Overview
Gravel is sold by weight, not volume, so the planning density — not the number of wheelbarrow loads — determines your order.
How the calculation works
Gravel quantities follow volume × density: area in m² × depth in metres gives m³, and multiplying by 1.4-1.7 t/m³ (depending on gravel type) gives tonnes.
For coverage the same numbers invert: one tonne of 20mm gravel at 1.6 t/m³ covers about 0.63 m³, roughly 12.5 m² at a 50mm depth.
Rounded gravels such as pea shingle settle differently from angular gravels, which is why the planning density stays an assumption rather than a constant.
Worked example
A garden path 8m long and 1.2m wide is to be dressed with 20mm gravel at 50mm deep.
Area = 8 × 1.2 = 9.6 m². Volume = 9.6 × 0.05 = 0.48 m³. Weight = 0.48 × 1.6 = 0.77 tonnes.
With a 10% ordering allowance, order about 0.85 tonnes — one 850kg bulk bag, or 34 × 25kg bags.
20mm gravel / shingle at a glance
- Planning density1.60 t/m³
- Typical range1.40–1.70 t/m³
- CategoryGravel
- CompactionUsually not applied
- SourcePlanning value based on typical UK 20mm shingle bulk densities
- Reviewed2026-08-16
Planning values for typical UK supplies; your supplier product data takes precedence.
20mm gravel / shingle coverage guide
| Depth | Area per tonne | Area per 850kg bag | Weight per m² |
|---|---|---|---|
| 25mm | 25.0 m² | 21.3 m² | 40 kg |
| 40mm | 15.6 m² | 13.3 m² | 64 kg |
| 50mm | 12.5 m² | 10.6 m² | 80 kg |
| 75mm | 8.3 m² | 7.1 m² | 120 kg |
| 100mm | 6.3 m² | 5.3 m² | 160 kg |
| 150mm | 4.2 m² | 3.5 m² | 240 kg |
Computed from a 1.60 t/m³ planning density on level ground.
Assumptions and accuracy
A planning density of 1.6 t/m³ is used for standard 10-20mm gravel; decorative gravels default lower and slate higher.
Gravel depth for decorative finishes is usually 40-50mm; vehicle areas need a compacted sub-base beneath, not just deeper gravel.
Moisture content changes bag weights and loose loads by several percent; results are estimates for planning.
Planning density values
Use supplier product data whenever available. These figures are planning defaults shared by every calculator on the site.
| Material | Category | Planning density | Typical range | Compaction |
|---|---|---|---|---|
| MOT Type 1 | Sub Base | 2.20 t/m³ | 2.00–2.30 t/m³ | Relevant |
| MOT Type 3 | Sub Base | 2.10 t/m³ | 1.90–2.20 t/m³ | Relevant |
| Recycled Type 1 | Recycled | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| 6F2 / 6F5 crushed aggregate | Recycled | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| Road planings | Recycled | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| Sharp sand | Sand | 1.60 t/m³ | 1.50–1.70 t/m³ | Usually no |
| Building sand | Sand | 1.50 t/m³ | 1.40–1.60 t/m³ | Usually no |
| Kiln-dried sand | Sand | 1.50 t/m³ | 1.40–1.60 t/m³ | Usually no |
| 10mm pea shingle | Gravel | 1.60 t/m³ | 1.40–1.70 t/m³ | Usually no |
| 20mm gravel / shingle | Gravel | 1.60 t/m³ | 1.40–1.70 t/m³ | Usually no |
| Decorative gravel | Decorative | 1.50 t/m³ | 1.30–1.70 t/m³ | Usually no |
| Slate chippings | Decorative | 1.60 t/m³ | 1.40–1.80 t/m³ | Usually no |
| Gabion stone | Gravel | 1.50 t/m³ | 1.40–1.60 t/m³ | Usually no |
| Self-binding gravel | Gravel | 1.90 t/m³ | 1.80–2.10 t/m³ | Relevant |
| Crushed concrete | Recycled | 1.90 t/m³ | 1.70–2.10 t/m³ | Relevant |
| Limestone scalpings | Sub Base | 2.00 t/m³ | 1.80–2.20 t/m³ | Relevant |
| Ballast | Sand | 1.70 t/m³ | 1.60–1.80 t/m³ | Usually no |
| Generic aggregate | General | 1.70 t/m³ | 1.50–2.20 t/m³ | Usually no |
How to use this calculator
- Measure the length and width of the area (or use the custom area field for irregular shapes).
- Enter the gravel depth — 50mm suits most decorative finishes.
- Pick your gravel type so the correct planning density applies.
- Choose your bag format to see bulk bag or 25kg bag counts.
When gravel is commonly used
20mm gravel and shingle suit driveways, pipe bedding and general landscaping; 10mm pea shingle suits decorative paths and drainage backfill; decorative gravels and slate chippings finish borders, beds and features.
Frequently asked questions
How much gravel do I need per square metre?
At the usual 50mm depth, allow 0.05 m³ — about 80kg of standard gravel per m². A 25kg bag covers roughly 0.3 m², and a 850kg bulk bag covers about 10.6 m² at 50mm.
What depth should gravel be laid at?
Decorative gravel over a firm base is commonly laid 40-50mm. Deeper layers are for vehicle use with a proper sub-base; shallower layers show the membrane or base through.
How many m² does a bulk bag of gravel cover?
An 850kg bag of gravel at 1.6 t/m³ holds about 0.53 m³, covering roughly 10.6 m² at 50mm or 13.3 m² at 40mm.
10mm or 20mm gravel for a driveway?
20mm angular gravel is the common choice for vehicle use because it knits together and resists scatter; 10mm pea shingle rolls and is better suited to footpaths and decorative areas.
Should I put a membrane under gravel?
For decorative areas, yes: a permeable weed membrane stops stone sinking into soil and blocks most weeds. Under vehicle use, the sub-base does the structural work and a separation geotextile is used between sub-base and subgrade.
References and source notes
- Typical UK gravel and shingle bulk densities from supplier technical sheets (planning values).