How Much Concrete for a 20x24x36 Footing?
Estimate concrete for a 20" x 24" x 36" footing — volumes, cubic yards, and bag counts for planning.
Get exact volume and planning quantities for a 20" × 24" × 36" rectangular footing: 10 cu ft (0.3704 yd³) and recommended bag counts with 10% waste included.
Planning first: this 20" × 24" × 36" footing is small enough that bagged concrete is often the most practical choice for a one-person or weekend job. Knowing the exact cubic feet and a conservative planning allowance saves trips and money on site.
At 36" deep this footing suits light columns, small deck posts where depth meets local frost requirements, and short retaining posts when paired with proper reinforcement. Check your local building department for depth and reinforcement mandates before you pour.
Quick Answer
Planning answer: the footing volume with a 10% waste allowance is 11.0 cubic feet (about 0.4074 cubic yards). For planning, buy roughly 37 40-lb bags, 25 60-lb bags, or 19 80-lb bags of concrete mix.
Exact material in the hole is 10.0 cubic feet, which equals 0.3704 cubic yards. If you prefer to buy only bags and want a little buffer, use the waste-adjusted counts above rather than the exact bag counts (exact counts without waste are 34, 23, and 17 respectively).
Need a custom estimate? Use the Footing Calculator. Browse footing size pages. All project pages.
How to Use This Footing Estimate
Quick steps to plan and pour a single 20" × 24" × 36" footing:
- Verify permit and footing depth with your local building department.
- Measure and stake the excavation to the exact 20" by 24" footprint and 36" depth.
- Calculate volume (or use these numbers) and buy the waste-adjusted bag count: 37 of the 40-lb, 25 of the 60-lb, or 19 of the 80-lb bags.
- Prepare forms, compact the base, and place any required reinforcement per plan or inspector instructions.
- Mix and pour concrete, consolidate to remove voids, and finish the top level.
- Cure the concrete per mix instructions and protect from rapid drying or freezing.
Formula
Use the rectangular prism formula: volume = length × width × depth. Convert inches to feet first or use the provided feet conversions.
For this footing: length = 1.6667 ft, width = 2.0 ft, depth = 3.0 ft, so area = 1.6667 × 2.0 = 3.3333 sq ft and volume = 3.3333 × 3.0 = 10.0 cubic feet. That is the exact in-place volume.
Convert cubic feet to cubic yards by dividing by 27: 10.0 ÷ 27 = 0.3704 cubic yards. Adding a 10% planning allowance gives 11.0 cubic feet (11.0 ÷ 27 = 0.4074 cubic yards). Bag conversions use standard yields (about 0.30 cu ft per 40-lb bag, about 0.44–0.45 cu ft per 60-lb bag, and about 0.58–0.60 cu ft per 80-lb bag) and match the planning counts here: 37 40-lb, 25 60-lb, or 19 80-lb bags with waste included.
Worked Example
Single-footing example: one 20" × 24" × 36" footing is 10.0 cubic feet in the hole, which converts to 0.3704 cubic yards. With a 10% allowance you should plan for 11.0 cubic feet (0.4074 cubic yards) — roughly 37 40-lb bags if you buy bagged mix.
Multiple-footing example: three identical footings equal 30.0 cubic feet total. Converted that is 1.1112 cubic yards (30.0 ÷ 27). For bagged concrete plan 3 × 37 = 111 40-lb bags (or 3 × 25 = 75 60-lb bags) including the 10% allowance.
Bag Count Estimate
- 19 bags of 80 lb concrete mix
- 25 bags of 60 lb concrete mix
- 37 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 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 | 10 | 17 | 23 | 34 |
| +5% typical DIY | 10.5 | 18 | 24 | 35 |
| +10% recommended | 11 | 19 | 25 | 37 |
| +15% rough conditions | 11.5 | 20 | 26 | 39 |
When Bagged Concrete vs Ready-Mix Makes Sense
For a single 20" × 24" × 36" footing the volume is small: 0.3704 cubic yards. That size often makes bagged concrete the convenient option—buying 37 40-lb bags and mixing on site avoids the minimum-order issues of some ready-mix suppliers.
Ready-mix can save labor when you have several footings or need speed and consistency, but most plants have a minimum delivery (commonly around 0.5 yd³ or higher). If you only pour one footing, bagged concrete usually costs less in waste and truck fees; for three or more footings, re-check costs and timing to see if ready-mix becomes competitive.
Common Uses for This Footing Size
A 20x24x36 footing is often used for supporting deck posts, pergola posts, fence posts, or small porch columns. Its rectangular shape and size fit well under single-point supports where the load isn’t too heavy. This footing can help keep wooden posts stable and off the ground, reducing moisture damage and shifting. It’s common in residential projects where posts need a solid base but the structure isn’t carrying a lot of weight.
Whether this footing size will work depends on several factors. The weight of what’s being supported, the type of soil, how deep frost goes in your area, and if any reinforcement is needed all matter. Local building codes also set rules that can affect footing dimensions. It’s important to consider these before deciding on this footing size for your project.
Common Mistakes to Avoid
Common errors to avoid when estimating and pouring this footing:
- Skipping the 10% waste allowance and running short of concrete on pour day.
- Assuming bag counts are the same with and without waste—exact counts differ from planning counts.
- Not verifying required depth or reinforcement with the local building department before excavating.
- Underestimating handling time for mixing many bags by hand or with a small mixer.
- Failing to compact the base or install specified reinforcement before pouring.
- Poor curing or leaving the new concrete exposed to rapid drying or freezing.
Frequently Asked Questions
How much concrete is in a 20" x 24" x 36" footing?
The exact in-place volume is 10.0 cubic feet, which is 0.3704 cubic yards. With a 10% planning allowance plan for 11.0 cubic feet (0.4074 cubic yards).
How many bags of concrete mix should I buy for this footing?
Plan on roughly 37 40-lb bags, 25 60-lb bags, or 19 80-lb bags including a 10% waste allowance. Exact counts without waste are 34, 23, and 17 respectively.
How much extra should I add for waste?
A 10% allowance is a practical planning margin for small footings to cover spillage, uneven forms, and small measurement errors.
Is this footing size adequate for a deck post?
It depends on the post load, soil bearing, frost depth, and local code requirements. A 36" depth may be suitable where that depth aligns with local frost-depth requirements, but check load calculations and your building department before deciding.
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.