Overview
Flat slate chips knit into a stable, dark surface that suits contemporary planting schemes.
How the calculation works
Slate weight = area × depth × 1.6 t/m³, the same arithmetic as gravel but with slate's planning density.
One tonne of slate chippings laid 50mm deep covers about 12.5 m²; an 850kg bulk bag covers about 10.6 m².
40mm slate spreads further for the same visual coverage in mulch applications but is coarser underfoot on paths.
Worked example
A contemporary border 8m × 2m mulched with 20mm plum slate at 50mm.
Area = 16 m². Volume = 0.8 m³. Weight = 0.8 × 1.6 = 1.28 tonnes.
Order 1.4 tonnes with a 10% allowance — two 850kg bulk bags.
Slate chippings at a glance
- Planning density1.60 t/m³
- Typical range1.40–1.80 t/m³
- CategoryDecorative
- CompactionUsually not applied
- SourcePlanning value based on typical UK slate chipping bulk densities
- Reviewed2026-08-16
Planning values for typical UK supplies; your supplier product data takes precedence.
Slate chippings coverage guide
| Depth | Area per tonne | Area per 850kg bag | Weight per m² |
|---|---|---|---|
| 25mm | 25.0 m² | 21.3 m² | 40 kg |
| 40mm | 15.6 m² | 13.3 m² | 64 kg |
| 50mm | 12.5 m² | 10.6 m² | 80 kg |
| 75mm | 8.3 m² | 7.1 m² | 120 kg |
| 100mm | 6.3 m² | 5.3 m² | 160 kg |
| 150mm | 4.2 m² | 3.5 m² | 240 kg |
Computed from a 1.60 t/m³ planning density on level ground.
Assumptions and accuracy
Welsh slate, Cambrian slate and imported slates differ slightly in density; 1.6 t/m³ is the planning midpoint.
Plum, blue and green colour variants share the same grading and density.
Supplier coverage figures take precedence for specific products.
How to use this calculator
- Measure the border or path area.
- Enter 40-50mm as the dressing depth.
- Slate chippings is preselected at 1.6 t/m³.
- For large beds use the custom area field from a tape-and-string measurement.
When slate chippings is commonly used
Slate chippings mulch borders and beds, surface garden paths, edge ponds, top pots and fill gabion features. The flat chips lock together better than rounded gravel on gentle slopes.
Frequently asked questions
How much slate do I need?
At 50mm deep allow 80kg per square metre. A 20 m² border needs about 1.6 tonnes plus allowance — two bulk bags.
How many m² does a bulk bag of slate cover?
An 850kg bag covers roughly 10.6 m² at 50mm. At 40mm the same bag stretches to about 13.3 m².
20mm or 40mm slate chippings?
20mm suits paths and finer mulch work; 40mm covers larger beds more economically and resists movement on slopes, but feels coarser underfoot.
Do slate chippings stop weeds?
They suppress weeds well when laid 40–50mm over a permeable membrane. Wind-blown seed still germinates in debris that builds up over time, so occasional topping-up and clearing keeps the finish clean.
References and source notes
- Typical UK slate chipping bulk densities from supplier technical data (planning values).