Concrete Calculator

Please enter all sizes greater than 0.
–cubic yards
–cubic feet
–80 lb bags
–60 lb bags

Estimate only. Bag yields vary by brand (about 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag), so check the label.

How to use this calculator

Choose the shape, enter the size in feet and the thickness in inches, then press Calculate. You get cubic yards for a ready-mix truck and the number of 80 lb and 60 lb bags if you mix it yourself.

The formula

Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Cubic yards = cubic feet ÷ 27
Circle area = π × diameter × diameter ÷ 4

Example: A 10 ft × 10 ft slab that is 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. With 10% extra you need about 1.36 cubic yards, or 62 bags of 80 lb concrete.

Planning your concrete pour

The calculator turns your slab, footing or round pad size into cubic yards for ready-mix and bag counts for mixing on site. Ready-mix trucks sell concrete by the cubic yard. Bags are sold by weight, and each one makes only a small amount, so the bag count climbs quickly.

A worked example

A 10 by 10 foot slab 4 inches thick is about 33.3 cubic feet, or 1.23 cubic yards. With the recommended 10% extra, that is about 1.36 cubic yards. Since an 80 pound bag yields about 0.60 cubic feet, the job needs about 62 bags. That much hand mixing makes for a long, heavy day, so ready-mix is often the better choice at this size.

What changes the amount

  • Thickness. Going from 4 to 6 inches adds 50% more concrete.
  • Uneven subgrade. Low spots in the base fill with concrete. Grade and compact the base carefully.
  • Thickened edges and footings. Slabs with deeper edges need extra. Calculate them as separate footings and add them in.
  • Round shapes. For posts and round pads, use the circle option and enter the diameter, not the radius.

Tips and common mistakes

  • Measure the forms after they are set, not the planned size, since forms often end up a bit larger.
  • Lay and compact a gravel base under slabs, often about 4 inches, to help drainage and support.
  • Do not add extra water to make concrete easier to spread. It weakens the finished slab.
  • Plan control joints. A common rule is to space them no more than about 2 to 3 times the slab thickness in feet, so about 8 to 12 feet for a 4 inch slab.
  • Keep the slab moist for several days while it cures, especially in hot or windy weather.

Ordering and cost

Enter the price per cubic yard to estimate ready-mix cost. Many suppliers charge a short-load fee for small orders, so ask about minimums. Have the crew, tools and wheelbarrows ready before the truck arrives, because waiting time may be billed. For bags, compare the total bag cost with ready-mix plus fees. Driveways, structural footings and slabs under buildings may need permits, rebar or mesh, and inspections, so check local building codes or ask a licensed contractor.

Frequently asked questions

How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb concrete or 60 bags of 60 lb concrete make one cubic yard.
How thick should a concrete slab be?
Use 4 inches for patios, walkways and shed floors, and 5 to 6 inches for driveways or heavy loads.
How much does one cubic yard of concrete cover?
About 81 square feet at 4 inches thick, 54 square feet at 6 inches thick, or 162 square feet at 2 inches thick.
Should I order extra concrete?
Yes. Adding 5 to 10 percent covers uneven ground, spillage and over-excavation.
When should I use bags instead of ready-mix?
Bags suit small jobs under about 1 cubic yard. For larger pours a ready-mix truck is usually cheaper and faster.
How long does concrete take to cure?
Concrete usually sets enough to walk on in about 24 to 48 hours. It reaches most of its design strength in about 28 days. Light vehicles can often use a driveway after about a week, but follow your contractor’s or supplier’s advice. Cold weather slows curing, and keeping the surface moist helps it gain strength.
Do I need rebar or wire mesh in a concrete slab?
Many patios and walkways use wire mesh or rebar to help hold cracks together and limit movement. Driveways and slabs carrying heavy loads usually benefit from rebar. Reinforcement does not stop cracks from forming, which is why control joints matter. Local building codes may require reinforcement for certain slabs, so check before you pour.
How much does a yard of concrete weigh?
A cubic yard of normal concrete weighs about 4,000 pounds, or roughly 2 tons. That is why a small trailer or pickup cannot safely haul much. A 1.36 cubic yard pour weighs about 5,400 pounds. Check your vehicle’s payload rating before hauling bags yourself. Forty 80 pound bags already weigh 3,200 pounds.