Concrete Calculator
Work out how much concrete you need for a slab, footing, or column — in cubic yards, cubic meters, and bags — before you head to the store.
Concrete volume calculator
33.33 cubic feet · 0.94 cubic meters
How concrete volume is measured
Concrete is ordered by volume. In the United States the unit is the cubic yard: a cube three feet on each side, or 27 cubic feet. Bagged concrete is sold by weight, and each bag yields a fixed volume once mixed — about 0.60 cubic feet for an 80 lb bag and 0.45 cubic feet for a 60 lb bag.
The math for any rectangular pour is simple: convert every dimension to feet, multiply length × width × thickness, then divide by 27 to get cubic yards.
Recommended slab thickness
| Application | Typical thickness | Notes |
|---|---|---|
| Footpath, patio, shed floor | 4 in | Compacted gravel base, control joints |
| Driveway (cars) | 5–6 in | Rebar or mesh recommended |
| Heavy vehicle or RV pad | 6–8 in | Engineered base and reinforcement |
| Deck pier footing | 12 in diameter × 36–48 in deep | Below frost line in cold climates |
Bags or ready-mix?
Bagged concrete is convenient for small jobs — under about one cubic yard, or roughly 45 bags of 80 lb mix. Above that, ready-mix delivery is usually cheaper and far less work. A single cubic yard weighs about 4,000 pounds, so 45 bags means lifting more than two tons of material by hand.
- Under 0.5 yd³: bags are fine.
- 0.5–1 yd³: bags are possible but tiring; compare delivery fees.
- Over 1 yd³: order ready-mix. Many suppliers have a 1-yard minimum and short-load fees.
How much waste should you add?
Order 5% to 10% extra as a rule. Concrete settles into uneven subgrade, spills during transfer, and fills low spots you did not account for. Running short in the middle of a pour creates a cold joint that weakens the slab and looks bad. For complex shapes, formwork with many corners, or a rough excavation, use 10% to 15%.
Bag yields and common slab sizes
| Bag size | Yield per bag | Bags per cubic yard | Typical use |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | Post holes, small repairs |
| 50 lb | 0.375 cu ft | 72 | Small pads, curbs |
| 60 lb | 0.45 cu ft | 60 | Walkways, patios |
| 80 lb | 0.60 cu ft | 45 | Slabs, footings |
Yields above are the figures printed by major US bagged-concrete manufacturers. Always check the bag you buy — a 10% difference in yield changes the count by several bags.
Common slabs at 4 inches thick
| Slab size | Area | Volume | Cubic yards | 80 lb bags |
|---|---|---|---|---|
| 6 × 6 ft | 36 sq ft | 12.0 cu ft | 0.44 | 20 |
| 8 × 8 ft | 64 sq ft | 21.3 cu ft | 0.79 | 36 |
| 10 × 10 ft | 100 sq ft | 33.3 cu ft | 1.23 | 56 |
| 12 × 12 ft | 144 sq ft | 48.0 cu ft | 1.78 | 80 |
| 20 × 20 ft | 400 sq ft | 133.3 cu ft | 4.94 | 223 |
These figures exclude waste. Add 10% for a typical pour, 15% for rough subgrade or complex formwork.
Odd shapes: break the slab into rectangles
Concrete volume formulas get unreliable fast, and most of them are unnecessary. Almost every residential slab is a union of rectangles, and the reliable method is to cut it into them.
An L-shaped patio 20 feet by 12 feet with a 6 by 8 foot corner removed: treat it as a 20 × 12 rectangle minus the 6 × 8 notch. At 4 inches that is 240 square feet less 48, or 192 square feet, which is 2.37 cubic yards. Adding the notch instead of subtracting it is the single most common arithmetic error on an L-shaped pour, and it doubles the order.
Curves are handled the same way with a little judgement: a rounded patio edge is approximated by a rectangle covering the same area, and a circular fire pit pad by its diameter squared times 0.785.
The thickened edge nobody counts
A flat slab calculation assumes uniform thickness. Slabs that carry a structure, or that sit at the edge of a driveway, usually turn the perimeter down to 8 or 12 inches so the edge does not spall and the load has somewhere to go.
That concrete is real and it is missing from every flat-area estimate. A 20 × 12 patio has 64 feet of perimeter. If the edge is turned down to 12 inches while the slab is 4, the extra depth is the 8 inches below the slab: 64 × 1 × 0.667 is 42.7 cubic feet, or 1.58 cubic yards — two thirds again on top of the 2.37 the slab itself needs.
If the forms show a deeper edge, measure the section and add it as a second volume. It is the difference between a pour that finishes with concrete to spare and one that stops two-thirds of the way across.
Frequently asked questions
How many 80 lb bags are in a cubic yard?
About 45. Each 80 lb bag yields around 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45. With a 10% waste allowance, plan on 50 bags.
How thick should a concrete slab be?
Four inches for patios, sidewalks, and shed floors; 5 to 6 inches for driveways and anything that carries a vehicle. Thicker slabs need more base prep and reinforcement, not just more concrete.
How much does a yard of concrete cost?
Ready-mix typically costs $125 to $175 per cubic yard in the US, plus delivery and short-load fees. Bagged mix is often $6 to $8 per 80 lb bag, which works out to $270 or more per cubic yard.
Do I need rebar or mesh?
For driveways, structural slabs, and anything over 4 inches, yes. Reinforcement controls cracking but does not replace a proper compacted base or control joints.
Can I pour concrete in cold weather?
Concrete needs to stay above about 40°F while curing. In cold weather use accelerated mix, keep the subgrade frost-free, and cover the slab. In hot weather, pour early and keep the surface moist.
Disclaimer: This calculator provides estimates only. Actual quantities depend on subgrade conditions, formwork, spillage, and the yield stated on your bag. Confirm with your supplier or a qualified contractor before ordering material.
Related: Bags of concrete per yard · Concrete slab cost · Does a 4 inch slab need rebar · Curing time · Control joint spacing · All material calculators