How Much Concrete for a 16x20x36 Footing?
Materials for a 16 x 20 x 36 footing: cubic feet, cubic yards, and recommended bag counts
Quantities for a 16" × 20" × 36" rectangular footing: exact volume 6.6667 cu ft (0.2469 yd³), waste-adjusted planning totals (7.3334 cu ft / 0.2716 yd³), and suggested bag counts.
This 16" × 20" × 36" rectangular footing works well for a single column or deck post base and small masonry piers. Think posts and short columns where a compact, deep footing is needed to spread load and resist settling.
The 36" depth helps with frost and lateral stability in many areas, and the footprint gives about 2.2222 sq ft of bearing area. Use this size when you need a deeper, relatively narrow footing for one or two support points rather than a wide slab.
Quick Answer
Exact excavation volume for one 16" × 20" × 36" footing is 6.6667 cubic feet, which equals 0.2469 cubic yards. To allow for on-site waste and slight over-excavation, add 10% and plan for 7.3334 cu ft (0.2716 yd³).
For bagged concrete, plan on about 25 40-lb bags, 17 60-lb bags, or 13 80-lb bags to cover the footing with a 10% buffer. Exact bag counts without waste are 23 (40-lb), 15 (60-lb), and 12 (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 place concrete for this footing:
- Excavate to the planned dimensions (16" × 20" × 36") and confirm level and compacted bottom.
- Set any required formwork and place rebar or a single vertical post anchor if specified by plans.
- Mix or stage bagged concrete or schedule readymix delivery based on the planning volume (7.3334 cu ft).
- Pour concrete, tamp to remove air pockets, and strike off level with the top of the form.
- Cure the concrete by keeping it moist for several days and protect from rapid drying or freezing.
- Remove forms after concrete reaches sufficient strength per engineer or local code guidelines.
Formula
The basic formula converts the footing's inches to feet and multiplies length × width × depth to get cubic feet. Using the provided conversions: length = 1.3333 ft, width = 1.6667 ft, depth = 3 ft.
Multiply those: 1.3333 ft × 1.6667 ft = 2.2222 sq ft (footing area). Then 2.2222 sq ft × 3 ft = 6.6667 cubic feet of concrete for one footing.
Convert cubic feet to cubic yards by dividing by 27: 6.6667 cu ft ÷ 27 = 0.2469 cubic yards. Add a standard 10% waste allowance: 6.6667 cu ft × 1.10 = 7.3334 cu ft (0.2716 yd³) to plan your order or bag counts.
Worked Example
Example 1 — Single footing: The footing needs 6.6667 cu ft of concrete, which is 0.2469 yd³. With a 10% allowance plan for 7.3334 cu ft (0.2716 yd³) and about 25 40-lb bags.
Example 2 — Three footings for a small porch: Total volume = 3 × 6.6667 cu ft = 20.0001 cu ft (rounded to 20.0001 to keep supplied precision), which equals 0.7407 yd³. With 10% waste plan for 3 × 7.3334 cu ft = 22.0002 cu ft (0.8148 yd³). That equals roughly 3 × 25 = 75 40-lb bags when using the planning bag counts.
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 16" x 20" x 36"
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 a single 16" × 20" × 36" footing the total concrete is small — under a third of a cubic yard after waste is included. That often makes bagged concrete the most practical choice: you can buy only what you need, carry it to the hole, and mix on site without a large minimum delivery.
Readymix is faster to place and ensures consistent mix quality, but many suppliers have minimum delivery quantities or truck minimum charges that exceed the planning volume of 0.2716 yd³. If you do choose readymix for several footings, consider ordering a single larger load and coordinating delivery time and placement to avoid waste.
Common Uses for This Footing Size
A 16x20x36 footing is commonly used for supporting posts in residential projects. You might see these footings under deck posts, pergola posts, or fence posts where the load isn’t too heavy. They can also work for small porch columns or similar structures that need a solid base but don’t carry large loads. The rectangular shape and these dimensions provide enough size to hold up typical wood posts in light-duty applications.
Whether this footing size is right depends on several factors like the weight it needs to carry, the type of soil, and how deep the frost line is in your area. Reinforcement with rebar and local building codes also affect what footing size you should use. It’s important to consider these details before deciding if this footing fits your project.
Common Mistakes to Avoid
Common mistakes when pouring a small footing like this:
- Underordering concrete by using exact volume and skipping the 10% waste allowance.
- Confusing exact bag counts with waste-adjusted planning bag counts when buying material.
- Skipping consolidation and not tamping, which leaves voids and weak spots in a narrow footing.
- Placing concrete without the specified reinforcement or anchor hardware called for on the plans.
- Not protecting the pour from rapid drying, rain, or freezing during the initial cure.
- Failing to confirm local frost depth, soil bearing capacity, and permit requirements before finalizing footing size.
Frequently Asked Questions
How much concrete does a 16" × 20" × 36" footing need?
The exact excavation volume is 6.6667 cubic feet, which is 0.2469 cubic yards. With a 10% waste allowance plan for 7.3334 cubic feet (0.2716 cubic yards).
How many bags of concrete should I buy for this footing?
Exact bag counts (no waste) are 23 40-lb bags, 15 60-lb bags, or 12 80-lb bags. For ordering with a buffer, plan on 25 40-lb, 17 60-lb, or 13 80-lb bags to cover small overages and spillage.
Why add 10% waste to the calculated volume?
A 10% allowance covers over-excavation, slight measurement errors, spillage, and the concrete left in mixing equipment. It helps ensure you won’t run short when placing concrete in the hole.
Is this footing adequate for a deck post?
It depends. Whether this footing is suitable for a deck post depends on the post load, soil bearing capacity, local frost depth and code, and reinforcement requirements. A 36" depth may meet frost-depth concerns in many areas, but check soil conditions and local building department requirements before using this size for a structural 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.