Tallyard help / concrete calculator
Concrete CalculatorCubic yards and bags for slabs, footings and piers
Pick what you are pouring, enter the size, and get the volume, a ready-mix order and the number of premix bags to buy, waste included and rounded up.
How much concrete do you need?
How it works
Every shape comes down to a volume, then the volume is turned into bags. All sizes are converted to one unit first (inches are divided by 12 to get feet, centimetres by 100 to get metres).
slab = length x width x thickness strip footing = length x width x depth pad footings = length x width x depth x number of pads round piers = pi x (diameter / 2)^2 x depth x number of piers with waste = volume x (1 + waste% / 100) cubic yards = cubic feet / 27 bags = with waste / yield of one bag -> rounded up
A worked example, a 10 x 10 ft patio slab, 4 in thick, 10% waste, 80 lb bags:
thickness = 4 / 12 = 0.333 ft volume = 10 x 10 x 0.333 = 33.3 cu ft (1.23 cu yd) with 10% waste = 33.3 x 1.10 = 36.7 cu ft (1.36 cu yd) 80 lb bag = 0.60 cu ft bags = 36.7 / 0.60 = 61.1 -> 62 bags ready-mix = 1.36 cu yd -> order 1.50 cu yd
Bag yield. The calculator assumes an 80 lb bag of standard premix makes 0.60 cu ft of placed concrete, and that yield is proportional to bag weight, so a 60 lb bag makes 0.45 cu ft and a 40 lb bag 0.30 cu ft. Metric bags use the same yield per kilogram. Brands differ a little; if your bag prints a different yield, treat the count as a close estimate.
Ready-mix. The ready-mix line is the volume with waste, rounded up to the next quarter cubic yard (or the next 0.1 m³ in metric).
| Bag | Makes about | Bags per cu yd (or m³) |
|---|
Common questions
How many bags of concrete make a cubic yard?
This calculator assumes an 80 lb bag of standard premix makes 0.60 cubic feet of concrete, and that yield scales with bag weight. A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb, before any waste allowance.
How many 25 kg bags of concrete are in a cubic metre?
At the same yield per kilogram, a 25 kg bag makes about 11.7 litres, so one cubic metre takes about 85.4 bags, which means buying 86. A 20 kg bag makes about 9.4 litres (107 bags per cubic metre) and a 40 kg bag about 18.7 litres (54 bags).
How do I work out cubic yards for a concrete slab?
Multiply length by width by thickness in feet, then divide by 27. A 10 x 10 ft slab 4 inches thick is 10 x 10 x 0.333 = 33.3 cubic feet, or 1.23 cubic yards. With a 10 percent waste allowance that becomes 1.36 cubic yards, or 62 bags of 80 lb premix.
How much concrete does a round pier or post hole need?
Use the cylinder volume: pi x (diameter / 2) squared x depth, times the number of holes. A 12 inch diameter hole 48 inches deep holds 3.14 cubic feet. With 10 percent waste that is 3.46 cubic feet, which rounds up to 6 bags of 80 lb premix per hole.
How much waste should I add?
5 to 10 percent is a usual range, because ground and trenches are rarely perfectly even and some mix is lost while mixing and finishing. The calculator starts at 10 percent.
Doing drywall too? The drywall sheets and concrete bags calculator covers boards and screws.