Deck Footing & Concrete Calculator

Concrete quantities that actually match the hole

Footings look simple until you're standing next to a 12-inch auger hole trying to figure out how many bags to buy. The calculator above computes the exact volume of each hole (as a cylinder), multiplies by your footing count, and converts to both 60-lb and 80-lb bag counts. Because concrete slumps and forms are rarely perfectly cylindrical, always buy 10–15% more than the calculated number — a bag over-buy is far cheaper than a mid-pour Home Depot run.

Sizing footings correctly

Footing diameter and depth depend on soil bearing capacity, frost depth, and the load coming down through the post. General residential guidance:

• Diameter: 10–12 inches for small ground-level decks, 12–16 inches for standard elevated decks, 16–24 inches for hot tub or heavy roof loads.
• Depth: at least 12 inches below the local frost line. That's 12" in the South, 42–48" in the Upper Midwest and Northeast, 60" in far-northern climates.
• Bell base: in weak soils, flare the bottom of the hole to 2× the shaft diameter for extra bearing.

60 lb vs 80 lb bags — which to buy

An 80-lb bag yields about 0.6 cubic feet; a 60-lb bag yields 0.45 cubic feet. Doing the math per hole: a 12-inch × 42-inch footing takes roughly 3.3 cubic feet of concrete — 6 bags of 80-lb or 8 bags of 60-lb. For anything over 6 footings, buy 80-lb bags and mix in a wheelbarrow or a rented mixer. For fewer than 6 or if you're mixing by hand in a bucket, 60-lb is easier on your back and faster to hydrate.

Fast-set vs standard mix

Fast-setting concrete (Quikrete Fast-Setting, Sakrete 5000 Fast) sets in 20–40 minutes and reaches structural strength in a few hours. Use it if you're pouring on a weekend and want to attach posts the same day. Standard concrete is cheaper and stronger long-term but needs 24–48 hours before you can start framing. Both meet code for residential deck footings when they hit their rated PSI.

Post-to-footing connection

Never bury a wood post directly in concrete — it will rot in 8–15 years no matter how treated it is. Set an approved post base (Simpson ABU or PB series) into the wet concrete with the top of the base at least 1 inch above finished grade. This lifts the post out of standing water and lets it dry between rains. Once your concrete is set, run the numbers on the framing above with our beam span calculator.