Concrete Calculator
Find out how much concrete you need for a slab, footing, pad or path — and exactly how many bags of pre-mix to buy. Enter the length, width and thickness, how many you are pouring, and a waste allowance. You'll get the volume in cubic yards or cubic meters and the bag count. Updates as you type.
How it works
volume = length × width × thickness × quantity × (1 + waste%)
- Volume is length × width × thickness with the units kept consistent (thickness is converted from inches/cm).
- Cubic yards = cubic feet ÷ 27 (imperial); metric is shown directly in cubic meters.
- Waste adds your allowance (10% by default) for spillage, uneven ground and over-dig.
- Bags = volume ÷ bag yield, rounded up. Yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45, 40 lb ≈ 0.30; 20 kg ≈ 0.009 m³.
Pouring more than about half a cubic yard (0.4 m³)? It's usually cheaper and stronger to order ready-mix delivered than to mix dozens of bags by hand.
FAQ
How do I calculate how much concrete I need?
Multiply length × width × thickness to get the volume, keeping the units consistent. In feet, divide the result by 27 to get cubic yards; in meters the volume is already in cubic meters. Add a small waste allowance for spillage and uneven ground. This calculator does it all and also works out the number of bags.
How many bags of concrete do I need?
Divide the total volume by the yield printed on the bag, then round up. An 80 lb bag of pre-mix yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft. A 20 kg bag yields roughly 0.009 m³. For example, a 10 × 10 ft slab at 4 inches thick needs about 56 eighty-pound bags.
When should I order ready-mix instead of bags?
Bagged pre-mix is fine for small jobs up to roughly half a cubic yard (0.4 m³) — beyond that the bag count and mixing time get impractical and ready-mix delivered by truck is cheaper and stronger. As a rough guide, more than about 30–40 bags means it is worth pricing up ready-mix.
How much extra concrete should I allow?
Add about 5–10% for waste — concrete is hard to top up once it starts curing, the ground is rarely perfectly level, and some is always lost to spillage and the mixer. The waste field defaults to 10%; the formula box shows the exact volume before waste for reference.
How thick should a concrete slab be?
A typical patio or shed base is 4 inches (10 cm) thick; driveways that take cars are usually 5–6 inches (12–15 cm); paths and edging can be 3 inches (7.5 cm). Footings are deeper and narrower — enter your own dimensions. Thicker slabs need more concrete, so the volume scales directly with thickness.
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