Measure the pad that will actually be placed
Use the finished length and width of the slab, not the excavation. A 20 ft × 12 ft garage pad that the forms hold at 20 × 12 is 240 ft², even if the overdig was larger. Depth is the design thickness — interior slabs are often 4 in, driveways 4–6 in, garage slabs 4–5 in with thicker edges. Confirm the spec; this method does not size the slab.
Split unlike thicknesses. A 4 in interior with a 12 in thickened edge is two prisms, or an edge takeoff plus the field, not one average depth.
- Rectangle: length × width × depth.
- Circle: π × (diameter ÷ 2)² × depth. Diameter is the finished span in feet, depth still in inches.
- L-shape: outer box minus the missing inner rectangle (notch length × notch width), then times depth.
Convert to cubic yards before you phone the plant
Cubic feet ÷ 27 = cubic yards. A 20 × 12 × 4 in rectangle is 80 ft³, which is 2.96 yd³ before waste. Ready-mix is sold in steps — commonly 0.25 yd³ in the US, 0.1 m³ metric — so a net figure is not an order.
Waste is its own input. 8% is a fair start on a rectangular pour with decent access. Pump lines, steps, and over-excavated grade all eat more. Shorting the truck costs more than a leftover yard.
Do it in order
- Choose rectangle, circle or L-shape and enter finished dimensions. Keep thickness in inches.
- Apply a waste allowance that matches access and placement, not a hidden bump inside length.
- Round the waste-adjusted volume up to the plant increment, then decide bags or a truck.
Worked example — 20 ft × 12 ft × 4 in, 8% waste
Net volume is 20 × 12 × (4 ÷ 12) = 80 ft³ = 2.96 yd³. With 8% waste that is 3.20 yd³, which rounds up to a 3.25 yd³ ready-mix order. The same pour is 288 of 40 lb bags or 144 of 80 lb bags if you mix by hand — a crew day, not a saving once you are past about a cubic yard.
| Bag | Nominal bags / yd³ | Yield |
|---|---|---|
| 40 lb | 90 | 0.30 ft³ |
| 50 lb | 72 | 0.375 ft³ |
| 60 lb | 60 | 0.45 ft³ |
| 80 lb | 45 | 0.60 ft³ |
Planning boundary
This is a quantity method. It does not set compressive strength, reinforcement, joint layout, soil bearing or a guaranteed plant price.
Do I enter thickness in feet?
No. Slab depth is inches (or millimetres in metric). Length and width are feet. Circle diameter is also feet.
Can I use one rectangle for an L-shape?
You will over-order. Enter the outer box and subtract the notch, or split the L into two rectangles and add them.
Is 8% waste always enough?
It is a start on a simple rectangle. Pumping, stairs, cold joints and poor access need more. Record the percentage so a later estimator can change it.