On This Page
- Quick answers for common jobs
- Using the calculator
- How the math is checked
- Bags or ready-mix?
- Picking thickness and depth
- Three jobs, worked through
- Cost and weight
- FAQ
Quick Answers for Common Jobs
Every pour is one of two shapes. Slabs, footings, walls and driveways are rectangles: length × width × thickness, all in feet. Columns, Sonotubes and post holes are cylinders: π × radius² × depth. Divide cubic feet by 27 to get cubic yards, the unit ready-mix is sold in.
| Job | Size | Concrete (with 10% wastage) | 60 lb bags |
|---|---|---|---|
| Patio slab | 10 × 10 ft, 4 in thick | 1.36 cu yd | 82 |
| Retaining wall | 20 ft long, 4 ft tall, 8 in thick | 2.17 cu yd | 131 |
| Two-car driveway | 24 × 24 ft, 5 in thick | 9.78 cu yd | 587 |
| Deck post (Sonotube) | 10 in wide, 42 in deep | 2.10 cu ft | 5 |
| Fence post hole | 6 in wide, 24 in deep | 0.43 cu ft | 1 |
Using the Calculator
- Pick the shape: slab, footing, column, driveway or post hole.
- Enter lengths and widths in feet. Slab and driveway thickness and every diameter go in inches; footing, column and post-hole depth or height go in feet.
- Leave the 10% wastage box ticked for a real order. Untick it only to compare quotes.
- Add a price per 60 lb bag for a materials-only cost. Delivery, ready-mix and labor are not included.
Results show cubic yards, cubic feet, cubic meters and bag counts for both 60 lb and 80 lb bags. Measure inside the form: outside dimensions include the form walls and overstate the pour.
How the Math Is Checked
Footing volume multiplies length by width by depth directly, since depth is entered in feet, while slab and driveway volume convert a thickness entered in inches down to feet before multiplying: two rectangular shapes computed with different unit handling despite looking like the same L x W x H formula. Column and post-hole share the cylindrical formula, pi times radius squared times height, but the interface asks column for a "height" and post-hole for a "depth," matching the language people actually use for each job. A wastage checkbox multiplies the raw volume by 1.10 before the cubic yard, cubic meter, and bag-count figures are derived from it, so the 60 lb and 80 lb bag counts always reflect the padded volume.
Bags or Ready-Mix?
Bagged mix yields a fixed volume per bag, and Quikrete and Sakrete match each other size for size:
Always round up: 43.7 bags means buying 44. Bags suit fence posts, deck footings and pads under about half a cubic yard. A pickup carries roughly 40 to 50 bags of 60 lb, around 0.7 cubic yards. Past about 1.5 cubic yards, ready-mix is usually cheaper and faster once you count the mixing, though orders under 3 or 4 yards often carry a short-load fee.
Picking Thickness and Depth
Volume rises in a straight line with thickness, so an 8-inch slab takes exactly twice the concrete of a 4-inch one.
- 3.5 in: the code minimum; walkways only
- 4 in: patios and garage floors for cars
- 5 in: the usual residential driveway
- 6 in: RVs and trucks towing trailers
- 8 in or more: semi-trucks and heavy equipment
Footings and deck posts are sized by the frost line, not by load. The bottom has to sit below the depth where the ground freezes: 12 inches or less on the Gulf Coast, 36 to 48 inches across the Midwest and Northeast, 48 to 60 inches in the northern states. Your local building code gives the exact figure. For posts, 10-inch Sonotubes are the standard deck footing, with 8-inch for light posts and 12-inch for heavier loads. The cylinder math is the same as in the Cylinder Volume Calculator.
A wall is a rectangle stood on end: length × height × thickness. Run it through the footing mode with the wall height as depth. For a simple block-shaped form, the Cube Volume Calculator does the same length × width × height with unit conversions built in.
Three Jobs, Worked Through
Patio slab, 10 × 10 ft at 4 inches
4 ÷ 12 = 0.333 ft, so 10 × 10 × 0.333 = 33.33 cu ft. With wastage that is 36.67 cu ft, or 1.36 cu yd. In bags: ceil(36.67 ÷ 0.45) = 82 bags of 60 lb, or 62 bags of 80 lb. At $7 a bag the materials alone come to $574.
