Quick Answer: A single 20kg bag of pre-mixed concrete yields approximately 0.0094 m³ (9.4 litres) of mixed wet concrete. In terms of area, one 20kg bag covers roughly 0.19 m² (2.0 sq ft) at 50mm (2 inches) depth, or 0.094 m² (1.0 sq ft) at 100mm (4 inches) depth. You need approximately 106 bags of 20kg concrete to make one full cubic metre (1m³).
Working out exact material quantities before starting a pour prevents costly mid-project shortages or unnecessary waste. Pre-mixed concrete in 20kg bags is ideal for small domestic repairs, fence posts, and minor bases. However, calculating how many bags you need requires converting the bag’s weight into compacted wet volume.
20kg Concrete Bag Coverage Quick Reference Table
Use this reference table to see how much area different quantities of 20kg bags will cover at standard UK project thicknesses (50mm for paths/shed bases, 100mm for driveways/slabs):
| Quantity (20kg Bags) | Total Volume (m³) | Volume (Litres) | Coverage @ 50mm (2″) | Coverage @ 100mm (4″) |
|---|---|---|---|---|
| 1 Bag | 0.0094 m³ | 9.4 L | 0.19 m² (2.0 sq ft) | 0.09 m² (1.0 sq ft) |
| 5 Bags | 0.047 m³ | 47 L | 0.94 m² (10.1 sq ft) | 0.47 m² (5.1 sq ft) |
| 10 Bags | 0.094 m³ | 94 L | 1.88 m² (20.2 sq ft) | 0.94 m² (10.1 sq ft) |
| 25 Bags | 0.235 m³ | 235 L | 4.70 m² (50.6 sq ft) | 2.35 m² (25.3 sq ft) |
| 50 Bags | 0.470 m³ | 470 L | 9.40 m² (101.2 sq ft) | 4.70 m² (50.6 sq ft) |
| 106 Bags (1 m³) | 1.000 m³ | 1,000 L | 20.00 m² (215.3 sq ft) | 10.00 m² (107.6 sq ft) |
The Math: How to Calculate Concrete Bag Yield
Pre-mixed concrete typically has an average cured density of roughly 2,130 kg per cubic metre (kg/m³) to 2,300 kg/m³, depending on aggregate size and water ratio. Using the standard industry density of 2,130 kg/m³:
- Volume in Cubic Metres: Divide the bag weight by density:
20 kg ÷ 2,130 kg/m³ = 0.009389 m³ (rounded to 0.0094 m³) - Volume in Litres: Multiply cubic metres by 1,000:
0.009389 m³ × 1,000 = 9.39 litres - Area Coverage: Divide total volume by your planned depth (in metres):
At 50mm depth (0.05m):0.009389 ÷ 0.05 = 0.188 m²
At 100mm depth (0.10m):0.009389 ÷ 0.10 = 0.094 m²
When to Switch from Bags to Volumetric Ready Mix
While 20kg bags are practical for setting a few fence posts or patching steps, hand-mixing multiple bags quickly becomes uneconomical and physically demanding. Hand-mixing also introduces consistency variations between batches, which can weaken the cured slab.
If your project requires more than 0.5 cubic metres (roughly 50+ bags of 20kg), ordering volumetric ready-mixed concrete is typically faster, cheaper, and structurally superior. With volumetric delivery, concrete is mixed fresh on-site to your exact specification, and you only pay for the exact volume you pour—eliminating unused bags and leftover waste.
Not sure how much concrete your ground prep requires? Contact the team at R.T. Mycock & Sons with your length, width, and depth measurements, and our specialists will calculate the exact cubic volume needed for your pour.
Frequently Asked Questions
How many 20kg bags of concrete are in a cubic metre (1m³)?
There are approximately 106 bags of 20kg concrete in one cubic metre (1m³). This is based on standard compacted concrete density of roughly 2,130 kg/m³.
How many 20kg bags do I need to set a fence post?
For a standard 100mm (4-inch) fence post set into a hole 200mm wide and 600mm deep, you will typically need 1.5 to 2 bags of 20kg pre-mix concrete (or dedicated post-mix) per post hole.
How much water should I add to a 20kg bag of concrete?
Most standard 20kg pre-mixed concrete bags require approximately 1.8 to 2.2 litres of clean water. Always add water gradually while mixing; adding too much water weakens the final compressive strength and leads to surface cracking.