Deck Footing Calculator
Find out exactly how many bags of concrete you need per deck footing — and for the whole deck — plus water, weight, cost and a frost-depth check.
Your footings
Leave at 0 to skip the check. Confirm your exact frost depth with your local building department.
= 4 footings (3 spans + 1)
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Per-footing breakdown
5 × 80 lb bags per footing
Mixing time
2 h 45 m
One person, 30 bags
Frost depth check
Enter your local frost depth to check
The short answer
A typical 12 inch diameter, 42 inch deep deck footing needs about 5 × 80 lb bags of concrete. Work it out precisely by finding the footing’s volume (round or square), adding 5–10% waste, and dividing by the bag’s yield — 0.60 cu ft for an 80 lb bag. Multiply by however many footings your deck plan calls for, and always check the depth against your local frost line.
Bags per footing, by diameter
Figures below are per footing, at a 42 inch depth, no waste added.
| Diameter | Concrete needed | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 8 in | 1.22 cu ft | 0.05 | 3 | 3 | 5 |
| 10 in | 1.91 cu ft | 0.07 | 4 | 5 | 7 |
| 12 in | 2.75 cu ft | 0.10 | 5 | 7 | 10 |
| 16 in | 4.89 cu ft | 0.18 | 9 | 11 | 17 |
| 18 in | 6.19 cu ft | 0.23 | 11 | 14 | 21 |
| 20 in | 7.63 cu ft | 0.28 | 13 | 17 | 26 |
| 24 in | 11.00 cu ft | 0.41 | 19 | 25 | 37 |
Most residential decks use 10–12 inch footings unless a structural plan calls for wider ones under heavier beams or larger tributary loads.
How deep should deck footings be?
| Climate | Typical frost depth | Typical footing depth |
|---|---|---|
| Mild / no frost (e.g. Gulf Coast, Southern California) | 0–12 in | 12 in minimum, per code |
| Moderate (e.g. Mid-Atlantic, Pacific Northwest) | 12–24 in | 24–30 in |
| Cold (e.g. Midwest, Northeast) | 30–42 in | 36–48 in |
| Very cold (e.g. Northern Plains, interior Canada) | 48–60+ in | 48–60+ in |
The footing needs to sit below the local frost line so seasonal freeze-thaw cycles can’t heave the post. Frost depth varies by region and even by county, so confirm the exact figure with your local building department before digging.
How to calculate concrete for a deck footing
A round footing is a cylinder; a square footing is a rectangular prism. Neither has a post displacing concrete the way a fence hole does, since the post base or anchor sits on top rather than embedded through the pour.
- Find one footing’s volume. Use the cylinder formula for round tube forms, or side² × depth for square forms.
- Multiply by the number of footings. This gives the total concrete for the whole deck.
- Add waste. 5–10% covers spillage and tube form overage above grade.
- Divide by bag yield and round up. 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb — round up per footing, not just the total, so no single footing comes up short.
- Check the depth. Compare your footing depth against your local frost line before pouring.
Worked example
A 12 inch diameter, 42 inch deep round footing, using 80 lb bags:
- Footing volumeπ × 0.5² × 3.5 = 2.75 cu ft
- Plus 10% waste3.02 cu ft
- Divide by bag yield3.02 ÷ 0.60 = 5.0
- Round up5 × 80 lb bags per footing
For six footings spaced along a 24 ft beam run, that’s 30 bags total — about 2,400 lb to carry and just under 3 hours of mixing for one person.
Setting a deck footing: a quick checklist
- Call before you digHave utility lines marked before digging any footing hole.
- Dig below frost lineConfirm your local frost depth and dig deep enough to sit below it, with a level bottom.
- Add a gravel base2–4 inches of compacted gravel improves drainage and provides a stable bearing surface.
- Set the tube formFor above-grade portions, level and brace a sonotube so it stays plumb during the pour.
- Place rebar or anchorSet vertical rebar or a post base anchor per your local code before or during the pour.
