How Much Garden Slate Do I Need?
A complete UK guide to calculating garden slate coverage — covering bag sizes, pallet weights, depth recommendations, and worked examples for borders, pathways, and driveways.
Last updated: 4 May 2026 · Reading time: 6 minutes
Quick Answer
For most UK gardens, you need 1 × 20kg bag of garden slate per 0.25m² at 50mm depth — that's roughly 4 bags per square metre. A standard half pallet (25 bags) covers approximately 6.25m², and a full pallet (49 bags) covers around 12m². For exact quantities, multiply your area in m² by 4 to get the number of 20kg bags needed.
Skip the maths: use our free coverage calculator — enter your shape, dimensions, and material, and it returns the exact bag count, weight in tonnes, and pallet recommendation.
Coverage at a Glance
The table below shows approximate coverage for our most common slate chip sizes. Coverage assumes a level, well-prepared surface with a weed membrane laid underneath.
| Chip Size | Depth | Coverage per 20kg Bag | Bags per m² |
|---|---|---|---|
| 10–14mm | 40mm | ~0.30m² | 3.3 |
| 10–14mm | 50mm | ~0.25m² | 4.0 |
| 20mm | 40mm | ~0.31m² | 3.2 |
| 20mm | 50mm | ~0.25m² | 4.0 |
| 20mm | 75mm (drives) | ~0.17m² | 5.9 |
| 40mm+ | 50mm | ~0.20m² | 5.0 |
Coverage values are approximate and assume average slate density of ~1,500 kg/m³.
How to Calculate Slate Coverage in 3 Steps
Step 1 — Measure your area in m²
For rectangles: length × width. For circles: π × radius² (use 3.14 for π). For irregular shapes, break the area into rectangles and triangles, then add the totals.
Step 2 — Choose your depth
Borders and decorative beds: 40–50mm. Pathways: 50–75mm. Driveways: 75mm minimum on a hardcore sub-base.
Step 3 — Calculate volume and weight
Multiply area (m²) × depth (m) to get cubic metres. Multiply cubic metres by 1.5 to get tonnes (slate density is ~1.5 t/m³). Divide tonnes by 0.02 to get the number of 20kg bags.
Worked Examples
Example 1 — Front garden border
Area: 4m × 1.5m = 6m². Depth: 50mm. Calculation: 6m² × 0.05m = 0.3m³ = 0.45 tonnes. Result: ~23 × 20kg bags, or approximately one half pallet.
Example 2 — Garden pathway
Area: 8m × 1m = 8m². Depth: 60mm. Calculation: 8m² × 0.06m = 0.48m³ = 0.72 tonnes. Result: ~36 × 20kg bags, or approximately three-quarters of a full pallet.
Example 3 — Small driveway
Area: 5m × 4m = 20m². Depth: 75mm. Calculation: 20m² × 0.075m = 1.5m³ = 2.25 tonnes. Result: ~113 × 20kg bags, or approximately 2.3 full pallets.
Pallet Sizing Reference
| Pallet Size | Bags Included | Total Weight | Coverage at 50mm |
|---|---|---|---|
| Quarter pallet | ~12 bags | 240kg | ~3m² |
| Half pallet | ~25 bags | 500kg | ~6.25m² |
| Full pallet | ~49 bags | 980kg | ~12.25m² |
Key Terms Explained
- Chip size
- The diameter of individual slate pieces, measured in millimetres (e.g., 20mm). Smaller chips cover more area per bag and pack more tightly.
- Depth
- How thick a layer of slate is laid. Affects both coverage and durability — too shallow risks exposing the membrane.
- Density
- Mass per unit volume. Slate averages around 1,500 kg/m³, which determines how much weight you need per cubic metre.
- Sub-base
- A compacted layer of hardcore (typically MOT Type 1) below decorative slate. Required for driveways and high-traffic surfaces to prevent sinking.
- Weed membrane
- A permeable geotextile sheet laid between soil and slate. Stops weeds growing through and prevents slate from sinking into the soil.
Frequently Asked Questions
How much area does a 20kg bag of garden slate cover?
A 20kg bag of 20mm garden slate covers approximately 0.25m² at a 50mm depth. For a 40mm depth, the same bag covers around 0.31m². Smaller chip sizes (10–14mm) cover slightly more area per bag because they pack more densely.
How deep should I lay garden slate?
For decorative borders and ground cover, lay slate at 40–50mm depth. For pathways and high-traffic areas, use 50–75mm. For driveways, a minimum 75mm depth on a properly compacted sub-base is recommended. Going shallower than 40mm risks exposing the membrane below.
How many bags of slate are on a pallet?
A full pallet typically holds 49 × 20kg bags (980kg total), a half pallet holds around 25 bags (500kg), and a quarter pallet holds approximately 12 bags (240kg). Exact counts can vary slightly by product.
What size slate chippings should I choose?
10–14mm is best for decorative borders, pots, and fine ground cover. 20mm is the most versatile size — suitable for pathways, beds, and light-traffic driveways. 40mm and larger work well as feature stones, around water features, or for drainage applications.
Do I need a weed membrane under garden slate?
Yes — a permeable weed membrane laid before the slate prevents weed growth and stops slate sinking into the soil. Geotextile membranes allow water to drain through while blocking weeds. Without one, slate will gradually mix with soil and lose its decorative effect.
How do I calculate slate for a circular area?
Multiply π (3.14) by the radius squared to get the area in m². For example, a 2m diameter circle has a 1m radius, giving an area of 3.14m². Then multiply by your chosen depth (in metres) to get cubic metres needed. Our coverage calculator handles this automatically.
How much does pallet delivery cost in the UK?
Standard UK pallet delivery is £64.59. Extended zones cost £91.44 and remote zones £157.66. Local LL postcodes (North Wales) receive free delivery. Some Scottish Highlands and island postcodes are quote-on-request due to ferry and access requirements.
Can I lay slate directly onto soil?
Not recommended. Soil contact causes slate to sink, mix, and lose its decorative finish within months. Always lay a weed membrane on a level, lightly compacted base before adding slate. For driveways, a hardcore sub-base is also required.
Ready to order?
Use our coverage calculator to get exact quantities, then browse our range of premium slate sourced from the Nantlle Valley.