Quick Answer: For a rectangular slab, multiply Length (ft) x Width (ft) x Thickness (in) ÷ 12 ÷ 27 to get cubic yards. Concrete weighs 3,915 lbs per cubic yard. An 80-lb bag yields 0.6 cu ft (45 bags per cubic yard). A 10x10 ft slab at 4" thick needs 1.23 cu yd (56 bags of 80-lb mix). Add 10% for waste. Ready-mix costs $110-150/cu yd; bagged mix costs $150-270/cu yd equivalent.
Whether you are pouring a patio slab, setting fence posts, or building a foundation wall, this calculator tells you exactly how much concrete to buy. Enter your project dimensions to get cubic yards, number of bags, total weight, and estimated cost.
How to Calculate Concrete for Common Projects
The basic formula for any rectangular pour:
Here is how to apply it to the most common projects:
- Patio or sidewalk (4" thick): Length × Width × 4 ÷ 12 ÷ 27. A 10x12 ft patio = 1.48 cu yd.
- Driveway (6" thick): Length × Width × 6 ÷ 12 ÷ 27. A 10x20 ft pad = 3.70 cu yd.
- Post holes (round): π × (radius in ft)² × depth (ft) per hole, then multiply by the number of holes.
- Walls (rectangular): Length × height × thickness (all in feet) ÷ 27.
- Steps: Calculate each step as a small rectangular volume and add the platform on top.
Concrete Needed by Project Size
Quick reference for common slab sizes at standard thicknesses:
| Slab Size | 4" Thick | 6" Thick | 80-lb Bags (4") |
|---|---|---|---|
| 8 x 10 ft | 0.99 cu yd | 1.48 cu yd | 45 |
| 10 x 10 ft | 1.23 cu yd | 1.85 cu yd | 56 |
| 10 x 20 ft | 2.47 cu yd | 3.70 cu yd | 111 |
| 12 x 12 ft | 1.78 cu yd | 2.67 cu yd | 80 |
| 20 x 24 ft | 5.93 cu yd | 8.89 cu yd | 267 |
Bags vs. Ready-Mix: Which Should You Order?
The choice between bagged concrete and a ready-mix truck depends on your project size:
- Under 0.5 cubic yards (23 bags): Bagged concrete is practical and affordable. You can mix by hand or with a small mixer. Ideal for post holes, small repairs, and patch work.
- 0.5 to 2 cubic yards (23-90 bags): Either option works. Bags are cheaper in material cost, but mixing 90 bags is backbreaking work. A rental mixer ($50-75/day) helps. Consider ready-mix if you value your time.
- Over 2 cubic yards (90+ bags): Always order ready-mix. Mixing hundreds of bags by hand is impractical, and the material cost savings vanish. Ready-mix also gives you a consistent product with the right water ratio.
| Bag Size | Yield | Bags/Cu Yd | Typical Price |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | $2.50-3.50 |
| 50 lb | 0.375 cu ft | 72 | $3.00-4.00 |
| 60 lb | 0.45 cu ft | 60 | $3.50-5.00 |
| 80 lb | 0.60 cu ft | 45 | $4.50-6.50 |
Tips for Ordering the Right Amount
- Always add 10%: Uneven sub-grade, form bulging, spillage, and over-excavation mean your actual pour volume always exceeds the calculated volume.
- Round up, not down: Running short mid-pour creates a cold joint — a weak seam where old and new concrete meet. It is far better to have leftover concrete than to fall short.
- Measure twice: Recheck all dimensions before ordering. A 6-inch error on a 20-foot slab changes the concrete needed by 5%.
- Check the sub-grade: After excavating, measure the actual depth at several points. Low spots can add 10-20% to your volume. Fill major low spots with compacted gravel before pouring.
- Plan for the minimum: Ready-mix trucks typically have a 1-yard minimum order. If you need 0.75 yards, you are paying for 1 yard anyway — use the extra for a small pad or post holes.
Formula Summary
Slab/Footing Volume: V(cu yd) = (L(ft) × W(ft) × T(in)) / (12 × 27)
Round Column Volume: V(cu ft) = π × (d/2)² × h × N where d = diameter (ft), h = height (ft), N = number of columns.
Bag Count: Bags = ⌈V(cu ft) / yield⌉. Yields: 80-lb = 0.60 cu ft, 60-lb = 0.45, 50-lb = 0.375, 40-lb = 0.30.
Weight(lbs) = V(cu yd) × 3,915
Standard Assumptions: Concrete density 3,915 lbs/cu yd (145 lbs/cu ft). 10% waste factor. 1 cu yd = 27 cu ft. Ready-mix $110-150/cu yd. 1 ton = 2,000 lbs.
Related Calculators
- Concrete Calculator — full concrete calculator with all project types
- Concrete Block Calculator — estimate CMU blocks for walls and foundations
- Rebar Calculator — calculate rebar for concrete reinforcement
- Cubic Yard Calculator — convert any dimensions to cubic yards
Estimates are for planning purposes only. Consult a qualified contractor for critical projects. Actual material requirements may vary based on site conditions, waste, and installation methods.
Pricing last verified: February 2026. Material prices vary by region, season, and supplier. Prices shown are national averages for planning purposes.