How Much Concrete for a 12-Inch Round Footing 36 Inches Deep?
12" diameter × 36" deep round footing — 2.3562 cu ft (0.0873 yd³) and bag counts for ordering
Estimate for a 12" diameter × 36" deep round Sonotube footing: exact volume 2.3562 cu ft (0.0873 yd³), exact bag counts, and planning quantities with 10% waste.
Bagged concrete covers this 12" diameter × 36" deep round form tube with a very small, controlled pour. The radius is 0.5 ft, giving a single-footing volume of 2.3562 cubic feet (0.0873 cubic yards).
This size is often used for isolated post supports or narrow cylindrical footings where the footprint is small. Keep in mind excavation depth, reinforcement, soil bearing, and whether the footing layout matches the loads you expect.
Quick Answer
Exact volume: 2.3562 cubic feet (0.0873 cubic yards). Exact bag counts for one footing: 8 40-lb bags, 6 60-lb bags, or 4 80-lb bags.
Planning with 10% waste: 2.5918 cubic feet (0.096 cubic yards). Planning bag counts per footing: 9 40-lb bags, 6 60-lb bags, or 5 80-lb bags.
Need a custom estimate? Use the Footing Calculator. Browse footing size pages. All project pages.
How to Use This Footing Estimate
Quick steps to estimate and order for this 12" × 36" round footing.
- Measure the tube diameter and depth to confirm 12" diameter × 36" deep.
- Use the known volume (2.3562 cu ft) to check material needs.
- Add 10% waste to the volume for mixing losses and minor overpours.
- Decide between bags of concrete mix or readymix based on how many footings you have.
- If ordering multiple footings, multiply the planning volume and bag counts before placing the order.
- Round up bag orders to whole bags and confirm delivery timing or pick-up availability.
Formula
Use this round footing formula:
Volume (ft³) = π × Radius (ft) × Radius (ft) × Depth (ft)
3.1416 × 0.5 × 0.5 × 3 = 2.36 ft³
2.36 ÷ 27 = 0.09 yd³
If you add a 10% waste allowance, you should plan closer to 0.1 cubic yards.
Worked Example
Single-footing example (one 12" × 36" tube): before waste = 2.3562 cu ft (0.0873 yd³) and exact bags 8 40-lb / 6 60-lb / 4 80-lb. With 10% waste the planning volume is 2.5918 cu ft (0.096 yd³) and planning bags are 9 40-lb / 6 60-lb / 5 80-lb.
Multiple-footing example (four identical footings): before waste = 9.4248 cu ft (0.3492 yd³). With 10% waste = 10.3672 cu ft (0.384 yd³). Planning bag totals = 36 40-lb bags, 24 60-lb bags, or 20 80-lb bags.
Bag Count Estimate
- 5 bags of 80 lb concrete mix
- 6 bags of 60 lb concrete mix
- 9 bags of 40 lb concrete mix
Always verify the actual yield on the product label before buying materials.
When Bagged Concrete vs Ready-Mix Makes Sense
For this single 12" × 36" footing, bagged concrete is usually simpler. One person can mix a few 40-lb or 60-lb bags and place the small volume without waiting for a truck.
If you have several footings (three or more), ready-mix can save time on mixing and hauling, and it reduces variability in strength. For a handful of tubes, compare the total yardage and supplier minimums before deciding.
Common Uses for This Footing Size
This 12" diameter × 36" deep footing suits isolated post supports and narrow cylindrical footing layouts where the footprint is tight. It's commonly used for posts like pergola or deck posts in layouts that demand a small-diameter pier.
Limits depend on load, soil bearing capacity, frost depth, and whether reinforcement is required. The size may work in some cases, but final adequacy depends on the load, bearing values, frost conditions in your area, and any permit or design requirements.
Common Mistakes to Avoid
Common estimating and ordering mistakes for this size.
- Not separating exact counts from waste-adjusted counts when ordering bags.
- Underestimating waste for small pours and not rounding bag counts up.
- Ordering readymix for a single small tube and incurring unnecessary delivery minimums.
- Forgetting to multiply volumes and bags when planning multiple footings.
- Assuming the same reinforcement or depth will work without confirming loads and soil.
Frequently Asked Questions
What is the exact concrete volume for a 12" diameter × 36" deep round footing?
The exact volume is 2.3562 cubic feet, which equals 0.0873 cubic yards.
How many bags of concrete do I need for one footing?
Exact bag counts for one footing are 8 40-lb bags, 6 60-lb bags, or 4 80-lb bags.
What waste allowance should I plan for on this footing?
Use a 10% waste allowance. That raises the volume to 2.5918 cu ft (0.096 yd³) and the planning bag counts to 9 40-lb / 6 60-lb / 5 80-lb per footing.
Is this 12" × 36" footing big enough for a deck or pergola post?
It is sometimes used for isolated post supports, but whether it is adequate depends on the post load, soil bearing capacity, frost depth, reinforcement, and any local design requirements.
Keep Planning Your Concrete Project
Use the calculator, compare more footing pages, and review your estimate before buying materials.
Disclaimer: This page provides planning estimates only. Actual concrete needs vary with excavation accuracy, site conditions, local code requirements, bag yield, and supplier guidance. Always verify measurements before buying materials, and consult a qualified professional for structural or code-sensitive work.