How many cubic yards of concrete do I need for a 20x20 slab?

A 20 ft x 20 ft slab at 4 inches thick: 20 x 20 x (4 / 12) = 133.33 cu ft / 27 = 4.94 cubic yards. At 6 inches thick: 20 x 20 x (6 / 12) = 200 cu ft / 27 = 7.41 cubic yards. Always add 5 to 10 percent for waste.

How do I calculate yards of concrete for a slab?

To calculate concrete yards for a slab: Length (ft) x Width (ft) x Thickness (in / 12) = Cubic Feet, then divide by 27 to get cubic yards. Example: a 10x10 ft slab at 4 inches thick = 10 x 10 x 0.333 = 33.3 cu ft / 27 = 1.23 cubic yards. Always add 5-10% for waste.

How many bags of concrete do I need for a 10x10 slab?

For a 10x10 ft slab at 4 inches thick you need approximately 56 bags of 80lb concrete (60 bags with 10% waste). At 6 inches thick you need about 84 bags (92 with waste). Use 60lb bags instead and multiply bag count by 1.33.

Concrete Calculator – Slab, Bag, Yard & Cost Estimator

Q: How much concrete for a fence post?

A typical fence post hole is 12 inches wide and 24 to 30 inches deep. With a 4x4 post, each hole needs about 0.5 to 0.6 cubic feet of concrete. One 80lb bag per hole is usually sufficient.

Q: How long does concrete take to cure?

Concrete reaches about 70 percent of its strength in 7 days and 100 percent in 28 days under proper curing conditions. Keep concrete moist for at least 7 days for best results.

Free online concrete calculator by square footage for slabs, pads, columns, footings, walls, and post holes. Enter your project size to get concrete yards, bag counts, mix ratios, rebar, and cost — all in seconds.

Project Type

Unit

Slab

Length

ft in

Width

ft in

Thickness / Depth

Concrete Strength (PSI)

Waste Factor (%)

Concrete Bag Size

Price

$/bag

Rebar (Optional)

in spacing

$/lb

cubic yards

0 bags

Cubic Yards

Cubic Feet

Cubic Meters

Gallons

Bags Needed

Total Weight (lbs)

Cement (bags)

Rebar (lbs)

How Much Concrete Do I Need?

Whether you're pouring a patio slab, setting fence posts, building a retaining wall, or pouring footings for a new structure, knowing the exact amount of concrete required is critical. Order too little and your project is delayed — too much and you waste money with no easy way to return leftover concrete.

This concrete calculator goes beyond simple volume. It handles 6 project types (slab, round column, square column, continuous footing, wall, and post hole), automatically converts between feet, inches, and meters, calculates concrete bag counts for any bag size (40lb to 94lb), estimates mix ratios by strength, and even provides rebar quantities and cost breakdowns.

Quick Reference: A standard 4-inch thick slab requires 1 cubic yard of concrete per 81 sq ft (9 ft × 9 ft). One cubic yard = 27 cubic feet = ~46 80lb bags.

Pro Tip: Always order 5–10% extra concrete to account for spillage, uneven subgrade, and slight depth variations. For slabs, order 10% extra; for footings and columns, 5% is usually sufficient.

This tool works as a concrete slab calculator, concrete bag calculator, concrete yard calculator, concrete pad calculator, concrete block calculator, concrete footing calculator, concrete wall calculator, concrete volume calculator, and concrete mix calculator — all in one place. Whether you need to calculate concrete yards, estimate bags of concrete, or find the total concrete cost for your project, this free calculator handles it all.

Concrete Mix Ratios by Strength

PSI	Best For	Mix Ratio (C:S:A)	Cement (bags/cu yd)	Water (gal/cu yd)
2000 PSI	Footings, walkways, non-structural	1 : 3 : 5	4.5	35
2500 PSI	General purpose, garage floors	1 : 2.5 : 4	5.0	34
3000 PSI	Slabs, driveways, patios	1 : 2 : 3.5	5.5	33
4000 PSI	Structural beams, columns, foundations	1 : 1.5 : 3	6.5	31
5000 PSI	Heavy-duty industrial, bridges	1 : 1 : 2.5	7.5	29

C:S:A = Cement : Sand : Aggregate (by volume). Water is calculated at a 0.5 water-cement ratio using the actual PSI-rated recipe. 1 bag of cement = 94 lbs = 1 cubic foot.

