Hammer CalculatorHow to Calculate How Much Concrete You Need
Concrete is ordered by volume, not by surface area — so before you call a ready-mix supplier or load up on bags, you need to convert your project's dimensions into cubic feet or cubic yards. The formula depends on the shape you're pouring.
Slabs, Footings & Rectangular Pours
Multiply length by width by thickness (converted to feet), then divide by 27 to get cubic yards:
- Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)
- Cubic Yards = Volume (cu ft) ÷ 27
Example: a 10 ft × 10 ft slab at 4 inches (0.33 ft) thick = 33.3 cu ft, or about 1.23 cubic yards.
Round Columns & Post Holes
Use the cylinder formula, converting the diameter to a radius in feet:
- Volume (cu ft) = π × Radius² × Height
Example: a 12-inch diameter post hole (0.5 ft radius) that's 3 ft deep = π × 0.5² × 3 ≈ 2.36 cu ft.
Always Add a Waste Factor
Real job sites are never perfectly flat, and every pour loses some mix to spillage and form seepage. Add 5-10% to your calculated volume for simple slabs, and 10-15% for irregular shapes, sloped grades, or a hand-mixed pour. Running short mid-pour means a visible cold joint and a rushed second batch — always round up.
Concrete Bag Yield Reference
If you're mixing by hand or with a small mixer, here's how much finished concrete each bag yields:
| Bag Size | Yield (cu ft) | Bags per Cubic Yard |
|---|---|---|
| 40 lb bag | 0.30 cu ft | ~90 bags |
| 60 lb bag | 0.45 cu ft | ~60 bags |
| 80 lb bag | 0.60 cu ft | ~45 bags |
Ready-Mix vs. Bagged Concrete: Which Is Cheaper?
As a rule of thumb, bagged concrete makes sense for anything under about 1 cubic yard — post holes, small footings, repairs, and steps. Once you cross roughly 1 cubic yard, ready-mix delivery almost always wins on cost per yard, and it saves substantial labor since you're not hand-mixing 40+ bags. Most ready-mix suppliers also have delivery minimums (often 1 yard), so factor that in for very small pours.
Tips for a Successful Concrete Pour
- Order 5-10% more than your calculated volume — a short pour is far more expensive to fix than a little leftover mix.
- Check your local frost depth before setting footing depth; codes vary significantly by region.
- Have your forms, rebar, and finishing tools ready before the truck arrives — ready-mix starts setting fast.
- Wet the subgrade lightly before pouring in hot weather to prevent the ground from pulling moisture out of the mix too quickly.
- Plan your control joints (roughly every 8-10 ft for slabs) to manage cracking.