How to Calculate Concrete
Multiply length × width × depth (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For depth in inches, divide by 12 first. This calculator includes 10% extra for waste.
Quick Estimation Shortcut
For a 4-inch slab: divide your total square footage by 81 to get cubic yards. For 6-inch slabs, divide by 54. Example: a 200 sq ft patio at 4 inches = 200 ÷ 81 = 2.47 cubic yards.
Recommended Slab Thickness
- 4 inches: Sidewalks, patios, garden paths
- 5-6 inches: Driveways, garage floors
- 8 inches: Heavy vehicle traffic, RV pads
- 12+ inches: Foundation footings
Example: 20×20 Patio
- Area: 400 sq ft at 4 inches thick
- Volume: 400 × 0.333 ÷ 27 = 4.94 cubic yards
- With 10% waste: 5.5 cubic yards
- Cost at $140/yard: approximately $770
Bags vs Ready-Mix
Bagged Concrete
- 40 lb bag: 0.30 cubic feet (90 bags per yard)
- 60 lb bag: 0.45 cubic feet (60 bags per yard)
- 80 lb bag: 0.60 cubic feet (45 bags per yard)
Best for projects under 1 cubic yard. Costs about $6 per 80-lb bag (~$270 per cubic yard including waste).
Ready-Mix Delivery
Best for projects over 1 cubic yard. Costs $125-165 per cubic yard delivered. Mixing 5+ yards by hand takes a full day with a portable mixer vs 30 minutes from a ready-mix truck. Expect short-load fees ($50-150 per yard under minimum) for orders under 8-10 yards.
Pro Tips
- Add 5-10% extra for waste, uneven subgrade, and spillage
- Ideal pouring temperature: 50-70°F. Cold slows curing; heat speeds it and can cause cracking
- Have all prep done before concrete arrives — it sets quickly and cannot wait
- Reinforce slabs with wire mesh or rebar for driveways and areas over 4 inches thick
- Cure time: walk on it in 24-48 hours, drive on it after 7 days, full strength at 28 days
- Keep the surface moist for the first 7 days of curing for maximum strength