How to Calculate Concrete

Calculating concrete is straightforward once you know the formula. The key is converting all dimensions to feet first, then dividing cubic feet by 27 to get cubic yards.

How to Calculate Concrete Yards (Cubic Yards)

To calculate concrete yards: multiply Length × Width × Thickness (in feet), then divide by 27. For example, a 10×10 slab at 4 inches thick = 10 × 10 × 0.333 = 33.3 cu ft ÷ 27 = 1.23 cubic yards.

How to Calculate Concrete for a Slab

Formula: Length (ft) × Width (ft) × Thickness (in ÷ 12) = Cubic Feet ÷ 27 = Cubic Yards

How to Calculate Cubic Yards of Concrete (Round Column / Cylinder)

Formula: π × (Diameter/2)² × Height (ft) = Cubic Feet ÷ 27 = Cubic Yards

Square Column

Formula: Width (ft) × Depth (ft) × Height (ft) = Cubic Feet ÷ 27 = Cubic Yards

Continuous Footing

Formula: Length (ft) × Width (ft) × Depth (ft) = Cubic Feet ÷ 27 = Cubic Yards

Wall

Formula: Length (ft) × Height (ft) × Thickness (ft) = Cubic Feet ÷ 27 = Cubic Yards

Post Hole (subtracting post volume)

Formula: [π × (HoleDia/2)² × HoleDepth] − [PostWidth² × HoleDepth] = Concrete volume for one hole. All dimensions in feet.

How to Calculate Concrete Yardage — Quick Reference

Project Size	Thickness	Cubic Yards	80lb Bags
10×10 ft slab	4 in	1.23 cu yd	56 bags
10×10 ft slab	6 in	1.85 cu yd	84 bags
20×20 ft slab	4 in	4.94 cu yd	222 bags
20×20 ft slab	6 in	7.41 cu yd	334 bags
30×30 ft slab	4 in	11.11 cu yd	500 bags
12 in dia × 24 in post hole	—	0.065 cu yd	3 bags

Example — 10 ft × 12 ft Slab, 6 inches thick:
Volume = 10 × 12 × (6 ÷ 12) = 60 cubic feet ÷ 27 = 2.22 cubic yards
80lb bags needed = 60 ÷ 0.6 = 100 bags (before waste)
With 5% waste: 2.22 × 1.05 = 2.33 cubic yards → 105 bags

Concrete Bag Coverage Reference

Bag Size	Volume per Bag	Bags per cu ft	Bags per cu yd	4" Slab Coverage
40 lb	0.30 cu ft	3.33	90	~2.4 sq ft per bag
60 lb	0.45 cu ft	2.22	60	~3.6 sq ft per bag
80 lb	0.60 cu ft	1.67	45	~4.8 sq ft per bag
94 lb (1 cf)	0.70 cu ft	1.43	38.6	~5.6 sq ft per bag

Note: Actual yield varies by brand and mix. Always check the bag for the manufacturer's stated yield. The values above are industry standard for standard concrete mix.

Concrete Calculator by Square Footage

To find how much concrete you need by square footage, first get the area in square feet. Multiply length by width. For a 20x20 slab, that's 400 square feet. Then multiply by the thickness in feet. A 4-inch slab is 0.333 feet thick. So 400 x 0.333 = 133.2 cubic feet. Divide by 27 to get cubic yards: 4.93 cubic yards. Add 5-10% for waste.

Use our concrete calculator by square footage above. Just pick your project type — slab, column, footing, wall, or post hole — enter the size, and it does the math for you. It also tells you how many bags of concrete to buy and the total cost.

