Concrete Slab & Footing Calculator

What Your Results Mean

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 worse than 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.
• 25 kg / 80 lb Bags: Pre-mixed bag counts for hand pours, in the two common retail standards. An 80 lb bag yields approximately 0.60 cubic feet (17 litres); a 25 kg bag yields approximately 11.7 litres (0.0117 m³). Yields vary slightly by brand — check the bag.
• 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 Concrete Required: The waste-adjusted volume in both unit systems — the single number to read out to a ready-mix supplier.

Ordering from a Ready-Mix Supplier

For pours larger than about 1.5 cubic metres (2 cubic yards), ready-mix delivery is usually the practical choice over bagged concrete and dramatically easier on your back. Most ready-mix companies have a minimum load and charge a short-load fee below about 3 cubic metres. When you call, give them the volume shown in this calculator plus a little extra for safety. Specify the slump (100 mm / 4 in is standard for flatwork, 125–150 mm / 5–6 in for footings with rebar), and ask about air entrainment if you live in a freeze-thaw climate.

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.

Bag Yield Reference

Pre-mixed bag yields are published by manufacturers:

• 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:

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.

Assumptions & Limitations

Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.

• 80 lb bag yields 0.60 cu ft (17 L); 25 kg bag yields 0.0117 m³ (11.7 L) — manufacturer published values
• #4 rebar (1/2 in.), 10 ft sticks, counted in both grid directions
• Waste factor is added to volume before bags are calculated
• Quantities only — this site deliberately does not price labour, delivery or materials

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:

1. Volume: 20 × 16 × (6÷12) = 160 cubic feet = 5.93 cubic yards.
2. With 10% waste: 5.93 × 1.10 = 6.52 cubic yards. Round up to 6.6 yards for ordering.
3. 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.
4. 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.
5. Linear feet of steel: (16 × 16) + (13 × 20) = 256 + 260 = 516 linear feet.
6. In bags: 176 cu ft ÷ 0.60 = 294 eighty-pound bags — or 5.0 m³ ÷ 0.0117 = 427 twenty-five-kilo bags. Either way, that is a great deal of mixing by hand.

For comparison, the same pour is about 6.6 cubic metres from a ready-mix truck — one delivery instead of hundreds of bags and an entire weekend of mixing. The calculator gives you the volumes; the bags-versus-truck decision follows from them.

Practical Next Steps

• Read: How Much Concrete Do I Need? Volume, waste tiers and the bags-versus-truck decision.
• Read: Concrete Slab Thickness Choose 4, 5 or 6 inches with confidence.
• Plan the whole job See what else a patio or shed-pad project needs.