How Much Concrete for a 20x24x48 Footing?
Exact concrete volume, yards, and bag counts for a 20" x 24" x 48" footing with a 10% waste allowance.
Find cubic feet, cubic yards, and recommended bag counts (40-, 60-, 80-lb) for a 20" x 24" x 48" footing. Includes a 10% waste allowance and practical excavation considerations.
At 48 inches deep, this 20" x 24" x 48" rectangular footing is built for deep support where excavation depth and embedded reinforcement matter. The depth-first perspective means you should plan for shoring, safe excavation slopes, and a consistent base before forming.
This size works for heavier posts, small column footings, or situations that require extra embedment below frost lines. Expect to dig and prepare about 3.3333 square feet of footing area and confirm footing bearing with your local building department and a structural design where necessary.
Quick Answer
Exact concrete needed: 13.3333 cubic feet (0.4938 cubic yards). With a standard 10% waste allowance, plan for 14.6666 cubic feet (0.5432 cubic yards).
Bagged concrete planning counts: about 49 40-lb bags, 33 60-lb bags, or 25 80-lb bags (these are the waste-adjusted planning numbers). Exact counts without waste are 45 40-lb, 30 60-lb, or 23 80-lb bags if you prefer no added margin.
Need a custom estimate? Use the Footing Calculator. Browse footing size pages. All project pages.
How to Use This Footing Estimate
Quick steps to go from measurement to pour for this 20" x 24" x 48" footing:
- Mark and dig to the full 48" depth, keeping excavation sides safe and level at the bottom.
- Compact the subgrade and add a thin gravel bedding if required by the engineer or inspector.
- Set formwork or use a reusable form sized to 20" x 24" and check that it sits square and level.
- Place reinforcement (rebar or wire) per design, holding it off the subgrade with chairs so bars are centered in the concrete.
- Mix and place concrete or pour from bags/ready-mix, consolidating to remove voids.
- Strike off, finish minimal surface, and cover for proper curing for at least several days to reach strength.
Formula
The volume of a rectangular footing is length × width × depth. Convert all dimensions to feet first: 20 inches = 1.6667 ft, 24 inches = 2 ft, 48 inches = 4 ft.
Multiply those three: 1.6667 ft × 2 ft × 4 ft = 13.3333 cubic feet. To convert cubic feet to cubic yards divide by 27: 13.3333 ÷ 27 = 0.4938 cubic yards.
To allow for waste, small over-pour, and slight measurement differences add 10%: 13.3333 cu ft × 1.10 = 14.6666 cu ft, which is 0.5432 cubic yards. Use the waste-adjusted numbers when ordering bags or planning materials.
Worked Example
Single footing example: this single 20" x 24" x 48" footing needs 13.3333 cubic feet of concrete, which is 0.4938 cubic yards. With the 10% waste allowance plan on 14.6666 cubic feet (0.5432 yd³) — about 49 40-lb bags or 33 60-lb bags.
Multiple-footing example: three identical footings total 40.0 cubic feet of concrete (3 × 13.3333 = 39.9999 cu ft), which converts to about 1.4815 cubic yards. Using the waste-adjusted planning volume, 3 × 14.6666 = 44.0 cu ft (about 1.63 yd³), so plan for roughly 147 40-lb bags, 99 60-lb bags, or 75 80-lb bags for all three footings.
Bag Count Estimate
- 25 bags of 80 lb concrete mix
- 33 bags of 60 lb concrete mix
- 49 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 48"
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 | 13.33 | 23 | 30 | 45 |
| +5% typical DIY | 14 | 24 | 32 | 47 |
| +10% recommended | 14.67 | 25 | 33 | 49 |
| +15% rough conditions | 15.33 | 26 | 35 | 52 |
When Bagged Concrete vs Ready-Mix Makes Sense
For this footing the planning volume is about 0.5432 cubic yards. That size often sits in a gray area: small enough that bagged concrete is practical for a DIYer, yet small ready-mix orders may incur a minimum charge or delivery fee from suppliers.
Bagged concrete advantages: you can buy exactly the number of bags needed (about 49 40-lb bags or 33 60-lb bags with waste), store them on site, and mix in small batches. Ready-mix advantages: cleaner placement, consistent mix, and faster pouring when you have multiple footings or limited labor. Choose bags for single pours or tight access; choose ready-mix when you need speed, consistent slump, or are pouring several footings at once.
Common Uses for This Footing Size
A 20x24x48 footing is commonly used for supporting deck posts, pergola posts, fence posts, and small porch columns. Its rectangular shape and size fit well for single-point supports that don’t carry heavy loads. This footing can be placed below ground to help keep wooden or metal posts stable in typical residential settings. It’s often chosen when a footing needs to be deeper than wide, such as in areas with frost concerns or when space is limited around the post.
Whether this footing size is right depends on several factors like the weight of the structure it supports, the type of soil, how deep the frost line is in your area, and if reinforcement like rebar is needed. Local building codes also play a big role in determining if this footing meets the requirements for your project. It’s important to consider these details before deciding on footing dimensions.
Common Mistakes to Avoid
A few common missteps to avoid with a 20" x 24" x 48" footing:
- Underestimating waste — order the planning counts, not just the exact volume.
- Not preparing or compacting the base, which can cause settlement after pour.
- Skipping or undersizing reinforcement when the design requires it.
- Mixing inconsistent batches by hand and ending up with weak concrete.
- Pouring without proper form support or bracing, leading to bowed forms and uneven footing.
- Insufficient curing time or letting the surface dry out too quickly during hot, windy conditions.
Frequently Asked Questions
How much concrete does a 20 x 24 x 48 footing need?
The exact volume is 13.3333 cubic feet (0.4938 cubic yards). With a 10% waste allowance, plan for 14.6666 cubic feet (0.5432 cubic yards).
How many bags of concrete should I buy for this footing?
Use the waste-adjusted planning counts: about 49 40-lb bags, 33 60-lb bags, or 25 80-lb bags. Exact counts without waste are 45, 30, or 23 respectively.
Why should I add a waste allowance and how much is typical?
Add waste to cover small over-pours, measurement differences, and slump loss during mixing. A 10% allowance is common for single footings and is already included in the planning numbers on this page.
Is this footing size adequate for a deck post?
It depends on the deck load, soil bearing capacity, frost depth at your site, and local building department requirements. In some areas a 48-inch depth may meet frost-depth needs, but confirm footing width, reinforcement, and loads with a structural plan or the local building authority.
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.