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.
Last updated: June 22, 2026
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.