Grow Bag Calculator — How Many Grow Bags

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.

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

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

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.

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.