Why use this calculator
Bulk landscaping material is ordered by volume and delivered by the truckload, which makes running short genuinely expensive. A second delivery often costs more than the material itself. Estimating by eye is unreliable because volume scales with three dimensions and depth is the one people guess.
This calculator converts an area and a depth into the figure a supplier actually wants, adds the allowance that covers settling and spillage, and tells you what it weighs so you know whether it fits in a trailer or needs a truck. It also shows the bag equivalent, which is usually the moment people realise how much more bagged product costs.
How to use it
- Measure the length and width of the area, using metres for metric or feet for imperial, and treat an irregular bed as a series of rectangles you add together.
- Depth is entered in millimetres for metric and inches for imperial, which is how both suppliers and product guidance describe it. Choose the material, since bulk density varies enormously.
- Mulch is light and road base is five times heavier for the same volume, which matters a great deal for transport.
- Adding a price per cubic metre or yard gives an estimated material cost, though delivery is normally charged separately per load.
What each result means
- Volume needed
- The exact volume the area requires at the depth you entered, before any allowance.
- Order this much (10% allowance)
- What to actually order. The allowance covers settling, spillage, uneven ground and the inevitable slightly-deeper patches.
- Approximate weight
- What the load weighs, which decides whether you need a truck. Check your trailer or ute payload before collecting it yourself.
- Area covered
- The surface area being covered, useful for cross-checking against a quote or a plan.
- Depth
- The spread depth used in the calculation, repeated so you can sanity-check it against product guidance.
- Bags if buying in 25L bags
- How many standard bags the same volume represents. Bagged product typically costs several times more per unit than bulk.
- Estimated material cost
- The order volume multiplied by your supplier's rate, before delivery, which is usually charged per load.
The formula
Volume is length × width × depth, with depth converted into the same unit as the other two: millimetres divided by 1,000 for metric, or inches divided by 12 for imperial. Imperial results are divided by 27 to convert cubic feet into cubic yards. Weight is volume in cubic metres multiplied by the material's bulk density in tonnes per cubic metre. Roughly 0.35 for bark mulch, 1.2 for garden soil, 1.5 for gravel, 1.6 for sand and 1.8 for road base. A ten per cent allowance is added to the order figure.
How this number is used in practice
Depth is where people go wrong in both directions. Mulch laid thinner than about 50 millimetres does not suppress weeds or hold moisture, so the job has to be redone. Laid thicker than about 100 millimetres it can restrict air and water reaching the root zone, and piled against trunks and stems it holds moisture where it causes rot. Seventy-five millimetres is the usual compromise, kept clear of the base of plants.
Weight matters more than volume for transport. A cubic metre of bark mulch is around 350 kilograms and fits comfortably in a small trailer; a cubic metre of road base is around 1.8 tonnes and exceeds the payload of most utes and box trailers. Overloading is both illegal and dangerous, and suppliers will load what you ask for rather than what your vehicle can legally carry.
Bagged versus bulk is the largest cost difference in the whole job. A cubic metre is forty 25 litre bags, and buying that volume in bags typically costs several times the bulk delivered price. Bags make sense for small top-ups and for anyone without access for a truck; anything above about half a cubic metre is usually cheaper delivered loose, even after the delivery fee.
Worked examples
6 m by 3 m garden bed, 75 mm of mulch
1.35 cubic metres needed, order about 1.49, weighing roughly half a tonne.
10 m by 1.2 m gravel path, 50 mm deep
0.6 cubic metres needed, order about 0.66, weighing close to a tonne. More than a small trailer should carry.
20 ft by 10 ft bed, 3 inches of soil
About 1.85 cubic yards needed, order roughly 2.04 cubic yards.
When people use it
- Ordering mulch for garden beds without running short
- Working out topsoil for a new lawn or raised bed
- Costing gravel for a path, driveway or drainage trench
- Checking whether a load will fit in a trailer or needs delivery
- Comparing the cost of bagged product against bulk delivery
Tips
- Order about 10 per cent more than the exact figure. A second delivery usually costs more than the extra material.
- Mulch at 75 mm is the usual depth: thinner does not suppress weeds, thicker can starve roots of air.
- Keep mulch clear of trunks and stems; piling it against them holds moisture and causes rot.
- Check weight, not just volume. A cubic metre of road base is around 1.8 tonnes and will overload most trailers.
- Bulk delivery is far cheaper than bags above about half a cubic metre, even after the delivery fee.
- Treat an irregular bed as several rectangles, calculate each, and add the results together.
What it assumes, and where it stops being accurate
- The area is treated as a simple rectangle; irregular shapes must be broken into rectangles and added.
- Typical supplier bulk densities are used, and actual density varies with moisture content and material grade.
- The ground is assumed to be level; sloped or uneven sites need more material than the calculation suggests.
- A ten per cent allowance is applied, which suits most jobs but not heavily uneven ground.
- Compaction is not modelled. Road base and gravel reduce in volume substantially once compacted.
- Delivery fees, which are usually charged per load rather than per unit, are excluded from the cost estimate.
Terms used on this page
- Bulk density
- The weight of a material per unit of volume, used to convert cubic metres into tonnes.
- Cubic metre
- A volume one metre on each side, the standard bulk landscaping unit in Australia.
- Road base
- A compactable crushed rock blend used as a base layer under paths and driveways.
- Payload
- The maximum weight a vehicle or trailer is legally rated to carry.
Questions people ask
- How much mulch do I need for my garden?
- Multiply the bed area by the depth. A 6 by 3 metre bed at 75 millimetres needs 1.35 cubic metres, and you should order about 1.5 to allow for settling and spillage. As a quick rule, one cubic metre covers roughly 13 square metres at 75 millimetres deep.
- How deep should mulch be?
- Around 75 millimetres, or three inches, for most garden beds. Below about 50 millimetres it fails to suppress weeds or retain moisture, and above about 100 millimetres it can restrict air and water reaching the roots. Always keep it clear of trunks and stems, where held moisture causes rot.
- How many bags are in a cubic metre?
- Forty 25 litre bags, or twenty 50 litre bags. That comparison is usually the moment bulk delivery starts to look sensible: buying a cubic metre in bags typically costs several times the loose delivered price, and involves carrying forty bags.
- How much does a cubic metre of soil weigh?
- Garden soil and compost run around 1.2 tonnes per cubic metre, bark mulch about 0.35, sand about 1.6, gravel about 1.5 and road base about 1.8. Moisture content moves all of these, and wet material can be substantially heavier. Which matters if you are collecting it yourself.
- Should I order extra?
- Yes, around ten per cent. Ground is never perfectly level, some material is lost in handling, and mulch settles once watered in. Running short means either an uneven finish or a second delivery, and the delivery fee on a small top-up load frequently exceeds the cost of the extra material.
- Will a cubic metre fit in my trailer?
- It depends on the material. A cubic metre of bark mulch is around 350 kilograms and fits a typical box trailer. A cubic metre of gravel or road base is one and a half to nearly two tonnes, which exceeds the legal payload of most trailers and utes. Check your vehicle's rating before ordering to collect.
Use this with
Mulch, Soil and Gravel Calculator answers one part of the question. These pick up where it stops: Concrete Volume Calculator, Area Calculator, Paint Calculator and Flooring Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- Your Home: Australian Government guide to sustainable homes. Landscaping, materials and water use guidance
- Cement Concrete and Aggregates Australia: publications. Technical guidance on concrete and aggregate quantities
Mulch, Soil and Gravel Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.