How to Estimate Concrete for a Slab, Patio or Footing
Measure, convert to cubic yards, add the right waste factor and decide between bags and ready-mix, with worked examples for common DIY projects.
Ordering the right amount of concrete matters more than with most materials. Run short during a pour and you risk a cold joint, a weak seam where fresh concrete meets concrete that has started to set. Order far too much and you pay for material you have to dispose of. Here is how professionals estimate.
Step 1: measure in feet
Measure length and width in feet and the thickness in inches. Convert any inches in length or width to decimal feet: 10 ft 6 in is 10.5 ft. For irregular shapes, split the area into rectangles and circles and add them up.
Step 2: calculate the volume
Cubic feet = Length × Width × (Thickness ÷ 12) Cubic yards = Cubic feet ÷ 27
A 12 × 16 ft patio poured 4 inches thick is 12 × 16 × 0.333 = 64 cubic feet, or 2.37 cubic yards.
For round post holes or columns, use the area of a circle: π × radius² × depth. A 10-inch hole, 3 ft deep, holds 3.14 × (0.417)² × 3 ≈ 1.64 cubic feet.
Step 3: add waste
Ground is never perfectly flat, forms bow slightly, and some concrete is always lost. Add:
- 5% for well-prepared, level forms
- 10% for typical DIY slabs
- 15% or more for uneven ground or irregular shapes
The patio above becomes 2.37 × 1.10 = 2.6 cubic yards. Ready-mix suppliers usually sell in quarter-yard increments, so you would order 2.75 yards.
Step 4: bags or a truck?
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
2.6 cubic yards would take about 117 bags of 80 lb mix, over 9,000 lb to lift and mix by hand. For anything over about one cubic yard, ready-mix is usually cheaper per yard and produces a stronger, more consistent slab. For post holes, steps and small pads, bags are ideal.
Step 5: prepare properly
- Remove topsoil and organic material, then lay and compact 4 inches of gravel base.
- Set forms level, with a slight slope (about 1/8 inch per foot) away from buildings for drainage.
- Use wire mesh or rebar for driveways and larger slabs, and plan control joints every 8–10 feet.
- Check local codes; some areas require permits for slabs and footings.
The concrete calculator does the volume, waste and bag counts for slabs and post holes in one step.