Step 01
Measure the slab consistently.
Measure length and width in metres and depth in centimetres, then keep the units consistent. For an irregular pad, divide it into rectangles and total the volumes.
The calculated volume is the theoretical amount before spillage, compaction and site conditions.
Step 02
Understand the calculation.
The volume per bag is printed on the product and depends on the mix and water ratio, so use the declared figure rather than a generic default.
- Volume = length × width × depth.
- Depth in centimetres ÷ 100 converts to metres.
- Bags = volume ÷ declared volume per bag, rounded up.
- Cost = whole bags × price per bag.
Step 03
Worked example: a 2 m × 1 m slab at 10 cm deep.
The volume is 2 × 1 × 0.1 = 0.2 m³. At 0.01 m³ per bag, that is 20 whole bags.
At £5 per bag, the material estimate is £100 before delivery, reinforcement, mixers or labour.
Step 04
Order whole bags with a small contingency.
Always round up to whole bags and add a small contingency for spillage and uneven ground. Confirm the mix, batch and coverage with the supplier before ordering.
This guide is a planning aid; site conditions and product instructions take priority.
Free reusable resource
Keep a practical worksheet.
Download a blank, editable file and keep the plan with your own records. No email address is required.
Concrete quantity worksheet
Record each slab or area, dimensions, volume, whole bags, price and the coverage assumptions used.