Gravel Calculator

Two fields and a density dial

Gravel orders live and die on one multiplication: area × depth × density. The first two come from your tape; the third is a supplier fact most quotes already state (around 1.5 tonnes per cubic metre for the common gravels). This page runs the multiplication, prints the volume in cubic metres and litres, converts at the density you enter, and rounds up the count of 25 kg bags — which is the comparison point between a bagged pickup and a yard delivery.

Depth is where intuition fails. A 5 cm topping looks paper-thin on the ground and is 1 m³ on a 20 m² driveway; a 10 cm HVAC apron is 2 m³ in the same space. Because loose gravel settles and beds into soft ground, real coverage usually runs slightly short of the theoretical — order the exact figure, then lean on the supplier's rounding.

A worked example, step by step

Problem: a garden path 20 m², 5 cm deep, quoted at 1.5 t per m³.

1. Volume: 20 × 0.05 = 1.0 m³ (1,000 litres).
2. Weight: 1.0 × 1.5 = 1.5 tonnes.
3. Bag read: 1,500 kg ÷ 25 kg = 60 bags exactly.
4. Cars-and-trucks read: 60 bags is two wheelbarrow-heavy trips from a DIY store; one yard delivery, one trip.
5. Deeper variant: the same area at 10 cm is 2.0 m³ → 3.0 t → 120 bags. Doubling depth doubles everything.

The bag count is the honest tiebreaker. Bags are easy to move, dear per tonne; bulk deliveries are cheaper per tonne and need access. Sixty bags versus one tipper is the practical boundary most driveways see.

Depth choices, all for 20 m²

One read: depth is linear in tonnes, so every centimetre of extra depth on 20 m² adds about 0.3 t at 1.5 t/m³ — the density the common gravels land at. Quarried crushed stone runs slightly heavier (1.6–1.7), decorative slate lighter (1.3–1.4); the density field takes whatever the quote states.

Sub-base, compaction and the real world

A path's load-bearing layer is the compacted sub-base beneath, not the 5 cm of gravel above: a membrane plus compacted MOT-type stone is what stops ruts, while decorative gravel is the finish. Gravel that settles into soil needs topping next season; weed fabric plus a deeper gravel needs less maintenance. The tool's volume is the finish layer's volume — by design. When a quote says "spread 50 mm", the tool's answer is exact; when the ground is soft and uncompacted, expect the supplier's rounding (and a spare half-tonne of patience) to matter.

Mistakes that quietly double a driveway order

• Entering metres for the depth. 5 "metres" of depth on a 20 m² path is 100 m³ — read the field label; the tool wants centimetres.
• Guessing the density. 1.3 against 1.6 t/m³ is a 20% weight swing; the quote or the material sheet beats folklore.
• Skimping depth on driving surfaces. A 3 cm gravel finish under cars clocks into ruts within a season; depth is not the place to save.
• Counting the sub-base twice. Decide which layer the quote covers; this tool prices one layer at a time, cleanly.

Keep exploring

• Concrete Calculator — the pour instead of the finish: slab volume and weight.
• Square Footage Calculator — the same areas in feet and square yards, when a UK-style quote arrives.
• Area Converter — cross-system conversions when the plot is listed in acres.
• Guide: Unit Conversion Mistakes to Avoid.