Concrete Calculator
Cubic yards, bags of mix, rebar, gravel, forms, and cost for your project
How to Calculate Concrete
Concrete is ordered by volume. Multiply length Γ width Γ thickness (all in feet) to get cubic feet, then divide by 27 to get cubic yards β the unit ready-mix suppliers sell in. For round post holes and columns, the volume is Ο Γ radiusΒ² Γ depth. Then add 5β10% extra for spillage, uneven ground, and forms that bow slightly. In metric, multiply meters together to get cubic meters directly.
To convert volume into bags, divide by the bag's yield. An 80 lb bag of standard concrete mix makes about 0.60 cubic feet, a 60 lb bag about 0.45, a 50 lb bag about 0.375, and a 40 lb bag about 0.30.
Worked Example: A 10 Γ 10 Patio
A 10 ft Γ 10 ft patio poured 4 inches thick is 10 Γ 10 Γ 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Adding 10% brings it to about 36.7 cubic feet (1.36 ydΒ³). That's 62 bags of 80 lb mix β almost 5,000 lbs to haul and mix by hand β or a little under 1.5 yards of ready-mix from a truck. At that size, ready-mix is usually the easier and often cheaper choice.
Bags per Cubic Yard
Typical Thickness by Project
4 inches is standard for patios, sidewalks, and shed floors. 5β6 inches is typical for driveways and anything carrying vehicles. Footings are usually at least 8 inches deep and must reach below the frost line. Fence posts are commonly set in holes about three times the post's width. These are rules of thumb β your local building code and any manufacturer specs come first.
Frequently Asked Questions
- How many bags of concrete are in a cubic yard?A cubic yard is 27 cubic feet. An 80 lb bag of concrete mix yields about 0.60 cubic feet, so you need about 45 bags per cubic yard. That works out to about 60 bags of 60 lb mix, 72 bags of 50 lb mix, or 90 bags of 40 lb mix.
- How thick should a concrete slab be?4 inches is standard for patios, sidewalks, and shed floors. Driveways and slabs that carry vehicles are usually 5 to 6 inches thick. Check your local building code and any manufacturer requirements for structures like garages or hot tubs.
- Should I use bagged concrete or order ready-mix?Bagged mix makes sense for small jobs like post holes, steps, and small pads under about 1 cubic yard. Above that, ready-mix delivered by truck is usually cheaper per yard and far less labor, since 1 cubic yard is about 45 bags weighing 3,600 lbs. Many suppliers charge a short-load fee for small orders.
- How much extra concrete should I order?Add 5 to 10 percent for spillage, uneven subgrade, and forms that bow slightly. Use the higher end for irregular excavations like post holes and footings dug by hand. Running short mid-pour is far more costly than having a little left over.
- How much does a cubic yard of concrete weigh?Normal-weight concrete weighs about 150 lbs per cubic foot, or roughly 4,000 lbs per cubic yard (about 2,400 kg per cubic meter).
- How deep should a fence post hole be?A common rule of thumb is to bury one third to one half of the post's above-ground height, and to go below your area's frost line. Hole diameter is typically about three times the post width, so roughly 10 to 12 inches for a 4x4. Local codes and fence manufacturer specs take priority.
Results are estimates for planning and budgeting. Confirm quantities with your supplier and follow local building codes for footings, reinforcement, and slab thickness.