Inputs used
- Length (m)
- Width (m)
- Depth (cm)
- Aggregate density (t/m³)
- Bag size (kg)
gravel calculator
How the Gravel Calculator works
Assumptions, formula, inputs, and practical limits for the Gravel Calculator.
How this estimate works
The calculator turns the finished surface area and gravel depth into cubic metres, then converts that volume into weight using the density you choose. It includes a 10% allowance for settling, small measuring errors, and losses around edges, then rounds bag counts up because suppliers sell whole bags.
- Volume = length x width x depth, with depth converted from centimetres to metres.
- Weight = volume x aggregate density x a 10% waste and settling allowance.
- Bag count = estimated kg divided by bag size, rounded up.
- Bulk bag estimates depend on the bag weight entered, because supplier bag sizes vary.
Outputs generated
- cubic metres
- tonnes
- 20kg bag count
- bulk bag estimate
Worked example
A 5 x 2 m area at 5 cm depth is 0.5 cubic metres before allowance. At 1.6 t/m3, that is about 0.8 tonnes before allowance and about 0.88 tonnes after the 10% allowance. If the supplier sells 20 kg bags, divide 880 kg by 20 kg and round up, giving 44 bags.
Gravel coverage examples
These examples use a 10% allowance and 1.6 t/m3 density. They are useful for sense-checking, but supplier density, bag weight, moisture, compaction, and the finished build-up can change the actual order.
| Area and depth | Approximate volume | Approximate weight |
|---|---|---|
| 3 x 1 m at 4 cm | 0.12 m3 | 0.21 tonnes |
| 5 x 2 m at 5 cm | 0.5 m3 | 0.88 tonnes |
| 8 x 1 m at 5 cm | 0.4 m3 | 0.7 tonnes |
| 10 x 3 m at 5 cm | 1.5 m3 | 2.64 tonnes |
| 4 x 4 m at 7.5 cm | 1.2 m3 | 2.11 tonnes |
| 20 m2 at 5 cm | 1.0 m3 | 1.76 tonnes |
Before you order
- Measure the finished area after edging or borders are planned.
- Choose depth based on the intended use, not only appearance.
- Confirm aggregate density and bag size with the supplier.
- Compare small bags, bulk bags, and loose delivery for larger jobs.
- Check whether the supplier quotes by kg, tonnes, bulk bags, or cubic metres.
- Confirm where the delivery can be dropped and how far the gravel must be moved.
- Keep a note of the length, width, depth, density, bag size, and allowance used.
- For driveways or bases, confirm sub-base, compaction, and drainage requirements before ordering.
When this estimate may be wrong
- Uneven ground, compaction, and sub-base requirements can change the final quantity.
- Irregular shapes need separate area measurements or a conservative allowance.
- Driveways and load-bearing areas may need construction guidance beyond decorative gravel depth.
- Bulk bags are not always the same weight, even when they look similar in product photos.
- Moist aggregate can weigh more than dry aggregate, so supplier figures should take priority.
- Large stones may need a deeper layer than small decorative chippings to give even coverage.
Limits of the estimate
This is an estimate. Site conditions, compaction, and aggregate type can change the final quantity.
Check the depth for the intended use, measure the actual finished area after edging, and compare small bags with bulk-bag or loose-load delivery. For driveways, shed bases, and other load-bearing jobs, confirm the sub-base and drainage build-up before treating the decorative gravel quantity as the whole job.