Concrete Calculator
Calculate how much concrete you need for slabs and columns, in cubic yards, feet, and meters.
The Formula
Add 10% for waste
= 20 × 10 × (4/12) / 27
= 200 × 0.333 / 27
= 2.47 yd³ → order 2.75 yd³
How to Calculate How Much Concrete You Need
Concrete is ordered and sold by the cubic yard in the US. To calculate volume: multiply length x width x thickness (all in feet), then divide by 27 to convert cubic feet to cubic yards. For a 10 x 20 foot patio at 4 inches thick: 10 x 20 x 0.333 = 66.6 cubic feet, divided by 27 = 2.47 cubic yards. Always order 5–10% extra to account for spillage, uneven subgrade, and waste. Rounding up to the nearest quarter yard is standard practice.
Pre-Mix Bags vs. Ready-Mix Concrete
For small projects under about 1 cubic yard, pre-mix bags are practical. An 80-lb bag yields about 0.6 cubic feet of concrete — you would need roughly 45 bags per cubic yard. For anything over a cubic yard, ordering ready-mix delivered by truck is faster, stronger, and usually more cost-effective per unit. Short loads under 5–7 yards typically incur a surcharge from concrete suppliers.
Common Projects and Recommended Thicknesses
Different applications call for different depths. Walkways and patios: 4 inches minimum. Driveways: 4–6 inches, with 6 inches recommended for heavy vehicles. Garage floors: 4–6 inches with wire mesh or rebar reinforcement. Post holes: fill to at least 12 inches below the frost line for your region. Structural elements like footings and foundations require engineered specifications beyond what a volume calculator provides.
Frequently Asked Questions
How many bags of concrete do I need?
An 80-lb bag of premix concrete yields approximately 0.6 cubic feet. Divide your total cubic feet by 0.6 to get the number of bags, then add 10% for waste. For a 1-cubic-yard project you need roughly 45 bags of 80-lb mix. For anything over a yard, ready-mix delivery is usually more practical and cost-effective.
How thick should a concrete slab be?
For standard residential use: 4 inches for patios and walkways, 4–6 inches for driveways (6 inches if heavy vehicles will use it), and 4–6 inches for garage floors. Going from 4 to 6 inches increases material cost by 50%, so match the thickness to the actual load requirements.
Do I need rebar or wire mesh?
Reinforcement is recommended for any slab subject to significant weight or stress. Driveways and garage floors should have wire mesh or rebar. Patios and walkways can often get by without it on a well-compacted subbase, but adding fiber reinforcement to the mix is inexpensive and reduces cracking. Always check local building codes.
How long does concrete take to cure?
Concrete reaches about 70% of its design strength in 7 days and full strength around 28 days. You can walk on it after 24–48 hours. Do not drive on a new driveway for at least 7 days, and ideally 28 days for full cure. Keep it moist for the first week — drying too fast weakens the surface.
How much does concrete cost?
Ready-mix concrete typically runs $125–$175 per cubic yard for material, plus delivery charges. Short loads under 5–7 yards usually incur a surcharge. Pre-mix bags from home improvement stores cost $6–10 per 80-lb bag, which works out to $270–$450 per cubic yard — more expensive per unit but with no delivery minimum, making them practical for small repairs.