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 100 mm (4 in), driveways at 125–150 mm (5–6 in), and footings from 200 to 300 mm (8–12 in) — all varying with local building regulations and soil bearing capacity. If you're unsure about thickness, consult your local building authority or a qualified contractor; undersized slabs crack under load.

For the rebar spacing field, 400 mm (16 in) on centre is a common residential grid for 100 mm slabs, while 600 mm (24 in) on centre is acceptable for lighter-duty applications like shed pads. Set the value to 0 if you're pouring fibre-reinforced concrete without rebar, or if you're using welded wire mesh instead. The calculator assumes 12 mm (#4 / half-inch) bar, a common residential size — bar designations and stock lengths vary by country, so ask your supplier what is available locally.

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 metre 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 Metres / Cubic Yards: The total adjusted volume. Order this amount from a ready-mix supplier for pours over about 1.5 cubic metres (2 cubic yards).
• 25 kg / 80 lb Bags: Pre-mixed bag counts for hand pours, in the two common retail standards. A 25 kg bag yields approximately 11.7 litres (0.0117 m³); an 80 lb bag yields approximately 17 litres (0.60 cubic feet). Yields vary slightly by brand — check the bag.
• Rebar Pieces: Total number of grid pieces required, in both directions — cut from standard stock lengths (which vary by country).
• 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. In metric the units line up directly: metres × metres × metres gives cubic metres. If you measure in feet, multiply to cubic feet and divide by 27 for cubic yards (1 cubic yard = 27 cubic feet); 1 cubic metre = 1.30795 cubic yards.

Bag Yield Reference

Pre-mixed bag yields are published by manufacturers. Bag sizes differ by region — use the yield for the bag your supplier actually sells:

• 25 kg bag (common metric size) — approximately 11.7 litres (0.0117 m³)
• 20 kg bag (common in Australia and New Zealand) — approximately 9.5 litres
• 40 kg bag (common in Canada) — approximately 19 litres
• 40 / 60 / 80 / 90 lb bags (US sizes) — approximately 0.30 / 0.45 / 0.60 / 0.68 cubic feet (8.5–19 litres)

To convert volume to bags, divide by the yield for your bag. For example, 0.10 m³ ÷ 0.0117 = 8.5, so 9 bags of 25 kg 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 (in any consistent unit):

Each piece running along the length has a length equal to the slab width, and vice versa. Multiply and sum to get total linear length.

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.

• 25 kg bag yields 0.0117 m³ (11.7 L); 80 lb bag yields 0.60 cu ft (17 L) — manufacturer published values
• 12 mm (#4) rebar grid, counted in both directions — bar sizes and stock lengths vary by country
• Waste factor is added to volume before bags are calculated
• Quantities only — this site deliberately does not price labour, delivery or materials

Worked Example: 6×4 Metre Driveway Apron

Let's walk through a real pour. Suppose you're pouring a 6 by 4 metre driveway apron at 150 mm thick, with 12 mm (#4) rebar on a 400 mm grid, and you want a 10 percent waste factor. Plug these into the calculator and watch what happens:

1. Volume: 6 × 4 × 0.15 = 3.6 cubic metres (4.7 cubic yards).
2. With 10% waste: 3.6 × 1.10 = 3.96 cubic metres — call it 4.0 m³ (5.2 yd³) when ordering.
3. Bags: 3.96 m³ ÷ 0.0117 = 339 twenty-five-kilo bags (or 234 eighty-pound bags). That's a lot of mixing — this is exactly where ready-mix delivery becomes the practical choice.
4. Rebar: ceil(6 ÷ 0.4) + 1 = 16 pieces running width-wise, plus ceil(4 ÷ 0.4) + 1 = 11 pieces running length-wise. Total = 27 pieces, cut from standard stock lengths.
5. Linear steel: (16 × 4 m) + (11 × 6 m) = 64 + 66 = 130 linear metres.

For comparison, the same pour is about 5.2 cubic yards 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 100, 125 or 150 mm with confidence.
• Plan the whole job See what else a patio or shed-pad project needs.