Direct concrete answer
How much concrete for 12 inch footings?
Eight 12 inch deck footings at 4 ft deep requires 0.94 cubic yards of concrete, or 42 bags of 80-lb premix before field waste. With 12% overage, order about 1.1 cubic yards.
Quick answer
| Project dimensions | 8 holes x 12 in diameter x 4 ft deep |
|---|---|
| Area or count | 8 round footings |
| Volume | 25.13 cubic feet |
| Volume | 0.94 cubic yards |
| Metric volume | 0.71 cubic meters |
| 80-lb bags needed | 42 bags |
| 60-lb bags needed | 56 bags |
| 50-lb bags needed | 68 bags |
| Ready-mix order with 12% overage | 1.1 cubic yards |
| Ready-mix planning cost at $150/yd3 | $156 |
| Bagged planning cost at $5/80-lb bag | $210 |
Calculation steps
- Convert diameter to radius: 1.00 ft diameter / 2 = 0.50 ft radius.
- Calculate one hole: 3.1416 x radius x radius x 4 ft = 3.14 ft3.
- Multiply by quantity: 3.14 ft3 x 8 = 25.13 ft3.
- Convert to cubic yards: 25.13 ft3 / 27 = 0.94 yd3.
- Add waste for ordering: 0.94 yd3 x 1.12 = 1.1 yd3. Round according to the supplier's ordering increment.
This is the exact math behind the answer, but the field order should reflect the actual formed dimensions, the base condition, and the supplier's minimum order policy. Use the actual hole diameter and concrete depth. A hand-dug 12 inch hole can become wider quickly in loose soil.
Number of bags by size
Bag counts use common premix yields and round up to whole bags. Always check the yield printed on the exact product before buying because specialty mixes can differ.
| Bag size | Common yield | Calculation | Bags to buy |
|---|---|---|---|
| 80-lb bag | 0.6 ft3 | 25.13 / 0.6 | 42 |
| 60-lb bag | 0.45 ft3 | 25.13 / 0.45 | 56 |
| 50-lb bag | 0.375 ft3 | 25.13 / 0.375 | 68 |
Cost snapshot
At a planning price of $150 per cubic yard, the waste-adjusted ready-mix concrete is about $156 before short-load, delivery, pump, labor, forms, base stone, reinforcement, tax, or finish upgrades. The 80-lb bag material estimate is about $210 before mixer rental or labor.
Cost is where small concrete jobs surprise people. A short-load fee can make the delivered price look high, while bagged concrete can look cheap until the crew has to mix 42 heavy bags fast enough to place and finish one continuous surface. Use the material number as a quote starting point, not as the final installed price.
Ready-mix vs bagged concrete
Round footings are often bag-friendly when the quantity is small, but multiple deep piers can cross into ready-mix territory.
As a rule of thumb, ready-mix becomes easier once a project is near or above one cubic yard. This page's base volume is 0.94 cubic yards, so the practical choice depends on access, crew size, weather, finish timing, and whether the supplier charges a short-load fee. If the pour must be continuous, the truck often reduces risk even when the invoice includes a delivery charge.
Recommended PSI and reinforcement
| Use | Typical planning PSI | Notes |
|---|---|---|
| Patio or walkway | 3,000 PSI | Light foot traffic on compacted base. |
| Garage floor | 3,500 PSI | Verify vapor barrier, joints, and reinforcement. |
| Driveway | 3,500 to 4,000 PSI | Use higher strength for trucks, RVs, or weak soil. |
| Footings | 3,000 to 4,000 PSI | Plan and local code control final mix. |
Use the PSI shown on the plan or required by local code. Many small residential piers use standard 3,000 to 4,000 PSI concrete.
Deck piers and structural posts may require vertical bars, ties, post bases, and inspection. Confirm the detail before pouring.
Project-specific notes
This answer is for eight 12 inch deck footings at 4 ft deep. The geometry is straightforward, but the site details decide whether the estimate is clean in the field. Before ordering, compare the calculated dimensions against the actual form layout, the base depth, and the planned finish elevation.
- Deck footings are governed by frost depth, load, and soil, so the volume answer is only one piece of the design.
- If some posts carry beams or stairs, group those larger footings separately instead of using one average size.
- If the project includes thickened edges, a landing, a flare, a step, or a separate footing, calculate that concrete separately and add it to the base result.
Common mistakes
- Counting gravel depth as concrete depth. Subtract any gravel layer before estimating concrete.
- Using one average hole when corner posts, gate posts, and line posts are different sizes.
- Forgetting that round volume grows with radius squared, so a wider hole changes yardage fast.
The expensive mistake is running short during placement. The second most expensive mistake is ordering more concrete than the site can place before it starts setting. The best order is not the smallest number; it is the number that fits the measured work, the crew, the truck access, and the supplier's rounding policy.
What to say when ordering
I am estimating eight 12 inch deck footings at 4 ft deep. My calculated volume is 0.94 cubic yards, and I want to plan around 1.1 cubic yards with 12% overage. Can you confirm the order size, mix strength, short-load fee, delivery charge, chute reach, and whether this job needs a pump or different placement method?
Related calculations
Sources and methodology
BuilderCalc uses standard geometric volume formulas, the 27 cubic feet per cubic yard conversion, common premix bag yields, and planning cost ranges that should be verified with local suppliers.