Calculate how much concrete you need for slabs, footings, and columns. Results in cubic yards, cubic meters, and bags.
Slab: Multiply length x width x depth to get volume in cubic feet, then divide by 27 to convert to cubic yards. Cubic yards = (L x W x D) / 27
Round column: Calculate the area of the circle (pi x r^2), multiply by height, then divide by 27. Cubic yards = (pi x r^2 x H) / 27
Wall/footing: Multiply length x height x width and divide by 27. Same as a slab but oriented vertically.
Bag coverage: A standard 80 lb bag of concrete mix yields about 0.6 cubic feet. A 60 lb bag yields about 0.45 cubic feet.
Pro tip: Order 5-10% extra concrete to account for spillage, uneven ground, and form variations.
The amount of concrete you need depends on three measurements: length, width, and depth (thickness). For a typical 10x10 foot patio slab that is 4 inches thick, you will need approximately 1.23 cubic yards of concrete, which translates to about 56 bags of 80 lb pre-mixed concrete. That same slab at 6 inches thick jumps to 1.85 cubic yards -- nearly 50% more material.
For most residential projects -- patios, sidewalks, shed pads, and garage floors -- a 4-inch thickness is the standard minimum. Driveways should be poured at 5 to 6 inches because they bear vehicle weight. Post footings for fences and decks are typically 10 to 12 inches in diameter and 24 to 42 inches deep, depending on your local frost line.
A critical rule most beginners miss: always order 5-10% more concrete than your calculation shows. Ground is rarely perfectly level, forms can bulge, and some concrete always gets left in the truck or wasted during placement. Running short mid-pour is one of the worst situations in construction because a cold joint (where fresh concrete meets partially cured concrete) creates a permanent weak point in the slab.
Let us walk through a real example: calculating concrete for a 12 x 20 foot driveway slab, 5 inches thick.
Step 1: Convert all measurements to feet.
Length = 20 ft, Width = 12 ft, Depth = 5 inches = 5/12 = 0.4167 ft
Step 2: Multiply to get cubic feet.20 x 12 x 0.4167 = 100 cubic feet
Step 3: Convert cubic feet to cubic yards.100 / 27 = 3.70 cubic yards
(There are 27 cubic feet in one cubic yard: 3 x 3 x 3 = 27)
Step 4: Add 10% for waste.3.70 x 1.10 = 4.07 cubic yards
Round up to 4.25 cubic yards when ordering from a ready-mix plant (they typically deliver in quarter-yard increments).
Step 5: Convert to bags if doing it yourself.
One 80 lb bag of concrete mix yields approximately 0.6 cubic feet.100 cubic feet / 0.6 = 167 bags (80 lb)
At 80 lbs each, that is over 13,000 lbs of concrete. For anything over 1 cubic yard (about 45 bags), a ready-mix truck is almost always more practical and cost-effective.
80 lb bags vs. ready-mix truck -- when to use which: For small projects under 1 cubic yard (fence post footings, small pads, patching), bags are convenient and allow you to work at your own pace. Each 80 lb bag yields about 0.6 cubic feet of mixed concrete. For anything larger, a ready-mix truck delivers freshly mixed concrete directly into your forms. Most ready-mix companies have a minimum order of 1 cubic yard and charge short-load fees for orders under 3-5 yards.
Concrete strength (PSI): Standard residential concrete is mixed at 3,000-3,500 PSI. Driveways and garage floors should be 3,500-4,000 PSI. Bagged mix from hardware stores is typically 4,000 PSI when properly mixed.
Curing timeline: Concrete reaches about 70% of its rated strength after 7 days and full strength at 28 days. You can walk on it after 24-48 hours. Do not drive vehicles on a new driveway for at least 7 days (14 days is better). Keep the surface moist for the first week by misting with water or covering with plastic sheeting.
Reinforcement: For slabs, use welded wire mesh (6x6 W1.4/W1.4) or #3 rebar on 18-inch centers. For footings and walls, rebar is essential. Fiber mesh can be added to the mix for additional crack resistance but does not replace structural reinforcement.
Ready-mix concrete: $130-$175 per cubic yard delivered, depending on your region and the mix design. Most deliveries include a truck charge of $50-$100 on top of the per-yard price. Short-load fees ($30-$50 per yard under minimum) apply for small orders.
Bagged concrete: 80 lb bags cost $5.50-$7.50 each at home improvement stores. At roughly 45 bags per cubic yard, the material cost alone is $250-$340 per cubic yard -- significantly more expensive than ready-mix, but without the delivery fees and minimum order requirements.
Total installed cost: If hiring a contractor, expect $8-$16 per square foot for a standard 4-inch slab including forming, pouring, finishing, and cleanup. Complex work like stamped or colored concrete runs $12-$25 per square foot.
Prices are approximate US averages for 2026 and vary by region, season, and market conditions.
| Project | Dimensions | Cubic Yards | 80 lb Bags | Est. Cost |
|---|---|---|---|---|
| Small patio slab | 10 x 10 ft, 4 in | 1.23 | 56 | $160-$215 |
| Sidewalk (4 ft wide, 30 ft) | 4 x 30 ft, 4 in | 1.48 | 67 | $190-$260 |
| Two-car driveway | 20 x 20 ft, 5 in | 6.17 | 280 | $800-$1,080 |
| Fence post footing (each) | 10 in dia, 36 in deep | 0.06 | 3 | $17-$23 |
| Garage floor | 20 x 24 ft, 5 in | 7.41 | 336 | $960-$1,300 |
| Large patio | 16 x 20 ft, 4 in | 3.95 | 179 | $510-$690 |
Always round UP -- running short mid-pour is far worse than having a little extra. Leftover concrete can be used for stepping stones or small pads.
Concrete sets faster in hot weather. On days above 90 degrees F, you may have less than 60 minutes of working time. Consider scheduling pours for early morning or adding a retarder to the mix. In cold weather (below 50 degrees F), use heated water in your mix and protect fresh concrete from freezing for at least 48 hours.
Do not add extra water to make concrete easier to work with. Excess water weakens the final product. If you need better workability, ask the ready-mix plant about adding a plasticizer (water reducer) instead.
Grade the subbase properly. Compact a 4-inch gravel base under your slab to prevent settling and improve drainage. This is one of the most commonly skipped steps and one of the biggest causes of cracking.
Use proper forming. 2x4 lumber works for 4-inch slabs; use 2x6 for 5-6 inch slabs. Stake forms every 2-3 feet and oil them before pouring so they release easily.
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