GardenCalcs.uk
Hanging Basket Calculator — Plants & Compost
Work out compost, liner, and plant quantities for hanging baskets.
Measured across the rim; the most common sizes are 30-40cm (12-16in).
Wire baskets need a moss or coir liner and can be planted through the sides for a fuller display. Solid baskets and pots are top-planted only.
Share of the full hemisphere actually filled with compost after the liner, rootballs and watering head-gap (typically 60-80%; 65% matches maker capacity figures).
How We Calculate This
Basket volume is approximated as a half-sphere: V = (2/3) × π × r³. The default efficiency factor of 65% allows for the liner thickness, plant rootballs and the watering head-gap below the rim, so the figure stays close to real fill capacity — about 7.3 litres for a 35cm (14in) basket and 10.9 litres for a 40cm (16in) basket, in line with maker spec sheets that quote roughly 7.5 L and 10.5 L respectively.
Plant counts are based on standard UK basket planting guides: roughly 3, 5 and 7 top plants for small (25cm), medium (35cm) and large (40cm+) baskets.
Wire baskets allow side planting through the liner for a fuller display, adding approximately 1.5× the top plant count in the sides.
Frequently Asked Questions
As a guide for bedding plants: a 25cm basket takes about 3 top plants, a 35cm basket about 5 top plus roughly 8 side plants in a wire basket, and a 40cm+ basket about 7 top plus roughly 11 side plants. Wire baskets gain the side plants because you can plant through the moss or coir liner — roughly 1.5× the top-plant count. Use fewer for vigorous plants like geraniums, fuchsias or trailing tomatoes (about 3 per 35cm basket).
Yes. Use moss, coconut fibre, or pre-formed liners. Liners hold compost in and retain moisture.
Daily in summer, often twice in hot weather. Hanging baskets dry out very quickly due to wind exposure. Add water-retaining gel.
Plant in May after the last frost risk. For an early start, plant in April and keep in a frost-free greenhouse until late May.
In summer, often twice a day. They dry out faster than any other container because wind and sun hit them from all sides. Self-watering baskets reduce this to daily.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.