Concrete Deck Footing Calculator
A standard 12-inch diameter deck footing at 48 inches deep needs about 6 bags of 80-lb concrete. A typical 12×16 ft deck with 6 footings needs 36 bags total. Enter your footing dimensions and count below for the exact number.
Pro Tips
- →Deck footings must extend below frost line
- →Typical footing diameter is 12 inches for deck posts
- →Set posts on concrete piers above footing
- →Number of footings depends on deck size and span
- →Check local codes for specific requirements
Includes 10% waste factor
That's typically a professional pour. See costs ↓
4 short emails from Dave: what a fair quote should land at for your slab, the scope changes that swing it ±$500, and whether DIY is actually cheaper at your volume. Reply anytime — he'll review your real quote.
Cost Estimate
Estimated material costs for your project
Recommendation: Bagged Concrete
For smaller projects, bagged concrete gives you more control and less waste.
26 bags × 80lb
0.58 cubic yards + delivery
Prices vary by location and time. Contact local suppliers for accurate quotes.
For general step-by-step instructions, read our complete When To Use Rebar and Permit Requirements.
Sizing Deck Footings by Load and Post Spacing
A deck footing has two jobs: carry the vertical load from the post above it down to soil that can support it, and stay below the frost line so it does not heave. The footing diameter you need is driven by tributary area — the slice of deck each post carries — and by the bearing capacity of your soil.
The IRC deck provisions (R507) size footings from the post's tributary area and an assumed soil load of 1,500 psf, the conservative default for undisturbed native soil. Each post carries roughly (beam span ÷ 2) × (joist span ÷ 2) square feet of deck. At a typical 40 psf live load plus 10 psf dead load (50 psf total), that tributary area sets the minimum bearing area at the bottom of the footing.
| Tributary Area per Post | Total Load (50 psf) | Min Bearing Diameter* | Common Footing |
|---|---|---|---|
| 24 sq ft | 1,200 lb | 12 in | 12 in tube |
| 36 sq ft | 1,800 lb | 15 in | 12 in tube + bell |
| 48 sq ft | 2,400 lb | 17 in | 16 in tube or bell base |
| 64 sq ft | 3,200 lb | 20 in | 16 in tube + 24 in base |
| 80 sq ft | 4,000 lb | 22 in | 18 in tube + 24 in base |
*Bearing diameter assumes 1,500 psf soil. Soft, clay, or fill soils have lower capacity and require larger bases — verify with your building department.
Keeping post spacing to 6–8 feet keeps tributary areas (and footing sizes) reasonable. Tightening spacing is often cheaper than digging oversized holes.
Footing Diameter Table: Concrete per Footing
Once you fix the diameter and depth, the concrete volume is just the cylinder. A 12-inch diameter, 48-inch deep footing holds 3.14 cubic feet (0.116 cu yd) — about 6 bags of 80-lb mix with 10% waste. Use the table to size each footing, then multiply by your footing count.
| Diameter | Depth | Volume per Footing | 80-lb Bags | 60-lb Bags |
|---|---|---|---|---|
| 12 in | 36 in | 2.36 cu ft | 4–5 | 6 |
| 12 in | 48 in | 3.14 cu ft | 6 | 7–8 |
| 16 in | 48 in | 5.59 cu ft | 10–11 | 14 |
| 20 in | 48 in | 8.73 cu ft | 16 | 21 |
| 24 in | 48 in | 12.57 cu ft | 22–23 | 30 |
An 80-lb bag yields about 0.60 cu ft; a 60-lb bag about 0.45 cu ft. For deeper frost-zone footings, scale linearly — a 60-inch deep 12-inch footing is 25% more concrete than the 48-inch row above. For the cylinder math behind any size, our Sonotube Calculator and Concrete Footing Calculator handle tube forms and spread footings respectively.
