- Compare bags by their litre capacity, not just their individual price.
Frequently asked questions
Why does 1,000 cm³ equal one litre?
One litre is one cubic decimetre. A cube with 10 cm sides occupies 10 × 10 × 10 = 1,000 cm³.
Does the number of litres tell me the weight?
No. Weight depends on the material and its moisture content. This tool calculates volume only.
Method and references · Editorial review: 25 October 2026
The geometric formulas are explained here. For a product, its packaging dimensions and coverage rate remain the reference inputs.