Ordering concrete starts with one question: how much concrete do I need? For a flat slab, the answer comes from three measurements — length, width, and thickness — plus a small allowance for waste. This guide walks through the arithmetic, shows a worked example, and explains when bags make sense and when ready-mix is the better choice.
If you already have your measurements, you can check the result with the Concrete Slab Calculator. For a price estimate, use the Concrete Slab Cost Calculator, and for unit conversions, the Cubic Yard Calculator.
How to work out slab volume
Work in feet throughout, and convert thickness from inches to feet first:
Thickness in feet = thickness in inches ÷ 12. Volume in cubic feet = length (ft) × width (ft) × thickness (ft). Volume in cubic yards = volume in cubic feet ÷ 27, because one cubic yard is 27 cubic feet.
Measure the longest and widest points of the slab area, and use the planned finished thickness — not the depth of any loose base underneath it. For an irregular shape, split it into rectangles, work out each rectangle separately, and add the results together.
Worked example: a 20 ft × 12 ft slab, 4 inches thick
Thickness in feet: 4 ÷ 12 = 0.333 ft. Volume: 20 × 12 × 0.333 = 80 cubic feet. In cubic yards: 80 ÷ 27 = 2.96 cubic yards.
Now add a 10% waste allowance for spillage, uneven ground, and formwork variation: 80 × 1.10 = 88 cubic feet, and 88 ÷ 27 = 3.26 cubic yards. Order about 3.26 cubic yards, not 2.96 — running short mid-pour is far harder to fix than ordering a little extra.
For bagged mix, the bag count depends on the yield per bag. Using the illustrative yield of 0.60 cubic feet per 80 lb bag shown on the calculator: 88 ÷ 0.60 = 146.7, so round up to 147 bags. Always round bag counts up, and check the yield printed on the bag you actually buy, because yields vary by product. You can compare bag options with the Concrete Mix Calculator.
Slab volume at a glance
The table below shows volumes before any waste allowance. Add 10% — or more for uneven ground — to the figures here before ordering.
| Slab size | Thickness | Volume (cubic feet) | Volume (cubic yards) |
|---|---|---|---|
| 10 ft × 10 ft | 4 in | 33.3 | 1.23 |
| 12 ft × 12 ft | 4 in | 48.0 | 1.78 |
| 20 ft × 12 ft | 4 in | 80.0 | 2.96 |
| 20 ft × 20 ft | 6 in | 200.0 | 7.41 |
Bags, waste, and ready-mix
Bags suit small jobs, repairs, and places a truck cannot reach, but the count — and the mixing labour — grows quickly. Ready-mix is usually the practical choice once a job passes about one cubic yard, and suppliers commonly charge for short loads or part loads, so confirm those terms before you order. Prices vary by region, supplier, mix strength, and delivery distance, so treat any price a calculator shows as an estimate to confirm with a local quote, not a fixed price.
Common mistakes are forgetting the inches-to-feet conversion, measuring the excavation instead of the finished slab, and ordering exactly the calculated volume with no waste allowance. A 5–10% allowance is a sensible starting point for a straightforward slab; use the higher end for irregular shapes or uneven subgrade.
Check your numbers
- Concrete Slab Calculator — volume, cubic yards, and bag count for a slab.
- Concrete Slab Cost Calculator — a cost estimate from your slab size.
- Cubic Yard Calculator — convert between cubic feet, cubic yards, and other volume units.
- Concrete Mix Calculator — compare mix and bag options.
Frequently asked questions
How many cubic yards of concrete do I need for a 10 ft × 10 ft slab?
At 4 inches thick, a 10 ft × 10 ft slab is 33.3 cubic feet, or 1.23 cubic yards before waste. With a 10% allowance, that becomes about 1.36 cubic yards.
How much waste should I allow for concrete?
Allow 5–10% for a straightforward slab. Use 10% or more for uneven ground, irregular shapes, or deep formwork, and round the final order up rather than down.
Should I buy bags or ready-mix concrete?
Bags suit small jobs and hard-to-reach areas. For larger slabs, ready-mix is usually less labour and often cheaper per cubic yard, but confirm minimum-load and delivery terms with the supplier. Prices vary by region.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. For example, 80 cubic feet ÷ 27 = 2.96 cubic yards. The Cubic Yard Calculator can do the conversion for other units as well.