How to calculate concrete for a slab
Concrete is bought by volume, so the first step for any pour is converting your slab's dimensions into cubic feet, then cubic yards. For a rectangular slab, the formula is:
Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)
Since slab thickness is almost always measured in inches, divide it by 12 to convert to feet before multiplying. For example, a 10×10 ft patio poured at the common 4-inch thickness works out to:
10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 yd³
Concrete rarely goes exactly to plan — subgrade is never perfectly level, some mix spills or over-fills the forms, and thickness varies slightly across the pour. That's why it's standard practice to add a waste allowance, typically 10%. For the same patio, that brings the order up to about 1.36 cubic yards, which works out to 62 bags of 80 lb concrete mix after rounding up to whole bags. This calculator applies that same allowance automatically, and it works the same way for footings and columns — just swap in the formula for that shape.
Bags vs. ready-mix: which should you buy?
Once you know your cubic yardage, the next decision is how to buy it. As a rule of thumb, projects under about 1 cubic yardare usually cheaper and more practical to handle with bagged concrete mix from a home center — you mix it in a wheelbarrow or mixer as you go, and there's no minimum order.
Once a pour crosses roughly a yard, the math tends to flip. Bagged mix for a full yard means hauling and mixing around 45 bags of 80 lb concrete by hand, which is slow and physically demanding. A ready-mix concrete truck delivers pre-mixed concrete straight to your forms and is usually cheaper per yard at that volume — the trade-off is that most ready-mix suppliers have a delivery minimum (often 1 yard or more) and may charge a short-load fee below that, so it's worth calling a couple of local suppliers to compare before you decide.
Bag yield table
Concrete mix bags are labeled by weight, but what actually matters for your calculation is the cured volume each bag yields. These are the standard yields printed on most Quikrete-style bags:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | ~90 bags |
| 60 lb | 0.45 ft³ | ~60 bags |
| 80 lb | 0.60 ft³ | ~45 bags |
To get the number of bags for your project, divide your total cubic feet (including waste allowance) by the yield of the bag size you're buying, then round up. This calculator does that automatically for all three sizes so you can compare cost per bag against how many you'd need to carry.
Common thickness guidance
Thickness has a big effect on volume — doubling thickness doubles the concrete you need — so it's worth getting it right before you order. These are common, widely-used starting points, not a substitute for your local building code:
- Sidewalks and walkways: 4 inches is the typical standard for foot traffic only.
- Driveways and patios: 4 to 6 inches, with the thicker end used where vehicles will drive or park.
- Footings: depth and width depend on soil type, frost line, and the load above them, and are usually set by local code rather than a single rule of thumb.
Because footing requirements vary so much by region and by what the footing is supporting, always check with your local building department or a structural engineer before finalizing footing dimensions — this calculator will get you the volume once you have the dimensions, but it can't tell you what your code requires.