How much soil does a raised bed actually take? Internal volume in litres and cubic metres, multiplied across every bed in the row, then converted to 40, 50 or 60 L bags — metric first.
What Your Results Mean
Measure inside the walls. The soil fills the internal cavity, and the timber displaces a surprising amount: a bed measuring 2.4 m outside with 25 mm (1 in) boards is about 2.35 m inside, which trims the order by roughly the same fraction. Depth is the distance from the bottom (or the ground, for bottomless beds) to your intended fill line — usually a couple of centimetres below the rim so water doesn't sheet over the edge.
Depth is a cropping decision as much as a construction one. Typical guidance: 15–20 cm (6–8 in) for salads, herbs and other shallow-rooted crops; around 30 cm (12 in) for general vegetables; 40–45 cm (16–18 in) or more for carrots, parsnips and deep-rooted plants like tomatoes. Beds sitting on open ground can run shallower, because roots carry on into the soil beneath; beds on paving or membrane need full depth. Requirements vary by crop, so err deep if in doubt.
What goes in the bed matters as much as how much. A mix of roughly half topsoil and half compost is the common choice and suits most vegetables. Dedicated square-foot-gardening mixes also exist — typically a light, free-draining blend of compost with a mineral component such as vermiculite or coarse sand and an organic-matter base. Whatever route you take, the mix should hold moisture but drain freely; a bed that turns to soup in winter or concrete in summer is a mix problem, not a volume problem.
Plan on settling. A fresh fill of soil and compost typically drops 10–15% over its first season as it consolidates and the organic matter starts breaking down. The bag count rounds up to whole bags, which absorbs a little of that drop; the rest is best handled as a light top-up next season — usually with compost, which the bed will want anyway — rather than overfilling on day one.
The bed-count field assumes identical beds sharing dimensions, which is how most people build: one measurement session, one order. If your beds differ, run the calculator once per size and add the results. And once the total climbs past roughly a cubic metre, compare bag counts with bulk delivery — soil and compost suppliers typically deliver by the cubic metre, and the per-bag equivalent of 2.6 m³ is over fifty bags to move by wheelbarrow.
What the Results Mean
- Total Soil Volume (m³): the bulk-delivery figure if you order by the trailer or scoop.
- Total Soil (litres): the same volume in the unit bagged soil is sold in.
- Per Bed (litres): one bed's share — useful for mixing and topping up individually.
- Bags Needed: whole bags at your selected size, rounded up.
- Total: litres and bags in one line for the garden centre or supplier.
The Math Behind the Calculator
A raised bed is a box: internal length × width × depth gives cubic metres, and multiplying by 1,000 converts to litres, the unit bagged soil is labelled in.
per_bed_L = length × width × depth × 1000
total_L = per_bed_L × bed_count
total_m³ = total_L ÷ 1000
bags = ceil(total_L ÷ bag_size_L)
The bag conversion is a plain division, rounded up — you can't buy four-fifths of a bag. Bag labels are nominal, and actual contents vary a little by brand and moisture, so check the print on the bag you're actually buying. Settling is deliberately not added to the formula: the 10–15% drop happens gradually and is better handled with a seasonal top-up than by over-ordering the initial fill.
Worked Example: Three 2.4 × 1.2 × 0.3 m Beds
Three identical beds, each measuring 2.4 m by 1.2 m internally with a 0.3 m fill depth, using 50 L bags of a half-topsoil, half-compost mix.
- Per bed: 2.4 × 1.2 × 0.3 = 0.864 m³ → 0.864 × 1000 = 864 L per bed
- Total: 864 × 3 = 2,592 L ≈ 2.6 m³
- Bags: 2,592 ÷ 50 = 51.84 → 52 bags at 50 L
- Settling check: 10–15% of 2,592 L is roughly 260–390 L — about 5–8 further bags, best added as compost top-ups over the first season rather than upfront.
- Mix split: at half topsoil and half compost, that's about 1,296 L of each — or order the whole 2.6 m³ as a blended vegetable mix if the supplier offers one.
At this volume, bulk delivery is the practical route for most gardens: 2.6 m³ is a standard bulk order, where 52 bags is a lot of trips with a trolley. For a single bed at 864 L, the answer flips — 18 bags (864 ÷ 50 = 17.28) is car-boot territory.
Reminder
Bed dimensions, fill depth and bag sizes are the inputs that move the answer; soil chemistry isn't covered here. If you're growing anything demanding, a soil test after the first season tells you what the mix actually needs.
Assumptions & Limitations
Every number above rests on these constants. If your beds differ, change the matching input — or read the linked guide for the full reasoning.
- Internal, rectangular beds measured to the fill line — walls displace soil, so outside dimensions overstate the order
- 1 m³ = 1,000 L; bag sizes as printed (40 / 50 / 60 L are the common retail options — check the label)
- Settling of 10–15% is typical but not added to the bag count — plan a seasonal top-up
- Identical beds share one calculation; run the calculator per size and add if they differ
- Quantities only — this site deliberately does not price soil, compost or delivery