How Much Concrete for an 18-Inch Round Footing 36 Inches Deep?
Plan concrete for an 18" diameter × 36" deep round footing — 5.3014 cu ft, 0.1963 yd³, and bag counts.
Quick concrete numbers for an 18" round Sonotube footing 36" deep: exact volume 5.3014 cu ft (0.1963 yd³), exact bag counts, plus 10% waste planning totals.
Bag planning first: this 18" diameter × 36" deep round footing is a narrow tube-form pour that uses about 5.3014 cubic feet of concrete. That’s a small, single-footing volume best handled as a short, measured mix job.
At 36" deep you’ll need to confirm reinforcement and bearing requirements for the load you intend to carry. The page lists exact bag counts and a 10% waste adjustment so you can order or buy the correct number of bags or compare to ready-mix options.
Quick Answer
Exact volume: 5.3014 cubic feet (0.1963 cubic yards).
Planning (10% waste): 5.8315 cubic feet (0.2159 cubic yards). Exact bag counts: 18 40-lb bags, 12 60-lb bags, or 9 80-lb bags. Planning bag counts with 10% waste: 20 40-lb bags, 13 60-lb bags, or 10 80-lb bags.
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How to Use This Footing Estimate
Quick steps to estimate and plan material for this 18" × 36" round footing.
- Confirm the footing diameter (18") and depth (36").
- Use the volume number 5.3014 cu ft as the exact pour volume.
- Add a 10% waste allowance to get 5.8315 cu ft for ordering.
- Decide bagged mix or ready-mix based on how many footings you’ll pour.
- Convert to bags and round up to whole bags (use the planning bag counts listed).
- If pouring more than one, multiply the planning totals by the number of footings before you buy.
Formula
Use this round footing formula:
Volume (ft³) = π × Radius (ft) × Radius (ft) × Depth (ft)
3.1416 × 0.75 × 0.75 × 3 = 5.3 ft³
5.3 ÷ 27 = 0.2 yd³
If you add a 10% waste allowance, you should plan closer to 0.22 cubic yards.
Worked Example
Single footing: exact volume = 5.3014 cu ft (0.1963 yd³). With 10% waste the planning volume = 5.8315 cu ft (0.2159 yd³). Exact bag counts are 18 40-lb, 12 60-lb, or 9 80-lb; planning bag counts are 20 40-lb, 13 60-lb, or 10 80-lb.
Multiple footings example (3 footings): total = 15.9042 cu ft, 0.5889 yd³. Exact bag totals = 54 40-lb, 36 60-lb, 27 80-lb. Planning bag totals with 10% waste = 60 40-lb, 39 60-lb, 30 80-lb.
Bag Count Estimate
- 10 bags of 80 lb concrete mix
- 13 bags of 60 lb concrete mix
- 20 bags of 40 lb concrete mix
Always verify the actual yield on the product label before buying materials.
Bag Counts by Waste Buffer for 18" 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 | 5.3 | 9 | 12 | 18 |
| +5% typical DIY | 5.57 | 10 | 13 | 19 |
| +10% recommended | 5.83 | 10 | 13 | 20 |
| +15% rough conditions | 6.1 | 11 | 14 | 21 |
When Bagged Concrete vs Ready-Mix Makes Sense
For a single 18" × 36" footing, bagged concrete is often the simpler option: you can buy 9–18 bags and mix what you need for a single small pour. It keeps handling straightforward and avoids ordering a truck for a fraction of a yard.
When you have multiple footings in one run, ready-mix can become more practical because it eliminates batch-to-batch variation and onsite mixing time. If you’re doing several footings in quick succession, compare the total yardage and how much continuous placement you need to decide which route saves time.
Common Uses for This Footing Size
This 18" diameter by 36" deep round footing is most often used for isolated post supports and narrow cylindrical layouts: pergola posts, deck posts, smaller sign posts, or single-column supports where a compact footing footprint fits the layout. The round tube shape keeps the hole size small while still providing a concentrated mass under a post.
Actual suitability depends on the load to be carried, soil bearing capacity, frost depth at the site, whether reinforcement is required, and any local permit or inspector requirements. Treat this as a planning estimate for material quantities rather than a final structural design.
Common Mistakes to Avoid
Common estimating and planning mistakes to avoid:
- Rounding bag counts down instead of up and arriving short on material.
- Forgetting the 10% waste allowance for small pours and leftovers.
- Using the wrong diameter or depth when calculating volume.
- Mixing different bag strengths or types and assuming identical yield.
- Not accounting for reinforcement or anchor hardware displacement inside the tube.
- Buying just the exact number of bags instead of allowing a couple extra for adjustments.
Frequently Asked Questions
What is the exact concrete volume for an 18" round footing 36" deep?
The exact volume is 5.3014 cubic feet, which is 0.1963 cubic yards.
How many bags of concrete do I need for one 18" × 36" footing?
Exact bag counts are 18 40-lb bags, 12 60-lb bags, or 9 80-lb bags. For planning with 10% waste order 20 40-lb, 13 60-lb, or 10 80-lb bags.
Why use a 10% waste allowance for this footing?
Small pours lose volume to forms, waste in mixing, and minor overfill. A 10% allowance raises the volume from 5.3014 cu ft to 5.8315 cu ft to reduce the chance of running short.
Will this footing size support a deck post or pergola column?
An 18" × 36" footing is sometimes used for deck or pergola posts, but whether it’s adequate depends on the load, soil bearing capacity, frost depth, and required reinforcement. Confirm structural adequacy before final design or installation.
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.