- Pour, consolidate and crownFill the form, tap or vibrate to remove air pockets, and crown the top slightly so water sheds away.
- Wait before loadingPost bases can typically go on after 24–48 hours; full strength arrives at about 28 days.
Common mistakes
- Not digging below frost lineA footing that sits above the frost line can heave and move the whole deck as the ground freezes and thaws.
- Guessing at frost depthFrost depth varies by region and even by county — always confirm with your local building department rather than assuming.
- Undersizing the footing diameterFootings sized without checking beam loads or soil bearing capacity can settle unevenly over time.
- Rounding up the total instead of per footingRound up bags per footing, then add them up — rounding the grand total first can leave individual footings short.
- Skipping the anchor or rebarMany codes require a post base or vertical rebar embedded in the footing — check before the pour, not after.
- Loading the post too soonFraming on a footing within a day of pouring risks disturbing it before it has developed enough strength.
What this calculator assumes
Adjust your result if your project differs from these.
- Bag yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags
- Round footings treated as true cylinders; square footings as rectangular prisms
- No post displacement subtracted, since post bases and anchors sit on top of the footing rather than through it
- Standard concrete mix, not fast-setting or high-strength products
- Water of roughly 3 quarts per 80 lb bag
- Mixing at about 6 minutes per 80 lb bag, per person
- Footing count from beam length rounds up to a whole number, spaced evenly
- Frost depth check is a rule-of-thumb comparison, not a substitute for your local building code
- Prices are whatever you enter; they are not live market rates
Common questions
How many bags of concrete per deck footing?
A common 12 inch diameter, 42 inch deep footing needs about 5 × 80 lb bags. Larger diameters or deeper footings can need considerably more.
How deep should deck footings be?
Footings need to sit below the local frost line, which ranges from about 12 inches in mild climates to 48 inches or more in cold northern regions. Always confirm with local code.
What diameter should a deck footing be?
Residential deck footings commonly range from 8 to 24 inches in diameter, sized to the load, soil bearing capacity and local code, with 12 inches being a frequent default for standard decks.
Do I need a tube form for a deck footing?
Above grade, a cardboard or plastic tube form (sonotube) keeps the pour round and clean. Below grade, many codes allow pouring directly against undisturbed soil without a form.
How much water does a bag of footing concrete need?
About 3 quarts (2.8 L) per 80 lb bag, 2.25 quarts for a 60 lb bag, and 1.5 quarts for a 40 lb bag. Add gradually to a firm, workable consistency.
Can I use fast-setting concrete for deck footings?
Yes for small footings, though standard mix poured and vibrated in one continuous batch generally gives a more consistent footing for load-bearing posts.
How long before I can build on deck footings?
Post bases can usually be set after 24 to 48 hours, but full load capacity is generally reached at about 28 days, so heavy framing loads should wait.
Do deck footings need rebar?
Many codes require vertical rebar or a post anchor embedded in the footing, especially for taller decks. Check your local building code for requirements.
How far apart should deck footings be spaced?
Spacing depends on beam size and joist span, but 6 to 8 feet on center is common for residential decks with standard lumber beams. Always confirm against your beam span tables.
How many footings for a whole deck?
Multiply the bags needed per footing by the total number of footings your deck plan requires, typically set by beam runs spaced 6 to 8 feet apart.
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Deck Footing Estimate
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materialcalculate.com/tools/deck-footing-calculator/
Project
| Footing size | — |
| Shape | — |
| Number of footings | — |
| Frost depth check | — |
| Waste allowance | — |
Per footing
| Concrete per footing | — |
| 80 lb bags per footing | — |
Total shopping list
| 80 lb bags | — |
| 60 lb bags | — |
| 40 lb bags | — |
| Water needed | — |
| Dry weight to move | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes cylindrical or square footings and standard concrete mix with yields of 0.60, 0.45, 0.375 and 0.30 cu ft for 80, 60, 50 and 40 lb bags. Confirm quantities and frost depth with your local supplier and building department before buying.