Gravel, sand and mulch calculator
Two inches of gravel is a decorative dressing that shows the membrane through it within a season. Four inches is a path that stays put. A drive that carries a car wants six inches over a compacted sub-base, laid in two layers rather than one, because gravel dumped deep never consolidates properly in the middle.
Loose material is sold by weight but used by volume. A tonne of gravel is about 0.6 cubic metres and covers roughly 12 square metres at 50 mm deep; sand runs near 1.6 tonnes per cubic metre and bark mulch only about 0.4. Multiply area by depth, then by the density of what you are actually buying.
How to work out gravel
The number that surprises people is bulk density. Gravel is not solid rock; a loose heap is roughly 40% air, so a tonne occupies far more space than the same weight of stone would. Suppliers quote tonnes because that is what the lorry weighs, and garden centres quote litres because that is what the bag holds, which means the same job can be priced two incompatible ways. Converting between them is the whole point of this page. Bulk bags nominally hold 850 kg but arrive anywhere between 700 and 900 depending on how wet the stone was when it was filled.
Sand needs a second decision the calculator cannot make for you, because three sands do three different jobs and are not interchangeable. Sharp sand is coarse and angular, for laying slabs, screeding and mixing concrete. Building sand, also called soft sand, is fine and rounded, for mortar where workability matters more than strength. Kiln-dried sand is sharp sand dried and graded for brushing into block-paving joints, and it is the only one that will actually run into a 3 mm gap. Buying the wrong one does not become obvious until the mortar will not hold or the paving beds unevenly.
Mulch and topsoil behave differently again: they are light for their bulk, sold by volume, and the crossover to buying loose comes sooner than people expect. One cubic yard is about fourteen of the two-cubic-foot bags, so a bed needing more than eight or ten bags is usually cheaper delivered loose even after the delivery charge. The catch is that a loose delivery arrives as a heap on the drive and has to be moved that day.
What people use it for
- Ordering a bulk bag or a loose load for a driveway
- Costing decorative gravel for a border
- Filling a soakaway or French drain
- Working out sub-base under a shed or patio
- Bedding slabs or block paving with sharp sand
- Topping up borders with bark at the depth that suppresses weeds
- Filling a sandpit with washed play sand to a depth worth digging in
- Working out topsoil or compost for a raised bed
- Costing bark chippings for a play area
- Comparing a bagged price against a loose delivery before ordering
- Working out the sand for a floor screed before tiling
Questions
About 12 square metres at 50 mm deep, or 130 square feet at 2 inches. Halve the coverage if you double the depth.
At least 100 mm (4 inches) of angular gravel over a compacted sub-base, laid in two passes. Rounded pea gravel migrates under tyres and is a poor choice for a drive.
A standard bulk bag is sold as 850 kg, so a tonne is one bag and about a sixth of another, and two bags is 1.7 tonnes. Actual weights vary with moisture, which is why a bag is usually sold by volume and quoted by weight rather than the other way round.
For anything decorative, yes. A geotextile membrane stops gravel sinking into soil and stops weeds coming up. It is the cheapest part of the job and the one most often skipped.
Yes, noticeably: up to 10% more. That matters when you are checking a delivery weight but not when you are working out how far it goes.
About 10 square metres at 60 mm deep, or 20 square metres at 30 mm. A bulk bag is roughly 0.5 cubic metres.
Sharp sand, either dry-laid at 40–50 mm or in a semi-dry mortar bed. Building sand is too fine and will not hold a level.
Weight, almost always: tonnes or bulk bags. Small bags are quoted in litres or kilograms, which is why this page converts between all of them.
Yes. Expect to lose 5–10% of the loose depth to compaction, more if it is vibrated with a plate compactor.
200–300 mm gives enough depth to dig. Use washed play sand rather than builders sand, which stains skin and clothes.
About 13.5 bags of 2 cubic feet, so buy 14. In metric, about 20 bags of 50 litres to the cubic metre.
Three inches (75 mm) for beds and borders. Two inches is enough around shallow-rooted plants; four inches or more starts to keep rain off the roots.
Organic mulch breaks down, so expect to add an inch or so annually. That is the point; it feeds the soil as it goes.
No. Piling mulch against a trunk traps moisture and invites rot and rodents. Leave a clear collar of a few inches around each stem.
Around 300 kg (650 lb) dry, considerably more after rain. It is light for its bulk, which is why it is sold by volume rather than weight.