Concrete Calculator
Free concrete calculator. Calculate cubic yards or bags of concrete needed for slabs, footings, sidewalks, and pads — from length, width, and thickness.
Quick answer
Cubic yards = length × width × depth ÷ 27 (with depth converted from inches to feet). A 60-lb bag yields ~0.45 ft³, an 80-lb bag yields ~0.6 ft³. Always order 5-10% extra to cover spillage and uneven forms.
Concrete Calculator
How it works
Calculates the volume of concrete you need in cubic yards (the standard unit for ordering ready-mix). Volume = length × width × depth, then divided by 27 to convert from cubic feet to cubic yards.
When to use it
Use this before pouring a slab, footing, sidewalk, patio, driveway, or fence-post foundation. Order at least 5–10% extra to cover spillage, uneven grading, and forms that aren't perfectly flat.
Common mistakes
Forgetting to convert depth from inches to feet. A 4-inch slab is 0.33 feet thick, not 4 feet. The math fails dramatically if you skip the conversion. Also: ordering exact volume — concrete trucks don't refund partial loads.
How the concrete calculator works
Concrete is sold by the cubic yard for ready-mix delivery and by the bag for small DIY pours.
The math is the same either way: volume = length × width × depth. The only catch
is unit conversion. Length and width are typically measured in feet, but depth is given in
inches because slabs and footings are usually 4-12 inches thick. Multiply length × width by
(depth ÷ 12) to get cubic feet, then divide by 27 to get cubic yards.
For bagged concrete, a 60-lb bag of standard mix yields about 0.45 cubic feet, and an 80-lb bag yields about 0.6 cubic feet. The calculator divides total cubic feet by those yields and rounds up. Cost is computed from the cubic-yard price you enter — the U.S. national average is $130-$170 per yard delivered, with surcharges for short-load fees on pours under 3 yards.
When to use it
Use this calculator before placing a ready-mix order or buying bags. Concrete trucks bill by the yard delivered, and most suppliers charge a short-load fee if you order less than 3 yards. If your project is between 0.5 and 1 yard, bagged concrete is usually cheaper — for anything 2+ yards, a delivered truck is faster, cheaper per yard, and gives you a better, more consistent pour.
Standard depths to plan around: 4 inches for sidewalks and patios, 4-6 inches for driveways, 6-8 inches for garage slabs, and 12+ inches for footings (depending on your local frost line). Always check your local building code — frost-protected footings in cold climates often need to extend 30-48 inches below grade.
Common mistakes
- Forgetting to convert depth from inches to feet. A 4-inch slab is 0.33 feet thick, not 4 feet. The math is off by a factor of 12 if you skip the conversion.
- Ordering exact volume. Concrete trucks won't refund a partial load, and you can't add more after the pour starts setting. Order at least 5% extra (10% for irregular shapes), and have a small form ready as a sacrificial dump.
- Mixing 60-lb and 80-lb bag math. An 80-lb bag yields ~33% more than a 60-lb bag — they are not interchangeable on a per-bag basis. Pick one bag size and stick with it.
- Ignoring rebar and gravel base cost. The cost figure here is concrete only. A typical slab also needs 4-6 inches of compacted gravel base ($25-$40/yd³) and rebar or wire mesh ($0.50-$1.00 per linear foot of rebar).
- Skipping the slope. Patios and driveways need to slope ~1/4 inch per foot away from the house. That extra slope adds 1-3% to your total volume on a typical pour.
Frequently asked questions
How many bags of concrete are in a cubic yard?
About 60 bags of 60-lb mix or 45 bags of 80-lb mix. Bagged concrete is significantly more expensive per cubic yard than ready-mix delivered concrete, so it only makes sense for small pours (under 1 cubic yard).
What is the standard thickness for a concrete slab?
Sidewalks and patios are typically 4 inches thick. Driveways are 4-6 inches. Garage slabs are 6 inches. Footings vary by load and frost depth — usually 8-16 inches thick and at least 30 inches below grade in cold climates.
How much extra concrete should I order?
Order 5-10% extra for most projects. Add 10% for irregular shapes, slopes, or thicker pours. Concrete trucks do not refund partial loads, and stopping the pour to wait for a second truck creates a cold joint that compromises strength.
Is bagged concrete or ready-mix cheaper?
Ready-mix is cheaper per cubic yard for any pour over about 1.5 yards. Bagged concrete is convenient for under 0.5 yards (about 1.5 bags of 80-lb mix per cubic foot) but costs roughly 2-3× more per yard. Most ready-mix suppliers also charge a short-load fee under 3 yards.
Do I need rebar or wire mesh in my slab?
For most residential slabs, yes. Rebar (#3 or #4) on 16-inch centers is standard for driveways and garage slabs. Wire mesh is acceptable for sidewalks and patios. Skipping reinforcement saves a few dollars per square foot but dramatically increases the chance of cracking.