Estimate cubic yards, cubic meters, 60 lb / 80 lb bag counts, #4 rebar grid requirements, and total material cost — including a waste factor. Built for patios, driveways, footings, shed pads, and walkways.
How to Use This Concrete Calculator
Start by measuring the longest side of your slab — this is your length. Then measure the perpendicular side for width. For thickness, residential patios and walkways are typically poured at 4 inches, driveways at 5 to 6 inches, and footings vary from 8 to 12 inches depending on local building code and soil bearing capacity. If you're unsure about thickness, consult your local building department or a licensed contractor; undersized slabs crack under load.
For the rebar spacing field, 16 inches on center is the most common residential grid for 4-inch slabs, while 24 inches on center is acceptable for lighter-duty applications like shed pads. Set the value to 0 if you're pouring fiber-reinforced concrete without rebar, or if you're using welded wire mesh instead. The calculator assumes #4 rebar (1/2-inch diameter), which is the standard for residential flatwork.
Waste factor accounts for spillage, over-excavation, and uneven subgrade. Five percent is fine for large flat pours on prepared ground; ten percent is a safe default for footings and irregular shapes; fifteen percent is wise for small pours under one cubic yard where a single overfill ruins your math. When in doubt, round up — running out of concrete mid-pour is far more expensive than buying one extra bag.
What the Results Mean
- Cubic Yards / Cubic Meters: The total adjusted volume. Order this amount from a ready-mix supplier for pours over 1.5 cubic yards.
- 60 lb / 80 lb Bags: Pre-mixed bag counts for hand pours. An 80 lb bag yields approximately 0.60 cubic feet of cured concrete; a 60 lb bag yields approximately 0.45 cubic feet.
- Rebar Pieces: Total number of 10-foot #4 sticks required to build the grid, including cross pieces in both directions.
- Rebar Length: Total linear length of steel needed. Useful if you buy by the bundle rather than per-stick.
- Total Cost: Bags at your input price plus rebar at your input price per stick. Does not include delivery, forms, vapor barrier, or finishing labor.
Ordering from a Ready-Mix Supplier
For pours larger than 1.5 cubic yards, ready-mix delivery is almost always cheaper than bagged concrete and dramatically easier on your back. Most ready-mix companies have a one-yard minimum load and charge a short-load fee below 3 yards. When you call, give them the cubic yards shown in this calculator plus an extra quarter yard for safety. Specify the slump (4 inches is standard for flatwork, 5 to 6 inches for footings with rebar), and ask for air entrainment if you live in a freeze-thaw climate.
Pro Tip
Ready-mix trucks are 8 to 10 feet wide and need at least 14 feet of vertical clearance. If your site can't fit a truck, plan for wheelbarrows or a concrete pump — pumps typically add $300 to $500 to the job.
The Math Behind the Calculator
Concrete volume is fundamentally length × width × depth. The trick is unit consistency: multiply in feet and divide by 27 to get cubic yards (because 1 cubic yard = 27 cubic feet), or multiply by 0.0283168 to get cubic meters.
volume_cubic_ft = length_ft × width_ft × depth_ft
volume_cubic_yd = volume_cubic_ft ÷ 27
volume_cubic_m = volume_cubic_ft × 0.0283168
volume_with_waste = volume × (1 + waste% / 100)
Bag Yield Reference
Pre-mixed bag yields are published by manufacturers and confirmed by ASTM standards:
- 40 lb bag — approximately 0.30 cubic feet
- 60 lb bag — approximately 0.45 cubic feet
- 80 lb bag — approximately 0.60 cubic feet
- 90 lb bag — approximately 0.68 cubic feet
To convert cubic feet to bags, divide by the yield. For example, 5.4 cubic feet ÷ 0.60 = 9 bags of 80 lb mix. Always round up — you cannot buy nine-tenths of a bag.
Rebar Grid Calculation
Rebar layout is a two-direction grid. The number of pieces running along the length equals the number of width-wise segments plus one (because both ends need a piece). Same goes for the width direction. Mathematically:
pieces_along_length = ceil(length_ft / spacing) + 1
pieces_along_width = ceil(width_ft / spacing) + 1
total_pieces = pieces_along_length + pieces_along_width
Each piece running along the length has a length equal to the slab width, and vice versa. Multiply and sum to get total linear feet.
Worked Example: 20×16 Driveway Apron
Let's walk through a real pour. Suppose you're pouring a 20 by 16 foot driveway apron at 6 inches thick, with #4 rebar on a 16-inch grid, and you want a 10 percent waste factor. Plug these into the calculator and watch what happens:
- Volume: 20 × 16 × (6÷12) = 160 cubic feet = 5.93 cubic yards.
- With 10% waste: 5.93 × 1.10 = 6.52 cubic yards. Round up to 6.6 yards for ordering.
- Bags: 160 × 1.10 = 176 cubic feet ÷ 0.60 = 294 eighty-pound bags. That's a lot of mixing — this is exactly where ready-mix delivery becomes economical.
- Rebar: ceil(20÷1.33)+1 = 16 pieces running width-wise, plus ceil(16÷1.33)+1 = 13 pieces running length-wise. Total = 29 pieces of 10-foot #4 rebar.
- Linear feet of steel: (16 × 16) + (13 × 20) = 256 + 260 = 516 linear feet.
- At $6.50 per 80 lb bag: 294 × $6.50 = $1,911 in bagged concrete alone. At $7.00 per rebar stick: 29 × $7.00 = $203. Total materials: $2,114.
For comparison: a ready-mix delivery of 7 yards at $145 per yard = $1,015, plus the same $203 in rebar = $1,218. On this pour, ready-mix saves nearly $900 and an entire weekend of mixing. The calculator gives you the numbers to make that call.
Reminder
The cost figure excludes forms, stakes, vapor barrier, fill, finishers' labor, and any pump fee. Treat it as a materials-only budget line, not a turnkey bid.