How many plants fit in a window box?

The plant count here is per single row along the length — a 60 cm box fits about 5-6 bedding plants in one row. Boxes wider than ~18-20 cm can take a second, staggered row, so double the figure for the back row and offset the plants for a fuller display (trailing at the front, upright at the back).

What depth should a window box be?

Aim for at least 15 cm of internal soil depth for bedding plants, and 20 cm or more for herbs and larger plants. Deeper boxes hold more compost and dry out more slowly.

How often should I water window boxes?

In summer, daily or even twice daily in hot weather. South-facing boxes dry out fastest. Mix water-retaining gel into the compost to extend the time between waterings.

What plants work best in window boxes?

Trailing petunias, lobelia, geraniums, fuchsia, herbs, and small bulbs. Choose plants suited to your aspect (sun or shade).

How do I stop window boxes drying out?

Use compost with water-retaining gel, add saucers if possible, and water morning and evening in hot weather. Self-watering window boxes are excellent for hot positions.

Why is container spacing closer than border spacing?

Window boxes and other containers are planted deliberately densely for instant, full impact rather than leaving room for long-term growth — the RHS recommends close planting for container displays. That is why the presets here (bedding 12 cm, trailing/herbs 15 cm, small bulbs 8 cm) are tighter than typical in-ground spacing.

GardenCalcs

Window Box Calculator — Compost & Plant Quantities

Calculate compost volume and plant quantities for window boxes.

Box Length

Internal length of the box (measure inside, not the outer frame)

Box Width

Internal front-to-back width — sets compost volume and whether a back row fits

Box Depth

Internal soil depth — at least 15 cm for bedding, 20 cm+ for herbs

Plant TypeContainer bedding is planted denser than borders for instant impact (RHS)

How We Calculate This

Compost volume = Length × Width × Depth (in cm) ÷ 1000 for litres, then × 85% (a 15% reduction for drainage gravel and not filling to the rim). For example a 60 × 18 × 18 cm box holds 19,440 cm³ ≈ 19.4 L, or about 16.5 L after the drainage allowance.

Plants per row = Floor(Box Length ÷ Spacing) + 1. This is a single front-row count along the length; for boxes wider than ~18-20 cm you can add a second staggered row at the back. Container bedding is planted denser than borders for instant impact (RHS), so the presets are tight: bedding 12 cm, trailing 15 cm, herbs 15 cm, small bulbs 8 cm.

Frequently Asked Questions

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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.