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Calculate concrete needed for deck footings.
With these inputs:
| Result | Value |
|---|---|
| Total concrete needed | 16.49 cu ft |
| Cubic yards | 0.61 yd³ |
| 60 lb bags | 37 bags |
| 80 lb bags | 28 bags |
| Per footing | 2.75 cu ft |
Add ~10% extra for spillage and form irregularities.
Footings are the part of a deck nobody sees, cannot be fixed afterwards, and are the reason decks move. Size and depth come from the load each post carries and the frost line where you live. This works out how many footings you need, how they should be spaced and what diameter suits the load, along with the concrete volume to order.
Below the frost line for your area, which your building department publishes and will not waive — 12 inches in parts of the south, 42 to 48 inches across much of the north, and deeper in the coldest regions. A footing that stops short gets lifted every time the ground freezes, which racks the frame and pulls the ledger away from the house.
One under each post, with posts at no more than eight feet along each beam and often six. A 16 by 12 ft attached deck typically needs three or four footings along a single beam; the same deck freestanding needs two beams and roughly double. Stairs need their own footing or landing pad at the bottom.
Twelve inches suits most residential decks on firm soil. Softer soils, taller decks and any post carrying a roof, a hot tub or a multi-level structure want more — 16 or 18 inches, or a flared base that spreads the load over a wider area. The footing is sized on bearing area, so doubling the diameter quadruples what it can carry.
Work out the hole volume and divide by the bag yield: an 80 lb bag makes about 0.60 cubic feet and a 60 lb bag about 0.45. A 12 in round footing 42 in deep holds 2.75 cubic feet, so 5 bags of 80 lb or 7 of 60 lb per post. A 10 in hole 36 in deep holds 1.64 cubic feet, or 3 bags of 80 lb. Buy one spare bag per few holes for spillage.
Eight if it is attached to the house, twelve if it is freestanding. A 20 ft joist span is too long for joists alone, so the deck needs two beam lines plus the ledger; each 20 ft beam needs four posts at no more than 8 ft apart (0, 6.7, 13.3 and 20 ft). Freestanding adds a third beam where the ledger would be. At 12 by 42 inch footings that is about 37 eighty-pound bags of concrete attached, or 55 freestanding.
For most residential decks on firm soil, a 12 inch diameter footing is the standard; 16 to 18 inches suits posts that carry more area, a roof or a hot tub, and 24 inches or a flared base is for heavy point loads or soft soil. Depth is set by your frost line, not by the load. A 12 inch footing 42 inches deep holds 2.75 cubic feet of concrete; a 16 inch one at the same depth holds 4.89.
Count the bags: a 12 by 42 inch footing holds 2.75 cubic feet, which is about 4.6 eighty-pound bags, so roughly 365 lb of dry mix goes into it. A 16 by 48 inch footing holds 5.59 cubic feet, around 745 lb of mix. That mass is what lets a poured footing resist frost heave and uplift in a way a surface deck block cannot.
Only for a low, freestanding, unpermitted deck. Precast blocks sit above the frost line by definition, so the deck rides seasonal ground movement instead of resisting it. That is acceptable on a ground-level floating deck and not acceptable on anything attached to the house, anything elevated, or anything a permit covers.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.