Quick Rule: A 4-inch slab needs 1 cubic yard of concrete for every 81 square feet. For a 6-inch slab, 1 cubic yard covers 54 square feet.

Frequently Asked Questions

How many cubic yards of concrete do I need for a 20×20 slab?

A 20 ft × 20 ft slab at 4 inches thick: 20 × 20 × (4÷12) = 133.33 cu ft ÷ 27 = 4.94 cubic yards. At 6 inches thick: 20 × 20 × (6÷12) = 200 cu ft ÷ 27 = 7.41 cubic yards. Always add 5–10% for waste and uneven subgrade.

How many 80lb bags of concrete do I need per cubic yard?

One cubic yard = 27 cubic feet. An 80lb bag yields approximately 0.60 cubic feet. So 27 ÷ 0.60 = 45 bags per cubic yard. For a 5% waste factor: 45 × 1.05 = 47.25 → 48 bags.

What is the best concrete mix for a driveway?

A 4000 PSI mix (1:1.5:3 ratio) is recommended for driveways. It offers good strength for vehicle loads and freeze-thaw resistance. Use a 6-inch thick slab for standard cars, or 8-inch for heavy trucks. Always include fiber mesh or wire reinforcement for crack control.

How much concrete for a fence post?

A typical fence post hole is 12 inches wide and 24–30 inches deep. With a 4×4 post (3.5″ × 3.5″ actual), each hole needs about 0.5–0.6 cubic feet of concrete. For a 4×4 post with a 10-inch wide hole at 24 inches deep: one 80lb bag per hole is usually sufficient.

How long does concrete take to cure?

Concrete reaches about 70% of its strength in 7 days and 100% in 28 days under proper curing conditions. You can typically walk on it after 24–48 hours and drive on it after 5–7 days (for a standard driveway). Keep concrete moist for at least 7 days for best results.

Can I use the concrete calculator for metric measurements?

Yes, toggle the unit switch to Meters. Input lengths in meters, and all results will show in cubic meters, liters, and metric bag equivalents. The calculator handles both imperial and metric inputs seamlessly.

How much rebar do I need for a concrete slab?

For a 10×10 slab with #4 rebar at 18 inches on center in both directions: roughly 7 bars × 10 ft each direction = 140 linear feet. #4 rebar weighs 0.668 lbs/ft → about 94 lbs total. Use our rebar estimator above to calculate exact quantities for your project.

Should I order ready-mix concrete or bagged concrete?

Ready-mix is best for projects over 2 cubic yards — it's delivered already mixed, more consistent, and costs less per yard. Bagged concrete (like Quikrete or Sakrete) is ideal for small projects under 2 yards or remote locations where delivery trucks can't access. A truck typically carries 8–10 yards minimum.

How do I calculate yards of concrete for a driveway?

To calculate concrete yards for a driveway: multiply Length × Width × Thickness (all in feet) then divide by 27. For a standard 20 ft × 10 ft driveway at 4 inches thick: 20 × 10 × (4÷12) = 66.67 cu ft ÷ 27 = 2.47 cubic yards. Add 10% for waste → order 2.72 yards. Use the calculator above to get the exact figure for your driveway dimensions.

How many bags of concrete do I need for a 10×10 slab?

For a 10×10 ft slab at 4 inches thick: volume = 10 × 10 × (4÷12) = 33.33 cu ft. Using 80lb bags (0.60 cu ft each): 33.33 ÷ 0.60 = 56 bags. At 6 inches thick: 50 cu ft ÷ 0.60 = 84 bags. Always add 5–10% extra — so plan for 60 bags (4") or 90 bags (6").

How do I calculate concrete yardage for a pad or patio?

Calculating concrete yardage for a pad is the same formula as a slab: Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = cubic yards. A 12×12 ft patio at 4 inches: 12 × 12 × 0.333 = 47.95 cu ft ÷ 27 = 1.78 cubic yards. Select "Slab" in the calculator above and enter your pad dimensions for instant results.

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