Slate Chippings Calculator
Find how many bulk bags or small bags of slate chippings you need for a border, bed or path.
Measure your project
Your answer
That is 0.50 m³.
If you buy bags
If you buy bulk bags
Berwyn Slate says 40 mm slate is not for walking on. For a path, use 20 mm.
Which costs less?
This price is for picking it up yourself. Add a delivery fee in "Change prices" if they bring it.
Bulk bag: £136.59 with VAT, the middle of only 4 prices from 2 sellers (Oct 2026). Small bag: Wickes, £10 with VAT. Treat both as rough guides.
Quick answers
- A 5 × 2 m bed, 50 mm deep
- About 0.80 tonnes. That is 1 bulk bag of 800 kg, or 40 small bags of 20 kg.
- One bulk bag covers
- About 10 m² at 50 mm deep, say Berwyn Slate (a Welsh quarry) and Wickes.
- Per m²
- About 80 kg at 50 mm deep. That is 4 small bags of 20 kg.
- Weight of 1 m³
- About 1.6 tonnes, from the cover that Berwyn and Wickes give.
How to work it out
- Measure the bed or path in metres. Length × width gives the area in m².
- Pick the depth in mm, then divide by 1,000 to get metres.
- Area × depth gives the volume in m³.
- Slate weighs about 1.6 tonnes per m³. So m³ × 1.6 = tonnes.
- Divide the tonnes by 0.8 for 800 kg bulk bags. For 20 kg bags, divide the kg by 20. Round up.
5 × 2 × 0.05 = 0.5 m³
0.5 × 1.6 = 0.80 tonnes
0.80 ÷ 0.8 = 1 bulk bag
Slate chippings coverage, by depth
A quarry and a merchant agree: an 800 kg bag of slate covers about 10 m² at 50 mm. That is 1.6 tonnes per m³. We use it for every sum here.
| Depth | kg per m² | 800 kg bag | 1 tonne | 20 kg bags per m² | Bags to buy for 1 m² |
|---|---|---|---|---|---|
| 25 mm | 40 | 20 m² | 25 m² | 2 | 2 |
| 40 mm | 64 | 12.5 m² | 15.6 m² | 3.2 | 4 |
| 50 mm | 80 | 10 m² | 12.5 m² | 4 | 4 |
For a bigger area, multiply the bags per m² by your m², then round up.
Small-bag covers can mislead. A seller on B&Q's site prints 0.5 m² for a 20 kg bag, with no depth. By our sums, that is only about 25 mm deep.
Slate chippings for common sizes
All rows are 50 mm deep. Costs use our middle bulk bag price and the Wickes small bag price, with VAT.
| Area | Tonnes | 800 kg bags (cost) | 20 kg bags (cost) |
|---|---|---|---|
| 3 × 1 m | 0.24 | 1 (£137) | 12 (£120) |
| 5 × 1 m | 0.40 | 1 (£137) | 20 (£200) |
| 3 × 3 m | 0.72 | 1 (£137) | 36 (£360) |
| 5 × 2 m | 0.80 | 1 (£137) | 40 (£400) |
| 5 × 4 m | 1.60 | 2 (£273) | 80 (£800) |
| 10 × 2 m | 1.60 | 2 (£273) | 80 (£800) |
Delivery not added. A part bag counts as a whole one.
Blue, plum and decorative slate chippings: 20 mm or 40 mm?
UK sellers sell slate in blue and plum. Berwyn Slate, a quarry, gives the sizes and uses:
- 20 mm (9–30 mm pieces): for paths, drives and borders.
- 40 mm (20–50 mm pieces): for borders and features. Not for walking on.
Berwyn gives one cover figure for both sizes. So the calculator uses the same weight for 20 mm and 40 mm.
Bulk bags or 20 kg bags of slate?
Small bags are handy for a small bed, but they cost far more per tonne:
- Bulk bags: £129.14 to £175, with VAT, from 2 sellers. Our middle price, £136.59, is about £171 a tonne for an 800 kg bag.
- Wickes 20 kg bag of 40 mm blue slate: £10, about £500 a tonne.
- 20 kg bag from a seller on B&Q's site (20 mm): £18.99 with VAT, about £949 a tonne.
With so few sellers, treat these as rough guides. Ask a local builders' merchant, with delivery.
Laying slate: membrane and washing
- Membrane: both small-bag sellers say slate keeps weeds down when used with a membrane. The RHS says choose a sheet that lets water through, as a waterproof one can cause run-off.
- Washing: Berwyn sends slate unwashed, so it is dusty. Hose it down once it is laid.
Thinking of bark instead? Our bark calculator works it out by the litre.
Questions people ask
How much slate chippings do I need?
Multiply the area in m² by the depth in metres, then by 1.6. That gives tonnes. A 5 × 2 m bed at 50 mm takes about 0.80 tonnes, or 1 bulk bag.
How much slate chippings per square metre?
About 80 kg at 50 mm deep, 64 kg at 40 mm, or 40 kg at 25 mm.
How much does a bulk bag of slate chippings cover?
An 800 kg bag covers about 10 m² at 50 mm, say Berwyn Slate and Wickes. At 25 mm, it covers about 20 m².
How many bags of slate chippings do I need?
For 20 kg bags, about 4 per m² at 50 mm. Each bag covers about 0.25 m² at that depth.
Do you need to put membrane under slate chippings?
Wickes and a seller on B&Q's site both say slate keeps weeds down when used with a membrane. The RHS says pick one that lets water through.
Do I need to wash slate chippings?
Berwyn Slate sends its chippings unwashed. It says hose them down after laying to wash the dust off.
How deep should slate chippings be?
Berwyn Slate gives its cover at 50 mm. JT Atkinson gives 40–50 mm of gravel for paths and borders.
Should I use 20 mm or 40 mm slate?
Berwyn says 20 mm suits paths, drives and borders. 40 mm is for borders and features, not for walking on.
Where our numbers come from
- Slate bulk bag covers 10 m² at 50 mm: Berwyn Slate quarry
- Slate weight: Wickes 20 mm slate chippings bulk bag
- Slate chippings price: the middle of 4 prices from only 2 UK sellers (Wickes, Tippers; Oct 2026, with VAT)
- Slate chippings 20 kg bag: Wickes Major Bag £10
- Slate chippings 20 kg bag on B&Q (marketplace seller)
- Mulch depth and membranes: RHS mulch guide
- JT Atkinson: gravel depth guide
- Mainland Aggregates: 850 kg bulk bags
Made by BuildersCount. Last checked 4 Oct 2026.
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