How Much Concrete for a 20-Inch Round Footing 36 Inches Deep?
20" round footing 36" deep — 6.545 cu ft and bag count estimates
Exact and planning volumes (10% waste) plus bag counts for a 20" diameter × 36" deep round footing. Includes single‑footing numbers and an example total for multiple footings.
This is a small, narrow tube‑form pour: a 20" diameter × 36" deep round footing. It’s a limited volume cylinder — the concrete in the tube only comes to about 6.545 cubic feet.
Plan excavation to the full 36" depth and have reinforcement or dowels ready if required by the project. Bagged concrete is straightforward for one pour; order a little extra material and keep the site measured so you don’t short the pour.
Quick Answer
Exact volume: 6.545 cu ft (0.2424 cu yd).
Planning with 10% waste: 7.1995 cu ft (0.2666 cu yd). Exact bag counts: 22 40‑lb bags, 15 60‑lb bags, or 11 80‑lb bags. Waste‑adjusted planning counts: 24 40‑lb, 16 60‑lb, or 12 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 get material for this single 20" × 36" round footing.
- Confirm the footing diameter and depth at the site: 20" diameter, 36" deep.
- Check the exact volume: 6.545 cu ft (see formula below).
- Add a 10% waste allowance and round up to whole bags for ordering.
- Decide bagged concrete or ready‑mix based on how many footings you pour.
- If pouring several footings, multiply the planning volume and bag counts before you call the supplier.
- Buy the extra bag count shown under planning to avoid a short pour.
Formula
Use this round footing formula:
Volume (ft³) = π × Radius (ft) × Radius (ft) × Depth (ft)
3.1416 × 0.83 × 0.83 × 3 = 6.55 ft³
6.55 ÷ 27 = 0.24 yd³
If you add a 10% waste allowance, you should plan closer to 0.27 cubic yards.
Worked Example
Single footing: before waste the footing is 6.545 cu ft (0.2424 cu yd). With 10% waste plan on 7.1995 cu ft (0.2666 cu yd) and the planning bag counts listed above.
Multiple footings example (4 footings): total before waste = 26.18 cu ft (0.9696 cu yd). With 10% waste the total is 28.798 cu ft (1.0664 cu yd). For bagged concrete that’s 96 40‑lb bags (24 × 4), 64 60‑lb bags (16 × 4), or 48 80‑lb bags (12 × 4) using the planning counts.
Bag Count Estimate
- 12 bags of 80 lb concrete mix
- 16 bags of 60 lb concrete mix
- 24 bags of 40 lb concrete mix
Always verify the actual yield on the product label before buying materials.
Bag Counts by Waste Buffer for 20" diameter × 36" deep
Pick a buffer based on site conditions: clean forms and accurate measurements can run leaner; rough subgrade or hard-to-reach pours need more cushion.
| Buffer | Volume (ft³) | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| No buffer | 6.55 | 11 | 15 | 22 |
| +5% typical DIY | 6.87 | 12 | 16 | 23 |
| +10% recommended | 7.2 | 12 | 16 | 24 |
| +15% rough conditions | 7.53 | 13 | 17 | 26 |
When Bagged Concrete vs Ready-Mix Makes Sense
For this single 20" × 36" footing, bagged concrete is usually simpler: you can mix what you need and carry the bags to the tube. The volume is small enough that a few dozen bags handle the job.
When you pour several footings that add up — for example the four‑footing planning total here is about 1.07 cu yd — a small ready‑mix truck can be more convenient and faster. If you only have one or two tubes, bags are often the cleaner option; for larger totals, favor ready‑mix.
Common Uses for This Footing Size
This 20" diameter by 36" deep footing size is most often used for isolated post supports and narrow cylindrical footing layouts. It works for single pergola posts, deck posts, sign supports, or other situations where a small round footing footprint is required and the axial load is moderate.
Actual suitability depends on the applied load, soil bearing capacity, frost depth in your area, and whether reinforcement is required. Those factors determine diameter, depth, and reinforcement needs; treat these numbers as volumetric planning only, not a structural prescription.
Common Mistakes to Avoid
Common estimating and ordering mistakes to avoid for this footing size.
- Using exact volume only and not adding the 10% waste allowance.
- Ordering exact bag counts as decimals instead of rounding up to whole bags.
- Forgetting the extra concrete needed for footing reinforcement pockets or sunk anchor hardware.
- Multiplying bag counts by footings incorrectly — double‑check your math when ordering multiple tubes.
- Assuming ready‑mix is automatically cheaper without comparing the total yards and delivery constraints.
- Not staging the bags or mixing area near the pour to avoid delays once you start.
Frequently Asked Questions
What is the exact concrete volume for this 20" diameter × 36" deep round footing?
The exact volume is 6.545 cubic feet, which is 0.2424 cubic yards.
How many bags of concrete mix do I need for the footing?
Exact bag counts are 22 40‑lb bags, 15 60‑lb bags, or 11 80‑lb bags for the unadjusted volume.
How much extra should I order to allow for waste?
Use a 10% waste allowance; the planning volume is 7.1995 cu ft (0.2666 cu yd). That raises the bag counts to 24 40‑lb, 16 60‑lb, or 12 80‑lb bags for one footing.
Will this footing size be enough for a deck or pergola post?
This size is sometimes used for isolated post supports and may work in some cases, but adequacy depends on the post load, soil bearing, frost depth, and required reinforcement. Confirm those project specifics before relying on this footing size.
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.