How Much Concrete for a 16x20x24 Footing?
Calculate concrete volume and bag counts for a 16" × 20" × 24" rectangular footing, including cubic feet, cubic yards, and planning bags.
Exact volumes and planning estimates for a 16" × 20" × 24" footing: 4.4444 cu ft (0.1646 yd³) exact, with waste-adjusted planning values and bag counts included.
16" × 20" × 24" describes a narrow rectangular footing with a 24-inch depth — often used for small residential posts, light support walls, or isolated column footings where a deeper bearing is required. The dimensions give an actual volume of 4.4444 cubic feet, which helps you decide between bagged concrete or ordering ready-mix.
This footprint is most relevant for homeowners and DIYers working on deck posts, small porch columns, or a compact pier for a garden structure. Because it’s 24" deep, it may be suitable where that depth aligns with local frost-depth requirements, but check local code and site conditions before pouring.
Quick Answer
Exact volume: 4.4444 cubic feet, which is 0.1646 cubic yards. Plan for waste: multiply by 10% to get 4.8888 cubic feet (0.1811 cubic yards) as the practical ordering target.
For bagged concrete, plan on about 17 40-lb bags, 11 60-lb bags, or 9 80-lb bags when you include the 10% waste allowance. The exact counts without waste are 15 (40-lb), 10 (60-lb), and 8 (80-lb).
Need a custom estimate? Use the Footing Calculator. Browse footing size pages. All project pages.
How to Use This Footing Estimate
Quick steps to pour this 16" × 20" × 24" footing correctly:
- Verify local footing depth and width requirements with the building department and check for underground utilities.
- Lay out and excavate to the full 16" × 20" footprint and a consistent 24" depth, keeping sides square and bottom level.
- Place any required reinforcement (rebar or wire) and compact the bottom if soil bearing and code call for it.
- Mix and place concrete: for bags, follow manufacturer water ratios; for ready-mix, confirm truck quantity and slump.
- Screed and finish the top, maintain moisture for curing as recommended for the mix used.
- Backfill after concrete reaches required strength per local code or engineer instructions.
Formula
The basic volume formula is length × width × depth. Use feet for each dimension to get cubic feet, then divide by 27 to convert to cubic yards.
Exact math for this footing: length 1.3333 ft × width 1.6667 ft × depth 2 ft = 4.4444 cubic feet. Converting: 4.4444 cu ft ÷ 27 = 0.1646 cubic yards.
Include a waste allowance (commonly 10%) to account for spillage, slight over-excavation, or uneven mixing. Multiply 4.4444 cu ft by 1.10 = 4.8888 cu ft. In cubic yards that’s 4.8888 ÷ 27 = 0.1811 yd³. Use the planning numbers for ordering bags or ready-mix.
Worked Example
Single footing example: one 16" × 20" × 24" footing requires 4.4444 cubic feet (0.1646 yd³) exact, and 4.8888 cubic feet (0.1811 yd³) when you add 10% waste. That equals about 17 40-lb bags (planning with waste).
Multiple-footing example: three identical footings need 3 × 4.4444 = 13.3332 cubic feet total, which is 13.3332 ÷ 27 = 0.4938 cubic yards. With 10% waste you’d plan for 3 × 4.8888 = 14.6664 cubic feet (about 0.5429 yd³) and roughly 51 40-lb bags, 33 60-lb bags, or 27 80-lb bags using the planning bag counts.
Bag Count Estimate
- 9 bags of 80 lb concrete mix
- 11 bags of 60 lb concrete mix
- 17 bags of 40 lb concrete mix
Always verify the actual yield on the product label before buying materials.
Bag Counts by Waste Buffer for 16" x 20" 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 | 4.44 | 8 | 10 | 15 |
| +5% typical DIY | 4.67 | 8 | 11 | 16 |
| +10% recommended | 4.89 | 9 | 11 | 17 |
| +15% rough conditions | 5.11 | 9 | 12 | 18 |
When Bagged Concrete vs Ready-Mix Makes Sense
For a single 16" × 20" × 24" footing the waste-adjusted volume is only 0.1811 cubic yards. Many ready-mix suppliers have minimum orders (commonly 1/4 or 1/2 cubic yard), so ordering ready-mix can be convenient but may come with a minimum quantity and delivery fee.
Bags of concrete mix offer tighter control for very small pours: 17 40-lb bags, 11 60-lb bags, or 9 80-lb bags are the planning targets after 10% waste. Use bagged concrete if you can handle mixing and have time to place and finish the small amount. Choose ready-mix if you prefer one pour and a uniform mix, but confirm the supplier’s minimums and pricing.
Common Uses for This Footing Size
A 16x20x24 footing is often used for supporting deck posts, pergola posts, fence posts, or small porch columns. Its rectangular shape and these dimensions make it suitable for single-point supports where the load isn’t too heavy. This size can handle typical residential outdoor structures that need a solid base without requiring a large concrete pour. It’s common in projects where the posts are spaced apart and the footing needs to fit within limited space.
Whether this footing size works depends on several factors like the weight of what’s being supported, the type of soil, how deep the frost line is in your area, and if reinforcement is necessary. Local building codes also influence footing size and depth. It’s important to consider these conditions before deciding if this footing fits your specific project.
Common Mistakes to Avoid
Common mistakes to avoid when estimating and pouring this footing:
- Underestimating waste — skip the 10% and you may run short mid-pour.
- Ordering ready-mix without checking supplier minimums and delivery timing.
- Failing to confirm local code or permit requirements for depth and reinforcement.
- Skipping reinforcement or using inadequate rebar for the actual load and soil bearing.
- Poor excavation — uneven bottoms or sloped sides reduce effective volume and bearing.
- Not wetting or curing the concrete properly, which weakens the finished footing.
Frequently Asked Questions
How much concrete in cubic feet and cubic yards does a 16" x 20" x 24" footing need?
The exact volume is 4.4444 cubic feet, which equals 0.1646 cubic yards. With a 10% waste allowance plan for 4.8888 cubic feet (0.1811 cubic yards).
How many bags of concrete mix do I need for this footing?
Exact counts are 15 40-lb bags, 10 60-lb bags, or 8 80-lb bags. For ordering and waste planning use the adjusted counts: about 17 40-lb bags, 11 60-lb bags, or 9 80-lb bags.
Why add a waste allowance and how much should I add?
Add waste to cover spillage, uneven excavation, and mix variability. A 10% allowance is typical for small footings; it increases the 4.4444 cu ft to 4.8888 cu ft for ordering.
Is a 16" x 20" x 24" footing adequate for a deck post?
It depends on the deck load, soil bearing capacity, frost depth, and local code. A 24-inch depth may be suitable where that aligns with local frost-depth requirements, but confirm reinforcement and size with your building department or a structural engineer before relying on this footing for a deck post.
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.