How Much Concrete for a 20x24x24 Footing?
How much concrete a 20" x 24" x 24" rectangular footing needs — cu ft, cu yd, and bag planning counts
Exact and planning quantities for a 20" x 24" x 24" footing: 6.6667 cu ft (0.2469 yd³) exact, 7.3334 cu ft (0.2716 yd³) with 10% waste, plus bag counts.
If you’re setting a column or heavy post on a 20" x 24" x 24" rectangular footing, this size is commonly used for deck posts, small porch columns, and light support bases where a 24-inch depth suits site requirements. The dimensions give a footprint of about 3.3333 square feet and a solid, narrow pad that’s easy to form and pour with hand tools.
This page focuses on practical ordering and on-site choices for that exact footing size. Before you dig, consider soil bearing capacity, expected structural loads, and whether a 24-inch depth aligns with local frost-depth requirements and permit rules — those factors affect whether this footing is adequate for your project.
Quick Answer
Exact concrete volume for a single 20" x 24" x 24" footing is 6.6667 cubic feet, which is 0.2469 cubic yards. If you prefer a safety margin, add the recommended 10% waste for a planning volume of 7.3334 cubic feet (0.2716 cubic yards).
For ordering convenience, plan on about 25 bags of 40-lb mix, 17 bags of 60-lb mix, or 13 bags of 80-lb mix when you include the 10% waste margin. The exact bag counts (without waste) are 23 of the 40-lb, 15 of the 60-lb, or 12 of the 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 measure, order, and pour this 20" x 24" x 24" footing:
- Stake and compact the hole to the required depth and check dimensions (20" by 24" by 24").
- Set up a simple form with straight sides and a level top; oil or line the form for easier removal.
- Mix bagged concrete or confirm ready-mix quantity; plan for 7.3334 cu ft (0.2716 yd³) including 10% waste.
- Place any required reinforcement and hold it off the bottom with chairs or stones.
- Pour and consolidate the concrete, then screed and finish the surface to the desired profile.
- Cure the concrete per manufacturer guidance and protect it from rapid drying or freezing for the first few days.
Formula
The basic formula is area × depth = volume. For rectangular footings that means (length × width) × depth, using feet for every dimension.
Using the supplied dimensions in feet: length = 1.6667 ft and width = 2 ft, so the area is 1.6667 × 2 = 3.3333 square feet. Multiply that area by the depth, 2 ft, to get 3.3333 × 2 = 6.6667 cubic feet of concrete.
To convert cubic feet to cubic yards divide by 27: 6.6667 ÷ 27 = 0.2469 cubic yards. Add the default 10% waste factor to plan conservatively: 6.6667 × 1.10 = 7.3334 cubic feet (7.3334 ÷ 27 = 0.2716 cubic yards). The bag counts shown are the quantities associated with those planning volumes.
Worked Example
Single-footing example: a single column base uses 6.6667 cubic feet (0.2469 yd³) of concrete. If you add the standard 10% ordering buffer, order 7.3334 cubic feet (0.2716 yd³), which translates to about 25 40-lb bags (or 17 60-lb or 13 80-lb bags) for that one footing.
Multiple-footing example for three deck posts: total volume = 3 × 6.6667 cu ft = 20.0001 cu ft, which is 0.7400 cubic yards. With the 10% planning buffer total = 3 × 7.3334 cu ft = 22.0002 cu ft, or 0.8148 cubic yards. That means roughly 75 40-lb bags (or 51 60-lb bags, or 39 80-lb bags) to have adequate material for three of these footings.
Bag Count Estimate
- 13 bags of 80 lb concrete mix
- 17 bags of 60 lb concrete mix
- 25 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" x 24" x 24"
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 | 6.67 | 12 | 15 | 23 |
| +5% typical DIY | 7 | 12 | 16 | 24 |
| +10% recommended | 7.33 | 13 | 17 | 25 |
| +15% rough conditions | 7.67 | 13 | 18 | 26 |
When Bagged Concrete vs Ready-Mix Makes Sense
For one 20" x 24" x 24" footing the total concrete volume is under a third of a cubic yard, so bagged concrete is often the most practical option — you can buy the exact number of bags needed and mix on-site with a wheelbarrow or portable mixer. Bagged concrete gives control for small jobs and avoids minimum ready-mix delivery amounts that some plants require.
If you’re pouring several footings at once or have helpers and a mixer, bagged concrete still works well and keeps costs predictable. For larger batches, or when you need very consistent slump and admixtures, a small ready-mix delivery can be considered — but check local plant minimums and account for extra volume to avoid short loads.
Common Uses for This Footing Size
A 20x24x24 footing is often used for supporting deck posts, pergola posts, fence posts, or small porch columns. Its rectangular shape and dimensions make it suitable for single-point supports where the load is relatively light. This size footing can help keep wooden posts stable and anchored in the ground for typical residential outdoor structures.
The actual use depends on factors like the weight of the structure it supports, soil conditions, frost depth in your area, and whether reinforcement is needed. Local building codes may also affect footing size and installation requirements. It’s important to consider these details before choosing this footing size for your project.
Common Mistakes to Avoid
Common mistakes to avoid when forming and pouring a 20" x 24" x 24" footing:
- Underestimating waste — not ordering the extra 10% can leave you short in the middle of a pour.
- Neglecting to check soil bearing and site conditions before sizing the footing.
- Skipping reinforcement or not supporting rebar on chairs, allowing it to sit on the soil.
- Poor form alignment that changes the intended dimensions and volume.
- Pouring in adverse weather without addressing rapid drying or freezing risks.
- Assuming one bag type interchangeably — exact and planning bag counts differ and should be treated separately.
Frequently Asked Questions
What is the concrete volume for a single 20" x 24" x 24" footing?
The exact volume is 6.6667 cubic feet, which equals 0.2469 cubic yards. With a 10% waste allowance for planning, use 7.3334 cubic feet (0.2716 cubic yards).
How many bags of concrete do I need for this footing?
Exact bag counts without waste are 23 of the 40-lb bags, 15 of the 60-lb bags, or 12 of the 80-lb bags. For planning with 10% waste, order about 25 40-lb, 17 60-lb, or 13 80-lb bags.
Why add 10% waste to the calculated volume?
The 10% waste covers spillage, slight measurement errors, uneven forms, and minor over-excavation. It helps ensure you don’t run short during the pour and is a common practical allowance for small concrete jobs.
Is a 20" x 24" x 24" footing suitable for a deck post?
It depends on the post load, soil bearing capacity, local frost depth, and code requirements. A 24-inch depth may be suitable where that aligns with frost-depth rules, but check structural loads and local building department guidance before relying on this footing size.
Keep Planning Your Concrete Project
Use the calculator, compare footing pages, and keep your estimate practical before you buy 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.