Concrete footing calculator
- – bags of 50 lb, or – bags of 40 lb
- – cu ft per footing, – cubic yards total
For deck footings, pole barns and fence posts. Put in how many holes, the tube size and how deep you're going, and it tells you how many bags to load. In Michigan, footings usually go to 42″ to get below frost, but your local building department sets the number.
What I'd have on hand
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How the math works
Each footing is a cylinder: π × radius² × depth, converted to cubic feet. Bag counts use typical yields for standard concrete mix: about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. Check the yield printed on the bag you buy, since it varies by product.
Augered holes are rarely perfect cylinders. If you're pouring without tubes, or the hole bells out at the bottom, add more than 10%.
Common questions
How many bags of concrete for a 12 inch sonotube?
A 12″ tube holds about 0.79 cu ft per foot of depth. A 42″ deep footing is about 2.3 cu ft, or four 80 lb bags.
How deep should deck footings be in Michigan?
Most Michigan jurisdictions require footings at least 42″ below grade to get under the frost line. Confirm with your local building department before you dig.
Should I use 80 lb or 60 lb bags?
80 lb bags are the cheapest per cubic foot. 60 lb bags are easier to lift all day and to mix in a wheelbarrow. For more than about 30 bags, price out a short-load truck.