Number of Footings by Deck Size
Footing count follows from your beam layout, not the deck's square footage alone. Most residential decks use a ledger against the house plus one or two beams carried on posts. Posts sit under each beam at 6–8 foot intervals; a second beam (for a freestanding or wraparound deck) doubles the post line.
| Deck Size | Beam Layout | Typical Footings | Total Concrete (12 in × 48 in) |
|---|---|---|---|
| 10×10 ft | 1 beam, ledger | 3 | ~9 cu ft (0.35 cu yd) |
| 12×16 ft | 1 beam, ledger | 4–6 | 13–19 cu ft (0.5–0.7 cu yd) |
| 16×20 ft | 1 beam, ledger | 6 | ~19 cu ft (0.7 cu yd) |
| 20×20 ft | 2 beams | 8 | ~25 cu ft (0.93 cu yd) |
| Freestanding 16×16 | 2 beams | 9 | ~28 cu ft (1.05 cu yd) |
Add footings for stairs (a landing needs its own 1–2 footings), bump-outs, and any concentrated load. A hot tub needs its own reinforced footing layout — a filled tub plus occupants can exceed 6,000 lb on a small footprint.
Bags per Footing vs. Ready-Mix
Deck footings sit in an awkward middle zone for concrete sourcing. Individual piers are small enough for bags, but a full deck can cross the threshold where ordering a truck makes sense.
| Total Volume | Job Example | Best Source |
|---|---|---|
| Under 0.5 cu yd | 3–5 small footings | Bagged mix, mixed on site |
| 0.5–1 cu yd | 6–8 standard footings | Bagged mix or trailer mixer |
| 1–2 cu yd | Large/freestanding deck | Short-load ready-mix or many bags |
| Over 2 cu yd | Deck + patio combined pour | Ready-mix truck |
A 12×16 deck with 6 standard footings is about 19 cu ft — roughly 36 bags of 80-lb mix, a half-day job for two people with a mixer. Above ~1 cubic yard, the labor of hauling and mixing 50+ bags usually outweighs the short-load fee for ready-mix. See our bagged vs. ready-mix cost comparison for the break-even math, and the Concrete Cost Calculator to price the finished volume.
Tube Forms, Bell Bases, and Frost Depth
Most deck footings use a cardboard tube form (Sonotube and similar) to shape the pier above grade and a wider base below it to spread the load. Two common base options:
- Bell-bottom base — the hole is belled out at the bottom, widening the bearing area without a separate form. Effective in cohesive soils that hold a belled shape.
- Plastic footing form (Bigfoot, Square Foot, or equivalent) — a bell-shaped plastic base snaps onto the bottom of the tube, giving a 20–28 inch bearing pad under a 10–12 inch pier. This is the cleanest way to meet bearing-area requirements in loose soil.
The footing must reach below the local frost line. The IRC requires footings to extend below the frost depth (R403.1.4) so seasonal freezing cannot lift them — heave on even one footing twists the whole frame.
| Region | Frost Line Depth | Min Footing Depth |
|---|---|---|
| Deep South (FL, GA, LA) | 0–6 in | 12–18 in |
| Mid-Atlantic, Pacific Coast | 12–24 in | 24–30 in |
| Midwest, Upper South | 30–42 in | 36–48 in |
| Northern states (MN, WI, ME) | 42–60 in | 48–66 in |
Frost depth is set locally — your building department publishes the authoritative number for your jurisdiction. Footings must bear on undisturbed native soil, never on backfill.
Post Anchors and Embedment
Never bury a wood post directly in concrete. Trapped moisture against the end grain rots the post within 10–15 years even with pressure treatment, and the IRC deck provisions call for posts to bear on footings through an approved connector. The durable method:
- Pour the footing flush with or slightly above grade so water drains away from the post.
- Set a post base anchor (Simpson Strong-Tie ABU44/ABU66 or equivalent) into the wet concrete and check plumb and alignment.
- Let the footing cure at least 48 hours before loading.
- Fasten the post into the standoff anchor with the manufacturer's structural fasteners.
The standoff base lifts the post about an inch off the concrete, keeping the end grain dry and pushing post life past 40 years in most climates. For fence and railing posts that are set in concrete, see our concrete fence post guide, and use the Post Hole Calculator for those holes.
Ready to size your job? Enter your footing diameter, depth, and count above for an exact bag and cubic-yard total, then confirm frost depth and bearing requirements with your local building department before you dig.