Eight deck posts in 10-inch Sonotubes, 42 inches deep
The radius is 5 inches, 0.417 ft, and the depth is 3.5 ft. π × 0.417² × 3.5 = 1.91 cu ft per post, 2.10 cu ft with wastage. ceil(2.10 ÷ 0.45) = 5 bags per post, 40 bags for all eight.
Two-car driveway, 24 × 24 ft at 5 inches
24 × 24 × 0.417 = 240 cu ft, 8.89 cu yd before wastage and 9.78 cu yd after. Order 10 yards of ready-mix. The same pour would take 440 bags of 80 lb, which is why nobody bags a driveway.
Cost and Weight
Ready-mix typically runs $120 to $200 per cubic yard delivered, plus $50 to $100 on short loads. A 60 lb bag of Quikrete or Sakrete cost $6 to $9 at Home Depot or Lowe's in 2024. For a finished slab with labor and basic finishing, $6 to $10 per square foot is a common budgeting range, so $600 to $1,000 for a 10 × 10 patio.
Normal-weight concrete weighs about 150 lb per cubic foot, around 4,050 lb per cubic yard. A 4-inch slab over 20 × 20 ft is 133.3 cu ft, roughly 20,000 lb of dead load. If the structure will also hold water, such as a cistern, the Tank Volume Calculator converts the same rectangular volume to liters and gallons.
Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length × width × thickness (all in feet) to get cubic feet. Divide by 27 for cubic yards. Add 10% for wastage. For cylindrical shapes like columns or post holes, use π × radius² × depth, where radius is the diameter divided by 2, and convert inches to feet first by dividing by 12.
How many bags of concrete per cubic yard?
One cubic yard (27 cubic feet) requires approximately 90 bags of 40 lb, 60 bags of 60 lb, or 45 bags of 80 lb concrete. Add 10% to those figures for a real job with normal wastage: 99 bags (40 lb), 66 bags (60 lb), or 50 bags (80 lb).
How deep does a concrete footing need to be?
At minimum, below the frost line for your area. In southern states that may be 12 inches; in the upper Midwest and Northeast, 36 to 48 inches is common. Check your local building code, which specifies the minimum footing depth for your region and soil type. Most codes also require footings to rest on undisturbed or compacted soil, not fill.
How many bags of concrete per fence post?
For a standard 4×4 fence post in a 6-inch-diameter hole at 24 to 30 inches deep: roughly 1 to 2 bags of 60 lb or 80 lb per post, depending on hole size. For larger 6×6 posts in 10-inch holes at 36 inches: 3 to 4 bags per post. Use Quikrete Fast-Setting for fence posts: pour it dry, add water, and it sets in 20 to 40 minutes without mixing.
How do I calculate the dry volume of concrete?
Multiply the wet volume (the finished concrete volume the formwork needs to hold) by 1.54. This factor accounts for the voids between aggregate particles and the compaction that happens during mixing. If you need 10 cu ft of finished concrete, your dry ingredients must total 10 × 1.54 = 15.4 cu ft of loose material before mixing. This calculation applies when ordering raw cement, sand, and gravel rather than pre-mixed bags.
How long does concrete take to set and cure?
Standard mix is hard enough not to disturb after 24 to 48 hours, fast-setting mix after 20 to 40 minutes. Foot traffic is fine after 24 hours, cars after 7 days, and the concrete reaches its full design strength at 28 days. Keep the surface moist for the first week so it does not dry too fast and crack. Once it has cured, sealing it helps prevent the oil, paint and rust stains covered in how to remove stains from concrete.
References
- Portland Cement Association: Concrete Mix Design: Technical guidance on concrete mix proportioning, water-cement ratios, and strength design from the leading industry body.
- QUIKRETE Technical Data Sheets: Official yield, coverage, and mix specifications for Quikrete products including standard, fast-setting, and 5000 PSI mixes.
- International Residential Code (IRC) 2021: Foundation and footing requirements including minimum dimensions, frost depth references, and bearing capacity provisions for residential construction.