Order too little aggregate and you're looking at a second delivery, a delayed job, and a driveway half-finished over a weekend when the yard's shut. Order too much and you've got a heap of MOT Type 1 sat on the drive for months. Getting the sum right the first time isn't complicated, but it trips people up more often than you'd expect, usually because they measure the area correctly and then guess the depth.
Basic Aggregate Calculator Formula
The formula itself is simple:
Length (m) × Width (m) × Depth (m) = Volume in cubic metres (m³)
Aggregate is sold by weight, though, not volume, so you then need to convert that cubic metre figure into tonnes. Most building aggregates weigh roughly 1.5 to 1.7 tonnes per cubic metre once compacted, depending on the material. Sharp sand and building sand sit at the lower end of that range, around 1.5 to 1.6 tonnes per m³. Denser materials like MOT Type 1 or crushed stone sit closer to 1.7 to 2.0 tonnes per m³ once compacted.
A rough rule that covers most jobs: multiply your volume in cubic metres by 1.7 to get a safe tonnage figure. It's not exact for every material, but it errs on the side of ordering slightly more rather than coming up short.
Worked Example: A Driveway Sub-Base
Say you're laying MOT Type 1 as a sub-base for a driveway that's 5 metres long and 3 metres wide, at a compacted depth of 100mm (0.1m).
- Volume = 5 × 3 × 0.1 = 1.5m³
- Tonnage = 1.5 × 1.7 = 2.55 tonnes
Round that up to 2.6 or 2.7 tonnes when you order. Nobody's ever complained about having slightly too much sub-base material left over, but plenty of people have had to stop halfway through compacting a driveway because they were 200kg short.
Worked Example: Bedding Sand for a Patio
For a 20 square metre patio (say 5m × 4m) needing a 50mm (0.05m) bed of sharp sand:
- Volume = 5 × 4 × 0.05 = 1m³
- Tonnage = 1 × 1.6 = 1.6 tonnes
That's roughly two standard bulk bags, since most bulk bags hold around 800kg to 850kg. Always check the actual bag weight when ordering rather than assuming every "bulk bag" is a full tonne, because they're not.
Quick Reference Table
| Job | Typical depth | Aggregate | Rough tonnage per m² |
| Driveway sub-base | 100 to 150mm | MOT Type 1 | 0.17 to 0.25 tonnes |
| Patio bedding | 50mm | Sharp sand | 0.08 tonnes |
| Path sub-base | 75 to 100mm | MOT Type 1 | 0.13 to 0.17 tonnes |
| Decorative gravel top layer | 50mm | Gravel/chippings | 0.08 to 0.1 tonnes |
Multiply the "tonnage per m²" figure by your area in square metres for a quick estimate on smaller jobs where you don't want to work through the full formula each time.
Why Depth Is Where Most People Go Wrong
Measuring length and width is straightforward, most people get that part right without thinking about it. Depth is where the mistakes creep in, usually in one of two ways.
The first is measuring the finished depth rather than the compacted depth. Loose aggregate compacts down once you've whacked it with a plate compactor, sometimes by as much as 20 to 25%. If you order for a 100mm finished layer without accounting for compaction, you'll end up short.
The second is uneven ground. A lot of gardens around Swindon and the surrounding villages sit on ground that looks flat but isn't, especially older properties where previous landscaping has left dips and high spots under the topsoil. If your sub-base needs to be a consistent 100mm everywhere, but the ground varies by 30 or 40mm across the area, your actual volume needed is higher than a flat calculation suggests. Dig a few test holes across the area before you order rather than assuming the whole site is uniform.
Common Mistakes When Calculating Aggregate
- Forgetting to convert volume to weight. Aggregate is priced and delivered by the tonne, not the cubic metre, so stopping your calculation at m³ leaves you unable to actually place an order.
- Using finished depth instead of compacted depth. Add 15 to 20% to your depth figure for anything that gets compacted, like MOT Type 1 or hardcore.
- Assuming a "bulk bag" is always a tonne. Bag weights vary by supplier and material, always check before you calculate quantity needed.
- Not accounting for wastage. A small allowance, 5 to 10%, is sensible for anything with an irregular shape or awkward access, since you'll lose some material to spillage and uneven spreading.
- Measuring only one point of an uneven site. Take several depth measurements across a larger area rather than one and assuming it's consistent throughout.
Getting the Order Right
If your project doesn't fit neatly into a rectangle, an L-shaped patio or a driveway with a curve, break it down into smaller rectangular sections, calculate each one separately, and add them together. It's more reliable than trying to estimate an irregular shape in one go.
Once you've got a rough figure, it's worth checking it against the full range of building aggregates before you order, since the exact material you choose will affect the compacted density and therefore the tonnage you actually need. If you're not confident in your own numbers, give the yard your measurements and let the team check the sum before it goes on the delivery schedule; that's a five-minute phone call against a second delivery charge if the first load falls short.