How many tomato plants per grow bag?

The RHS recommends 2 tomato plants per standard 40L growing bag. Three is possible but restricts root space, reduces fruit size and noticeably increases watering and feeding. A larger bag holds more roots, so this calculator scales the count up proportionally with the bag size you choose.

Do grow bags need drainage holes?

Yes. Cut slits in the underside or poke drainage holes. Without drainage, roots sit in waterlogged compost and rot.

How long do grow bags last?

One growing season for heavy feeders like tomatoes. The spent compost can be used as a soil conditioner or mixed into borders.

Can I reuse grow bags?

Not for the same crop (disease risk). You can grow a lighter feeder like lettuce in year-two bags, or add the compost to your garden.

Does a bigger grow bag fit more plants?

Yes — plant density is per standard 40L bag, so a larger bag holds proportionally more roots. This calculator scales plants per bag with the size you choose (floored at 1), matching the RHS standard-bag densities.

GardenCalcs

Grow Bag Calculator — How Many Grow Bags

Calculate how many grow bags you need and plants per bag.

Number of Plants

Total plants you want to grow across all bags

CropCrop type sets the recommended plants per standard bag

How We Calculate This

Grow bags needed = Number of plants ÷ Plants per bag (rounded up).

Densities are quoted per standard 40-litre growing bag (RHS): Tomatoes 2/bag, Peppers 3/bag, Courgettes 2/bag, Cucumbers 2/bag, Strawberries 5/bag, Lettuce 8/bag.

Plants per bag scale with the bag size you choose — a 40L bag holds twice the roots of a 20L bag — so a larger bag fits proportionally more plants (floored at 1). Plants per bag = round(standard-bag density × bag size ÷ 40).

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.