How Much Soil for a Grow Bag or Planter?

Quick answer: A standard grow bag holds about 40–50 litres of compost — roughly one bag of multipurpose. For a planter, work out the volume as length × width × depth: a typical 1m long × 20cm × 20cm trough holds about 40 litres. Fill to a couple of centimetres below the rim, use fresh compost, and for deep planters you can part-fill the base to save compost and weight.

Grow bags and planters sit between pots and raised beds, and their long, shallow shapes make volumes easy to misjudge — a trough that looks small can hold as much as a couple of pots. This guide covers how much a grow bag holds, how to size compost for planters and troughs of any dimensions, what to use, and how to fill deep planters economically. Use the calculator above to total up your containers.

Grow bags: about one bag each

A grow bag — the flat, ready-filled bags sold for tomatoes, peppers, and salad — typically holds around 40–50 litres of compost, which is why they’re sold pre-filled at roughly the volume of one bag of multipurpose. If you’re filling your own reusable grow bags or fabric planters, budget around a bag of compost each.

Grow bags are shallow, so they dry out and deplete quickly under thirsty crops like tomatoes. Standing two plants per bag rather than three gives each more root room and compost, and you’ll usually need to feed through the season as the limited compost runs low on nutrients.

Planters and troughs: measure the volume

For a planter or trough, it’s a straightforward volume calculation: length × width × depth. A trough 1m long, 20cm wide, and 20cm deep is 100 × 20 × 20 = 40,000 cm³ = 40 litres. For a tapered planter, measuring the inside dimensions at the middle gives a fair average.

Keep units consistent — in centimetres, divide the result by 1,000 for litres. Fill to about 2cm below the rim so water soaks in rather than spilling over. Long window-box-style troughs hold more than they appear to, because the length adds up even when they’re shallow, so it’s worth calculating rather than guessing.

Adding up several containers

If you’re filling a row of planters or several grow bags, work out each volume and add them together. Three 40-litre troughs plus two grow bags at 45 litres each is 120 + 90 = 210 litres — about four to five 50-litre bags. The calculator lets you total a mix of containers and converts the result to bags in your region’s sizes, so you buy the right number in one trip.

What compost to use

Use fresh multipurpose or potting compost, not garden soil, which compacts and drains poorly in a container. For long-term planters with shrubs or perennials, a loam-based compost (John Innes type) holds structure and nutrients longer. For seasonal crops and bedding in grow bags and troughs, standard multipurpose is fine, ideally with added feed as the season goes on since shallow containers deplete fast.

Avoid reusing last year’s spent compost on its own — it’s lost structure and nutrients. You can refresh it by mixing in a good proportion of fresh compost, but for hungry crops, fresh is best.

Filling deep planters economically

Tall planters don’t need compost all the way down, since roots concentrate in the upper portion. For a deep planter, part-fill the base with lightweight bulky material — upturned pots, broken polystyrene, or coarse woody material — and fill the top two-thirds with compost. This saves compost, cuts the weight (useful on balconies and against fences), and improves drainage. Calculate compost only for the filled portion above the bulking material.

For shallow grow bags and troughs there’s no need for this — they’re already shallow enough that the whole depth is root zone. The trick is for genuinely deep planters where the lower third would otherwise be wasted compost.

Common mistakes to avoid

The main mistakes are under-estimating how much a long trough holds, using garden soil instead of compost, filling to the very rim so water overflows, and not feeding crops in shallow grow bags where nutrients run out fast. One more: over-planting a grow bag — three tomatoes in one bag compete for limited compost and water, so two per bag usually crop better.