Bulk Bag Basics
A bulk bag — also called a tonne bag, dumpy bag, or builder's bag — is the standard unit of delivery for topsoil in the UK. Understanding what you're getting helps you order the right quantity first time.
What's in a Bulk Bag?
A standard bulk bag contains approximately 0.5-0.6 cubic metres of topsoil. The exact volume varies by supplier and how tightly the bag is filled, but 0.5m³ is the figure most suppliers quote and the safest one to calculate with.
The bag itself is roughly 90cm x 90cm x 90cm, but the soil settles and compacts during transit, so it doesn't fill the bag to the brim.
How Much Does It Weigh?
A bulk bag of topsoil typically weighs 800-1000kg (roughly 0.8-1 tonne). The wide range is because moisture content makes a significant difference — dry summer topsoil is at the lighter end, winter topsoil straight from a wet stockpile is heavier. For more detail, see our guide on topsoil weight.
This weight matters for two reasons:
- Delivery logistics — a HIAB lorry carries 4-8 bags depending on the vehicle. Large orders may need multiple deliveries
- Surface protection — a bag this heavy will crack paving slabs and dent tarmac. See our delivery guide for protection tips
The Coverage Calculation
The formula is simple:
Volume needed (m³) = Length (m) × Width (m) × Depth (m)
Then divide by 0.5 (or your supplier's stated bag volume) to get the number of bags.
Coverage Per Bag at Common Depths
| Depth | Coverage per bag (0.5m³) |
|---|---|
| 25mm | 20 square metres |
| 50mm | 10 square metres |
| 75mm | 6.7 square metres |
| 100mm | 5 square metres |
| 150mm | 3.3 square metres |
| 200mm | 2.5 square metres |
Worked Examples
Example 1: Levelling a lawn Area: 8m × 6m = 48m² Average fill depth: 50mm (0.05m) Volume: 48 × 0.05 = 2.4m³ Bags needed: 2.4 ÷ 0.5 = 4.8 → order 5 bags
Example 2: New turf preparation Area: 10m × 5m = 50m² Topsoil depth: 150mm (0.15m) — see our turf preparation guide Volume: 50 × 0.15 = 7.5m³ Bags needed: 7.5 ÷ 0.5 = 15 bags At this quantity, a loose load is better value — see below
Example 3: Filling a raised bed Bed dimensions: 2.4m × 1.2m × 0.45m = 1.3m³ Bags needed: 1.3 ÷ 0.5 = 2.6 → order 3 bags (Consider a topsoil-compost mix rather than pure topsoil for raised beds)
Irregular Shapes
For L-shaped, curved, or irregular areas, split them into rectangles, calculate each separately, and add the totals. For circles (round beds, tree pits), the formula is: π × radius² × depth.
When to Choose Bulk Bags vs Loose Loads
Bulk Bags Are Better When:
- You need 1-5 bags
- Access is tight (bags can be craned over walls and fences)
- You can't use all the soil immediately and need it contained
- You need the soil placed precisely (e.g., near the back garden, not on the drive)
Loose Loads Are Better When:
- You need more than 5-6 cubic metres
- You have good access for a tipper lorry
- You can move the soil quickly from where it's tipped
The cost difference is significant. Bulk bags typically cost £60-£100 each (including delivery for multi-bag orders), while a loose load of 10m³ might cost £400-£600 — roughly half the per-cubic-metre price of bags.
Common Ordering Mistakes
Underestimating Depth
The most common error is underestimating how deep you need the topsoil. Check depth recommendations for your specific project before calculating. A lawn needs at least 100mm; raised beds need 300mm+; vegetable gardens need 200-300mm.
Forgetting Settlement
Fresh topsoil settles by roughly 10-15% over the first few weeks as it's watered and rained on. Order 10% more than your calculation to account for this.
Not Measuring Properly
Measure your garden, don't estimate. A garden that "looks about 10 metres long" might be 8 or 12. A tape measure costs less than an extra delivery charge for the bags you forgot.
Mixing Up Volume and Weight
Suppliers sell by volume (cubic metres) or by weight (tonnes). These aren't the same. One cubic metre of topsoil weighs approximately 1.3-1.5 tonnes depending on moisture — see our weight guide. If a supplier quotes per tonne, make sure you're comparing like-for-like.
Tips for Ordering
- Measure twice, order once. Use our quantity calculator guide for a thorough calculation
- Round up, never down. An extra half-bag costs £30-£50. A second delivery costs £50-£100+
- Ask about part-loads. Some suppliers offer a discount if you order, say, 6 bags at once instead of making two separate orders
- Check the volume guarantee. Confirm whether the supplier's bags are 0.5m³ or 0.6m³ — that 20% difference matters across a large order
- Plan for delivery access. Bulk bags need a HIAB crane. Check for overhead wires and clearance before ordering