Volume in litres = length (m) × width (m) × depth (m) × 1,000. A 2 m × 1 m bed filled 30 cm deep holds 600 L, which is 20 × 30 L bags or 0.6 m³ of bulk soil. Add 10–15 % for settling, and split the depth into layers (drainage, compost, topsoil) so you only pay premium prices for the top layer.
The planner below does this for every bed and pot you add, keeps the layers within the bed's real depth, and compares the bagged total with a bulk delivery.
Your garden structures
Inside measurements, in centimetres
ExampleWe've loaded a sample 2 m × 1 m bed, 30 cm deep, with gravel, compost and topsoil layers. Change any number to make it yours.
Your project totals
Bagged vs bulk delivery
Example prices – enter your supplier'sYour shopping list
| Material | Volume | Bags (approx.) | Or bulk |
|---|
How the calculator works out your soil layers
A raised bed is rarely filled with a single product. The common approach in Australian and New Zealand gardens is a thin drainage layer at the bottom (only if the base is sealed or heavy clay), a compost or manure-rich layer in the middle, and a premium garden mix or vegetable mix on top where the roots feed.
The planner treats the inside depth of the bed as a budget that the layers spend:
- It starts with the total inside depth you enter, for example 40 cm.
- Each layer you add (say 5 cm of gravel and 20 cm of compost) is subtracted from that depth, so you can always see how many centimetres are left to fill.
- If the layers add up to more than the bed's depth, totals pause and the bed is flagged until you adjust a number.
- Pots use the same rules with a round cross-section: π × radius² × depth.
bags = ⌈ volume (L) ÷ bag size (L) ⌉ · 1 m³ = 1,000 L
All internal arithmetic is done in millimetres so that 12.5 cm and 0.125 m give identical answers; only the display rounds. Bag counts always round up, because you cannot buy 19.4 bags.
Two raised beds, start to finish
Suppose you are building two raised beds, each 2 m long × 1 m wide × 30 cm deep, and your local landscape yard quotes a garden mix at $85 per cubic metre plus a $70 delivery fee, while 30 L bags cost $9 at the hardware store.
Bulk delivery wins by $188 even after paying for the truck. The break-even point with these prices is only about 0.33 m³ (11 bags): below that, bags are cheaper; above it, bulk pulls ahead quickly. Prices here are illustrative – type your own quotes into the planner to see your real break-even.
- Volume per bed
- 2 × 1 × 0.3 = 0.6 m³ = 600 L
- Total for 2 beds
- 1,200 L = 1.2 m³
- Using 30 L bags
- 1,200 ÷ 30 = 40 bags
- Bagged cost at $9/bag
- 40 × $9 = $360
- Bulk at $85/m³ + $70 delivery
- (1.2 × $85) + $70 = $172
Raised bed soil: frequently asked questions
How much soil do I need for a raised garden bed?
Multiply the inside length, width and depth in metres, then multiply by 1,000 to get litres. A 1.2 m × 0.9 m bed filled 40 cm deep needs 1.2 × 0.9 × 0.4 × 1,000 = 432 L, so 15 × 30 L bags or about 0.45 m³ of bulk mix. Add 10–15 % for settling if you are filling with a fresh, fluffy mix.
What size bags should I buy in Australia or New Zealand?
Garden soil, vegetable mix and compost are most often sold in 25 L, 30 L and 40 L bags at Bunnings, Mitre 10 and independent nurseries. Mulch usually comes in bigger 50 L to 70 L bags because it is light. Always compare the price per litre: a 40 L bag at $12 (30 c/L) beats a 25 L bag at $9 (36 c/L).
When is bulk delivery cheaper than bags?
Usually somewhere between 0.5 and 1 m³. Bulk soil costs a fraction of the bagged price per litre, but the delivery fee ($60–$120 in most metro areas) is fixed, so tiny jobs lose. Once you pass roughly 20–30 bags' worth, a delivery almost always wins – and you avoid carrying 20 kg bags. Our bagged vs bulk guide shows the break-even formula.
Do I need a gravel drainage layer at the bottom of a raised bed?
Not if the bed sits on open soil – water drains straight into the ground, and a gravel layer can actually hold water in the soil above it (a "perched water table"). A thin 3–5 cm drainage layer is worth it when the bed sits on concrete, paving or a sealed base, or when you are building a wicking bed. The planner lets you leave gravel out entirely.
What is the difference between garden soil, topsoil and potting mix?
Topsoil and "garden mix" are heavier, mineral-based products for beds and in-ground use. Potting mix is a lighter, composted-bark-based product designed to drain freely in containers; in Australia premium potting mix carries the black-tick mark of AS 3743. Never fill pots with garden soil – it compacts and drains poorly – and avoid filling a whole raised bed with potting mix, which slumps and dries out quickly.
Is my project saved? Do I need an account?
No account and no upload. Everything is calculated in your browser. Use the Share button to create a link that contains your beds and prices – open it on your phone at the hardware store, or print the shopping list.
Sources and method
- Standards Australia, AS 4419:2018 Soils for landscaping and garden use and AS 4454:2012 Composts, soil conditioners and mulches – the standards referenced when we describe "premium" garden mixes and compost. standards.org.au
- Standards Australia, AS 3743:2003 Potting mixes – source of the red-tick (regular) and black-tick (premium) potting mix marks.
- Bag sizes and typical price ranges are checked against the online catalogues of major Australian and New Zealand hardware chains and independent landscape suppliers; they change with season and region, which is why the planner uses your own prices.
- Volume formulas: rectangular prism (L × W × D) and cylinder (π r² × D). Cross-checked by hand against the worked examples on each page.
Written by Burak · Last reviewed 22 September 2026 · Report an error