Gravel Calculator
Estimate how much coarse aggregate a concrete pour requires. Enter your planned concrete volume in cubic metres and the gravel rate in cubic metres per cubic metre of concrete — the default 1.1 reflects that loose gravel occupies more volume than the concrete it produces — and get the volume to order.
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Gravel Calculator online
Work out the coarse aggregate a concrete volume needs from its mix rate — and translate the answer into the loose cubic metres and tonnes your supplier actually sells.
How it works
The tool multiplies concrete volume by the gravel rate: Gravel volume = Concrete volume (m³) × Gravel per m³ (m³/m³). With the default rate, 1 m³ of concrete needs 1 × 1.1 = 1.1 m³ of gravel. Gravel is the coarse aggregate that gives concrete its skeleton and compressive strength; because voids between stones are later filled by sand and cement paste, you need more than one cubic metre of loose stone per cubic metre of finished concrete.
Worked example
The same 3.6 m³ driveway slab (6 m × 4 m × 0.15 m) at the default 1.1 rate: 3.6 × 1.1 = 3.96 m³ of gravel. Add 5% for handling and subgrade variation: 3.96 × 1.05 = 4.16 m³. At a loose density near 1,550 kg/m³, that is about 3.96 × 1,550 = 6,138 kg (6.14 tonnes) before waste.
Units
Inputs and output are cubic metres: 1 m³ = 35.31 ft³, so 3.96 m³ is about 139.8 ft³. US suppliers often sell crushed stone by the US ton (2,000 lb); convert using the quarry’s stated density for the nominal size you ordered (for example 20 mm vs. 40 mm), because larger stone packs differently.
Practical tips
- Use 0.9–1.0 for rounded river gravel and 1.05–1.2 for angular crushed stone at the same mix, since shape changes void content.
- Check the maximum aggregate size against your reinforcement spacing and slab depth — 20 mm stone suits most slabs, larger stone suits mass footings.
- Keep separate piles for sand and gravel; mixing them on the ground throws off batching proportions.
- Use this quantity only for ordering — the choice of strength class, water-cement ratio, and aggregate grading needs review by a competent professional.
Edge cases and limitations
- Loose is not compacted. Aggregate is delivered loose and placed compacted; compacted gravel occupies roughly 85–90% of its loose volume. If the rate gives you a placed volume, order about 10–15% more loose material to land on it.
- Weight rides on moisture. The worked example uses about 1,550 kg/m³ loose. Rain-soaked gravel can run 1,650–1,750 kg/m³ — same volume, an extra tonne or two per load. When a quote is by the tonne, ask which density it assumes.
- Stone size has placement limits. As a rule of thumb, maximum aggregate should not exceed about one-third of a slab's depth, nor three-quarters of the clear gap between reinforcing bars. A rate cannot fix a mix whose stone physically cannot pass the steel.
- Grading gaps cost cement. Single-sized stone packs with more voids than well-graded aggregate; filling those voids takes mortar, which is why changing stone source mid-job can quietly change the cement demand the rate assumed.
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The calculation runs entirely in your browser — your figures are never uploaded.
Frequently asked questions
What gravel rate suits a 1:2:4 nominal mix?
About 1.1 m³ per m³ of concrete, the page default. For 2 m³ of concrete that is 2 × 1.1 = 2.2 m³ of gravel, plus waste.
Can the gravel volume really exceed the concrete volume?
Yes. Loose stone contains roughly 30–40% voids, so 1.1 m³ of gravel plus 0.6 m³ of sand compacts with cement paste into about 1 m³ of dense concrete. That is why the default gravel rate is above 1.0 while the sand default is 0.6.
How much should I add for waste?
Add 5% on level, well-managed sites and up to 10% where spillage, double handling, or soft ground is expected. On the 3.96 m³ example, 10% gives 3.96 × 1.10 = 4.36 m³ to order.
Does stone size change the result?
No — the tool multiplies volume × rate regardless of grading. If you switch from 20 mm to 40 mm crushed rock, keep the same 3.96 m³ quantity but confirm the new grading still meets your specification’s strength and workability requirements.