How Much Concrete Do I Need?
Volume in cubic feet divided by 27 gives cubic yards. That is the core math. Thickness in inches, round columns, and waste allowances still trip up many first-time estimators. When we poured a 500 sq ft driveway last spring, measuring the actual form depth instead of the plan specs saved us from a disastrous short load. Below: common project shapes, the standard 10% waste factor, and an instant bag and yard count from our calculator.
Use the Concrete Calculator →Steps
- Measure your project — For a slab: length × width in feet, thickness in inches. For a column: diameter and height.
- Convert to cubic feet — Multiply length × width × (thickness ÷ 12). Columns use π × r² × height.
- Add waste and convert to yards or bags — Add 10% for spillage. Divide ft³ by 27 for yards, or divide by bag yield (80 lb ≈ 0.60 ft³) for bags — round up.
Slab volume formula
For rectangular slabs, patios, and garage floors, multiply length (ft) × width (ft) × thickness (in) ÷ 12. That gives volume in cubic feet.
Example: a 12 ft × 10 ft patio poured 4 inches thick → 12 × 10 × 4 ÷ 12 = 40 ft³. Divide by 27 to get cubic yards: 40 ÷ 27 ≈ 1.48 yd³ before waste.
Why thickness matters
A 4-inch slab is typical for foot traffic; driveways often need 4–6 inches depending on soil and load. Changing thickness by one inch on a large slab can add a full cubic yard — always measure the actual form depth, not the nominal spec on your plan.
Bag yields and rounding
Bagged mix lists an approximate yield on the label. Typical US values: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³. Always verify the bag you are buying — high-strength mixes can differ slightly from standard sidewalk mix.
Bags needed = ceil(volume ft³ ÷ yield × (1 + waste)). We round up because you cannot buy a fraction of a bag, and partial bags opened on site often go to waste.
Why add 10% waste?
Spillage during the pour, uneven subgrades, and form deflection mean you rarely hit the theoretical volume. Ten percent is not padding for the supplier. It is a realistic buffer. In our experience, skipping this buffer usually leads to a frantic store run mid-pour.
For irregular excavations or steep slopes, bump that to 12–15%. For repeatable precast columns with tight forms, 5–8% works. When in doubt, 10% is the safe default for DIY slabs.
Ready-mix vs bags
Ready-mix trucks deliver by the cubic yard with a short pour window. Minimum orders are often 1 yd³; short-load fees apply below 4–6 yd³ depending on the plant. Bagged mix shines under ~1.5 yd³ or when access prevents a truck.
Use our calculator to compare both outputs side by side. If you are within a quarter yard of a truck minimum, ordering ready-mix usually saves labor and produces a more consistent mix.
Frequently asked questions
- How many bags for a 10×10 slab?
- A 10×10 × 4 in slab is ~40 ft³. With 10% waste and 80 lb bags (0.60 ft³ each), you need about 74 bags — use the calculator for your exact thickness.
- When should I order ready-mix?
- Most suppliers minimum is 1 yd³. At roughly 1.5+ yd³, ready-mix is usually faster and often cheaper than bags.