Overview
Every result is an estimate for planning only. Bulk density varies with source, grading, moisture and compaction, so confirm the final order with your supplier.
How the calculation works
Volume is calculated first: the area of your shape (length × width for a rectangle, π × radius² for a circle, 0.5 × base × height for a triangle) is multiplied by the depth converted into metres.
Weight is then volume × planning density in tonnes per cubic metre (t/m³). A 2.2 t/m³ sub-base and a 1.6 t/m³ gravel of the same volume differ by more than a third in weight, which is why the material choice matters.
Finally the ordering allowance is applied: recommended order = base weight × (1 + allowance ÷ 100). Bag counts divide the total kilograms by the bag weight and round up, because part bags cannot be bought.
Worked example
A rectangular patio sub-base measures 6m × 4m at a compacted depth of 50mm, using an aggregate with a planning density of 1.70 t/m³.
Area = 6 × 4 = 24 m². Depth = 50mm = 0.05m. Volume = 24 × 0.05 = 1.20 m³. Weight = 1.20 × 1.70 = 2.04 tonnes.
With a 10% ordering allowance the suggested order is 2.24 tonnes — about 3 × 850kg bulk bags (2,240kg ÷ 850kg = 2.6, rounded up).
Generic aggregate coverage guide
| Depth | Area per tonne | Area per 850kg bag | Weight per m² |
|---|---|---|---|
| 25mm | 23.5 m² | 20.0 m² | 43 kg |
| 40mm | 14.7 m² | 12.5 m² | 68 kg |
| 50mm | 11.8 m² | 10.0 m² | 85 kg |
| 75mm | 7.8 m² | 6.7 m² | 128 kg |
| 100mm | 5.9 m² | 5.0 m² | 170 kg |
| 150mm | 3.9 m² | 3.3 m² | 255 kg |
Computed from a 1.70 t/m³ planning density on level ground.
Assumptions and accuracy
Densities used are planning defaults for typical UK supplies, not exact physical properties of your particular material.
Real bulk density changes with source quarry, grading, moisture content and the degree of compaction achieved on site.
Results are estimates. Confirm ordering quantities and product-specific densities with your supplier or contractor before ordering.
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
- Pick a project type if one matches your job — it only suggests a depth, never overrides your entry.
- Choose the closest shape: rectangle, circle, triangle, L-shape or a directly entered area.
- Enter dimensions in metric or imperial units; everything is converted to metres internally.
- Select your material so the right planning density is used.
- Open Advanced options for a supplier density, compaction allowance, bag formats and pricing.
When aggregate is commonly used
This general calculator suits any aggregate job: sub-bases, decorative gravel, drainage backfill, general fill and concrete constituents. Specialist pages for gravel, MOT Type 1, sand and ballast pre-select the relevant materials and depths.
Common UK aggregate materials
How much aggregate should I order?
Match the delivery format to your site. Below about six bulk bags, bagged delivery usually wins on convenience; above that, a loose tipper load is normally cheaper per tonne. Restricted access tips the balance back towards bags.
Keep a small surplus rather than running short: a 5-10% ordering allowance for decorative finishes and bedding sands, and 10-20% for compacted sub-bases and hardcore ordered loose, covers uneven ground, spillage and compaction differences.
Order all layers for a layered build (sub-base plus surface) in one delivery where you can store it — single deliveries save on haulage and usually secure a better price per tonne.
See the full quantity guide for depth guidance by project type.
Frequently asked questions
How much aggregate do I need per square metre?
At a 50mm depth one square metre needs 0.05 m³ of aggregate. In weight that is roughly 80kg of gravel (1.6 t/m³) or about 110kg of MOT Type 1 (2.2 t/m³), before any ordering allowance.
How many cubic metres are in a tonne of aggregate?
It depends on density. A tonne of gravel at 1.6 t/m³ is about 0.63 m³; a tonne of MOT Type 1 at 2.2 t/m³ is about 0.45 m³. Use the tonnes-to-cubic-metres converter for exact figures.
How many 850kg bulk bags make a tonne?
Two 850kg bulk bags hold 1,700kg, slightly under two tonnes, so order bags by total weight rather than assuming one bag equals one tonne — many bulk bags are filled to 850kg, 800kg or another nominal weight.
Should I add extra aggregate when ordering?
A 5-10% ordering allowance is common to cover uneven subgrade, spillage and compaction differences. The calculator shows the allowance separately so you can decide.
Are these tonnage figures exact?
No. They are planning estimates from typical planning densities. Your supplier's product data takes precedence for any firm order.
References and source notes
- Planning density values compiled from typical UK supplier technical data (see the density table on this page).
- Standards for Highways — current Specification for Highway Works (MCHW, restructured September 2025; granular sub-base formerly under Series 800) (for sub-base material contexts) — standardsforhighways.co.uk