How Much Concrete for a 12x24x36 Footing?
Material and bag counts for a 12" x 24" x 36" footing — volumes in cubic feet, cubic yards, and bags
Get the exact 6 cu ft (0.2222 yd³) volume for a 12×24×36 footing, plus 10% waste planning: 6.6 cu ft (0.2444 yd³) and bag counts for 40-, 60-, and 80-lb mixes.
For a single 12" x 24" x 36" rectangular footing, bagged concrete is often the most practical choice — a small number of bags is easy to transport and mix on-site. This size yields a compact 6 cubic feet of concrete, so you can realistically plan on using bags unless you have several footings or a contractor bringing a truck.
Before you finalize materials, have the footing reviewed for the imposed loads and reinforcement requirements. A structural review will confirm footing thickness, bar size, spacing, and whether the 36" depth meets your local frost-depth and soil-bearing requirements.
Quick Answer
Exact volume: 6.0 cubic feet, which is 0.2222 cubic yards. Exact bag counts for this volume are 20 40-lb bags, 14 60-lb bags, or 10 80-lb bags.
With a 10% allowance for waste and minor overage, plan on 6.6 cubic feet (about 0.2444 cubic yards) — roughly 22 40-lb bags, 15 60-lb bags, or 11 80-lb bags for ordering and planning.
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How to Use This Footing Estimate
Steps to order and place concrete for a single 12" x 24" x 36" footing.
- Confirm footing dimensions and any reinforcement called out on the plans.
- Decide whether to use bagged concrete or ready-mix based on quantity and site access.
- If using bags, buy the waste-adjusted counts (22 40-lb bags, 15 60-lb bags, or 11 80-lb bags).
- Prepare the form and compact the subgrade before placing concrete.
- Mix and place concrete, consolidate with a rod or vibrator, then finish the top and let it cure per instructions.
- Protect the new concrete from rapid drying and follow curing time before loading.
Formula
The basic volume formula for a rectangular footing is length × width × depth. Convert all measurements to feet first: 12 in = 1 ft, 24 in = 2 ft, 36 in = 3 ft, so the volume is 1 ft × 2 ft × 3 ft.
Exact math for this footing: 1 × 2 × 3 = 6.0 cubic feet. Convert cubic feet to cubic yards by dividing by 27: 6.0 ÷ 27 = 0.2222 cubic yards.
To account for a standard 10% waste allowance, multiply the cubic feet by 1.10: 6.0 × 1.10 = 6.6 cubic feet, which equals 6.6 ÷ 27 = 0.2444 cubic yards. Bag yields for the exact 6.0 cu ft can be seen as 6.0 ÷ 20 = 0.30 cu ft per 40-lb bag, 6.0 ÷ 14 ≈ 0.4286 cu ft per 60-lb bag, and 6.0 ÷ 10 = 0.60 cu ft per 80-lb bag. Using those yields, the waste-adjusted counts are 22, 15, and 11 bags respectively.
Worked Example
Single-footing example: one 12" x 24" x 36" footing needs 6.0 cubic feet of concrete, which is 0.2222 cubic yards. For ordering with waste, plan on 6.6 cu ft (0.2444 yd³) — about 22 40-lb bags, 15 60-lb bags, or 11 80-lb bags. Bagged concrete is convenient here because the total is small.
Multiple-footings example: three identical footings total 6.0 × 3 = 18.0 cubic feet, which equals 0.6667 cubic yards. With 10% waste planning, that's 6.6 × 3 = 19.8 cu ft (about 0.7332 yd³), so order roughly 66 40-lb bags, 45 60-lb bags, or 33 80-lb bags if using bagged concrete; at that scale a small ready-mix delivery might become more practical.
Bag Count Estimate
- 12 bags of 80 lb concrete mix
- 15 bags of 60 lb concrete mix
- 23 bags of 40 lb concrete mix
Always verify the actual yield on the product label before buying materials.
When Bagged Concrete vs Ready-Mix Makes Sense
For this footing, the per-footing planning volume is only 0.2444 cubic yards, which is well below most ready-mix minimums. Bagged concrete is typically more economical and less wasteful for a single footing because you can buy exactly the number of bags needed and mix as you go.
If you have several footings or multiple nearby pours, the math flips: three or more footings push you toward a full yard of material and ready-mix becomes more efficient and faster to place. Also consider labor — bagged mixes require mixing time and handling, while ready-mix saves on onsite mixing but may have minimum-order, delivery, and access constraints.
Common Uses for This Footing Size
A 12x24x36 footing is often used for supporting deck posts, pergola posts, or fence posts in residential settings. Its rectangular shape and size can handle small porch columns or similar single-point supports where the load isn't too heavy. This footing size can fit into tighter spaces while providing a solid base for these types of structures.
Whether this footing size is right depends on several factors like the weight it needs to carry, the type of soil, frost depth in your area, and if reinforcement is necessary. Local building codes also play a role in determining if this footing meets requirements. It's important to consider these details before deciding on this footing size for your project.
Common Mistakes to Avoid
Common mistakes to avoid when planning concrete for a 12" x 24" x 36" footing.
- Underordering by ignoring the 10% waste allowance, which can leave you short during the pour.
- Confusing exact bag counts with waste-adjusted planning counts and ordering the lower number.
- Not confirming minimum ready-mix order sizes and assuming a truck can deliver a very small fraction of a yard.
- Skipping a structural check on reinforcement, bar size, or footing thickness called for by the plans.
- Failing to compact and level the subgrade before placing concrete, which reduces bearing capacity.
- Pouring in extreme temperatures without proper curing protection, which can weaken the concrete.
Frequently Asked Questions
How much concrete does a 12x24x36 footing need?
The footing requires 6.0 cubic feet of concrete, which equals 0.2222 cubic yards.
How many bags of concrete do I need for this footing?
Exact counts for 6.0 cu ft are 20 40-lb bags, 14 60-lb bags, or 10 80-lb bags. With a 10% planning allowance, plan on 22 40-lb, 15 60-lb, or 11 80-lb bags.
Why add 10% waste when ordering concrete?
A 10% allowance covers spillage, over-excavation, and uneven forms. For this footing it increases the volume from 6.0 cu ft to 6.6 cu ft (0.2222 yd³ to 0.2444 yd³).
Is a 12x24x36 footing adequate for a deck post?
It depends on the post load, soil bearing capacity, reinforcement, and local codes. A 36" depth may be suitable where that depth meets local frost-depth requirements, but have the footing and loads reviewed by the building department or a structural engineer.
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.