I once ordered the wrong aggregate for a job and did not realise until it was already tipped. I needed sharp sand for a patio bedding layer and asked for "sand" at the builders merchant. They sent soft builder's sand - fine, smooth, completely wrong for bedding paving slabs.
It took an extra delivery, a refusal charge and half a day of sorting out to fix. The difference between aggregate types matters, and so does getting the quantity right.
What type of aggregate do you need?
Before you calculate quantity, you need the right material. Here is a plain summary of the main types:
MOT Type 1 - Crushed stone with controlled particle sizing. The standard sub-base material. Self-binding, excellent compaction. Use it under driveways, patios, paths and concrete slabs. See the MOT Type 1 Calculator for sub-base-specific calculations including the compaction factor.
Ballast (all-in aggregate) - A blended mix of coarse stone and sharp sand. Used with cement and water to make concrete. Common in bags from builder's merchants under names like "all-in ballast" or "concrete aggregate."
Sharp sand (concreting sand) - Coarse, gritty, angular sand. Used in concrete mixes, as a bedding layer under paving slabs, and in mortar for paving. Do not confuse with soft/builder's sand - they are not interchangeable.
Scalpings - A by-product of stone quarrying, containing a mix of fines and chips. Cheaper than MOT Type 1. Good for general filling and less critical sub-base applications.
Crushed limestone - Lighter coloured aggregate for driveways, paths and decorative use. Good drainage characteristics.
Gravel / shingle (10-20mm) - Rounded or angular stone for driveways, drainage, garden use. Use the Gravel Calculator for driveway and decorative gravel calculations.
The formula
Volume (m3) = Length (m) x Width (m) x Depth (m)
Weight (tonnes) = Volume x Density (t/m3)
Tonnes to order = Weight x (1 + wastage fraction)
Density reference table
| Aggregate | Typical loose density |
|---|---|
| MOT Type 1 | 2.1 t/m3 |
| Ballast (all-in) | 1.75 t/m3 |
| Sharp sand | 1.6 t/m3 |
| Scalpings | 1.9 t/m3 |
| Crushed limestone | 1.55 t/m3 |
| Gravel / shingle | 1.7 t/m3 |
The Aggregate Calculator has these values pre-loaded - just select your material type.
Worked examples
Example 1: Ballast for a concrete footing
Trench 3m x 0.6m, 200mm deep ballast layer:
- Volume = 3 x 0.6 x 0.2 = 0.36 m3
- Weight = 0.36 x 1.75 = 0.63 tonnes
- Add 10% = 0.69 tonnes
About three-quarters of a bulk bag. Round up to one bag.
Example 2: Sharp sand bedding layer for a patio
Patio 5m x 4m, 25mm bedding layer:
- Volume = 5 x 4 x 0.025 = 0.5 m3
- Weight = 0.5 x 1.6 = 0.8 tonnes
- Add 10% = 0.88 tonnes
Approximately one bulk bag of sharp sand.
Example 3: Scalpings sub-base
Driveway 10m x 4m, 100mm compacted depth (no compaction factor needed for scalpings estimate here):
- Volume = 10 x 4 x 0.1 = 4.0 m3
- Weight = 4.0 x 1.9 = 7.6 tonnes
- Add 15% = 8.74 tonnes
A loose tipper load would be the right delivery option at this volume.
Bulk bags vs loose load - the economics
| Volume | Bulk bags | Loose load |
|---|---|---|
| Under 1t | 1-2 bags | Too small for most loose loads |
| 1-3t | 2-4 bags | Possible if supplier has small loads |
| 3-6t | 4-7 bags | Loose load starting to be better value |
| Over 6t | Many bags - expensive | Loose load clearly better |
Bulk bags typically cost 15-25% more per tonne than loose loads. For a job requiring over 4-5 tonnes, the extra convenience of bags rarely justifies the cost difference.
Sub-base aggregate: remember the compaction factor
If you are using aggregate as a compacted sub-base layer (MOT Type 1, scalpings, crushed stone), the standard calculation above is not quite right - you also need to apply the 1.3x compaction factor.
MOT Type 1 compacts by approximately 20-30% when properly laid and compacted. This means you need to order 1.3 times the compacted volume.
The Sub Base Calculator and MOT Type 1 Calculator apply this factor automatically. For decorative or uncompacted aggregate (gravel, sharp sand bedding), no compaction factor applies.
Coverage per bulk bag - quick reference
At 0.85 tonne bag weight and various depths:
| Aggregate | Depth | m2 per bulk bag |
|---|---|---|
| Sharp sand | 25mm | 21 m2 |
| Sharp sand | 50mm | 11 m2 |
| Gravel (shingle) | 50mm | 10 m2 |
| Scalpings | 100mm | 4.5 m2 |
| MOT Type 1 | 100mm compacted | 3-4 m2 |
MOT Type 1 figure accounts for compaction factor.
My tips for ordering aggregate
Here is what I have picked up after getting it wrong more than once:
Be specific with the merchant. "Sand" is not a useful order - specify sharp sand or soft sand. "Gravel" could mean anything - specify the size grade (10-20mm, 6-10mm etc). "Aggregate" is even more vague. The wrong material delivered is your problem, not theirs.
Ask about minimum order quantities. Some suppliers have minimum loose load quantities (often 4-6 tonnes). If you only need 2 tonnes, bags may be your only option from that supplier regardless of the cost difference.
Get the delivery sorted before you need the material. Loose loads from busy suppliers can be 3-5 days away, especially in summer when everyone is doing garden projects. Order ahead.
Check where the tipper can drop. A full tipper truck needs reasonable access and a suitable surface to tip onto. If access is tight, consider whether bags are actually more practical for your specific site.
For an accurate calculation for any aggregate, use the Aggregate Calculator.