How Much Concrete for an 18-Inch Round Footing 24 Inches Deep?
Estimate for an 18" diameter × 24" deep round footing — cubic feet, cubic yards, and bag counts
Exact volume (3.5343 cu ft / 0.1309 yd³) and bag counts for an 18-inch round Sonotube footing 24 inches deep, plus a 10% waste planning allowance for ordering.
Plan on a small, narrow tube-form pour when you size this footing: 18" diameter × 24" deep. The pour is a narrow cylinder that holds 3.5343 cubic feet of concrete.
This size is often used for an isolated post support or other narrow-footprint layouts. Expect to dig a full 24" depth in the tube form, and use the bag planning numbers below when ordering materials or deciding whether to mix by hand or use a truck for multiple footings.
Quick Answer
Exact volume: 3.5343 cubic feet (0.1309 cubic yards).
Planning with 10% waste: 3.8877 cubic feet (0.144 cubic yards). Planning bag counts: 13 40-lb bags, 9 60-lb bags, or 7 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 18" × 24" round footing.
- Measure the tube inside diameter and depth to confirm 18" by 24".
- Use the calculated volume (3.5343 cu ft) to compare product yields.
- Add the 10% waste allowance to get 3.8877 cu ft for ordering.
- Decide between bags or ready-mix based on how many footings you have.
- Round up bag counts to whole bags and confirm delivery or pickup timing.
- For multiple footings, multiply the planning volume and bag counts before ordering.
Formula
Use this round footing formula:
Volume (ft³) = π × Radius (ft) × Radius (ft) × Depth (ft)
3.1416 × 0.75 × 0.75 × 2 = 3.53 ft³
3.53 ÷ 27 = 0.13 yd³
If you add a 10% waste allowance, you should plan closer to 0.14 cubic yards.
Worked Example
Single-footing example: before waste the footing is 3.5343 cu ft (0.1309 yd³). With 10% waste the planning volume is 3.8877 cu ft (0.144 yd³) and the planning bag counts are 13 40-lb bags, 9 60-lb bags, or 7 80-lb bags.
Multiple-footing example (4 footings): before waste total is 14.1372 cu ft (0.5236 yd³). With 10% waste total planning volume is 15.5508 cu ft (0.576 yd³). Planning bag totals for four pours: 52 40-lb bags, 36 60-lb bags, or 28 80-lb bags.
Bag Count Estimate
- 7 bags of 80 lb concrete mix
- 9 bags of 60 lb concrete mix
- 13 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 × 24" 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 | 3.53 | 6 | 8 | 12 |
| +5% typical DIY | 3.71 | 7 | 9 | 13 |
| +10% recommended | 3.89 | 7 | 9 | 13 |
| +15% rough conditions | 4.06 | 7 | 10 | 14 |
When Bagged Concrete vs Ready-Mix Makes Sense
For one 18" × 24" footing, bagged concrete is usually simpler: you can buy 7 to 13 bags depending on bag size and mix small batches in a wheelbarrow. There is minimal waste and no minimum delivery for one small cylinder.
If you have several footings, ready-mix becomes worth considering once the total yardage approaches a fraction of a truck load. For example, four or more footings raise total yardage and may justify a short delivery instead of dozens of bags.
Common Uses for This Footing Size
This 18" diameter by 24" deep footing suits narrow, isolated supports such as a pergola post, a single deck post, a porch column with a small base, or a sign support where a cylindrical layout and compact footprint are required. The slender tube form keeps the poured volume low while providing a focused bearing area.
Limits include the load on the support, the site soil bearing capacity, frost depth at the location, and whether reinforcement is needed. Those factors affect whether this diameter and depth will work; sizing and reinforcement choices should align with load and ground conditions and any required permits or inspections.
Common Mistakes to Avoid
Common planning mistakes that add cost or delay.
- Ordering the exact bag count and not adding the 10% waste allowance.
- Mixing bag yields incorrectly by confusing 40-, 60-, and 80-lb bag yields.
- Rounding cubic yards separately from cubic feet and creating ordering mismatches.
- Not multiplying bag counts when planning multiple identical footings.
- Assuming the tube interior equals the actual poured diameter without accounting for form thickness or voids.
- Failing to schedule delivery or pickup so material arrives when you can place it.
Frequently Asked Questions
What is the exact concrete volume for an 18" round footing 24" deep?
The exact volume is 3.5343 cubic feet, which is 0.1309 cubic yards.
How many bags of concrete do I need for one footing?
Exact bag counts (no waste) are 12 40-lb bags, 8 60-lb bags, or 6 80-lb bags. Use the planning bag counts with waste for ordering.
Why add a 10% waste allowance and how does it affect ordering?
A 10% allowance covers small overpour, uneven forms, and mix variation. It raises the volume to 3.8877 cu ft (0.144 yd³) and planning bag counts to 13 40-lb, 9 60-lb, or 7 80-lb bags.
Is this footing size adequate for a deck or pergola post?
This size may work in some cases for isolated posts or pergola supports, but adequacy depends on the applied load, soil bearing, frost depth, and reinforcement. Confirm the required size and reinforcement with a qualified designer or authority before finalizing.
Keep Planning Your Concrete Project
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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.