Hardscape calculator
Deck footing calculator: count, depth and concrete
Footings are the only part of a deck you cannot fix later. Work out how many, how deep for your frost line, and how much concrete to buy.
Your project
The dimension the deck boards run along — usually the side parallel to the house.
Joists run this way, so this is the number that has to fit the joist span table.
8 ft is the usual maximum on a 2-ply beam. Closer spacing means smaller beam lumber.
12 inch tubes suit most residential decks; 16 inch for tall decks or soft soil.
Footings must bear below the frost line or the deck heaves every winter.
Footings required
3
12" diameter × 42" deep
- Footings per beam line
- 3
- Depth including 6 in below frost
- 42 in
- Concrete per footing
- 2.75 cu ft
- 80 lb bags per footing
- 5 bags
- Total bags of concrete
- 15 bags
- Total concrete volume
- 8.2 cu ft
- In cubic yards (ready-mix)
- 0.31 cu yd
- Deck area carried per footing
- 64 sq ft
- Installed footing cost estimate
- $480
Depth here is the frost line plus six inches, which is what most inspectors want to see. Soil bearing capacity, deck height and local amendments can all push the diameter up — check with your building department before you dig.
The shareable link reopens this calculator with your exact inputs.
How many footings does a deck need?
One under each beam post. Post spacing on a standard 2-ply beam tops out around 8 feet, so a 16 foot deck attached at the ledger needs three footings along the outer beam. Make the deck freestanding and you need a second row along the house, doubling the count.
Closer post spacing is not wasted money. Dropping from 8 feet to 6 feet adds a footing but often lets you use a smaller beam, and it always reduces beam deflection — the bounce people notice when they walk to the rail.
Corner and stair footings are extra. Stair stringers landing on grade need their own pad or footing, and a bump-out or angled corner adds one post each.
Depth is set by the frost line, not by convenience
A footing that bears above the frost line lifts every winter as the soil below it freezes and expands, and it does not come all the way back down in spring. Two or three cycles and the deck is visibly out of level and the ledger is being levered off the house.
Frost depth runs about 12 to 24 inches across the Deep South and coastal California, 36 inches through the mid-Atlantic and Midwest, 42 inches across the northern tier, and 48 inches or more in the upper Midwest and northern New England. Dig six inches past it so the bearing surface is genuinely below frost.
In expansive clay soils the frost line is not the whole story — helical piles or a belled footing may be required. That is a local question, and the building department will answer it in one phone call.
How much concrete each hole takes
A 12 inch tube 42 inches deep holds 2.75 cubic feet, which is five 80 lb bags. The same hole at 16 inch diameter holds 4.9 cubic feet and takes nine bags. That is why the diameter matters more to your back than the depth does.
Under about fifteen bags total, mixing on site is reasonable. Past that, a small ready-mix load or a trailer mixer is faster, more consistent and usually cheaper once you count the bags, the mixing time and the water.
Set the post base anchor in the wet concrete rather than drilling later, and keep the top of each footing an inch or two above grade so water drains away from the post rather than pooling at the end grain.
Concrete per footing by diameter and depth
Cubic feet, with 80 lb bags at 0.6 cu ft each in brackets.
| 8" tube | 10" tube | 12" tube | 16" tube | Depth |
|---|---|---|---|---|
| 0.7 (2 bags) | 1.09 (2 bags) | 1.57 (3 bags) | 2.79 (5 bags) | 24" |
| 1.05 (2 bags) | 1.64 (3 bags) | 2.36 (4 bags) | 4.19 (7 bags) | 36" |
| 1.22 (3 bags) | 1.91 (4 bags) | 2.75 (5 bags) | 4.89 (9 bags) | 42" |
| 1.4 (3 bags) | 2.18 (4 bags) | 3.14 (6 bags) | 5.59 (10 bags) | 48" |
Frequently asked questions
+How many footings do I need for a 12x16 deck?
Three along the outer beam if the deck is ledger-attached with posts at 8 feet, or six if it is freestanding with a second beam line along the house.
+How deep should deck footings be?
Six inches below your local frost line. That is roughly 30 inches in the Deep South, 42 in the mid-Atlantic and Midwest, and 48 to 54 in northern states.
+How many bags of concrete per deck footing?
A 12 inch tube at 42 inches deep takes about five 80 lb bags, or 2.75 cubic feet. An 8 inch tube at the same depth takes two, and a 16 inch tube takes nine.
+What diameter should deck footings be?
12 inches covers most residential decks on ordinary soil. Go to 16 inches for decks over 6 feet tall, large post spacing, or soft or sandy ground.
+Can I use deck blocks instead of footings?
Only for a low, freestanding, unattached platform — and only where your code allows it. Anything attached to the house, or above 30 inches, needs footings below the frost line.
+Do deck footings need rebar?
A plain tube footing in compression usually does not, but a belled or wide footing often does, and some jurisdictions require a vertical bar tying the footing to the pier. Ask before you pour.