How Much Concrete for a 12x12x36 Footing?
Material totals and bag estimates for a 12" x 12" x 36" footing: cubic feet, cubic yards, and bags with 10% waste
Find exact volume and planning totals for a 12" x 12" x 36" footing: 3 cu ft (0.1111 yd³), waste-adjusted totals, and bag counts for 40-, 60-, and 80-lb mixes.
For small vertical supports like a deck post or a pergola column, the 12" x 12" x 36" footing is a practical option where a deeper seat is needed. This page focuses on real-world use for that size—how much concrete to buy, whether bags or ready-mix make sense, and simple planning numbers you can use on site.
The footing measures 12 inches by 12 inches by 36 inches (1 ft × 1 ft × 3 ft), which yields an exact volume of 3 cubic feet. Because this depth and footprint are often used where frost depth or heavier loads matter, plan material and reinforcement accordingly for your soil and local code requirements.
Quick Answer
Exact volume: 3.0 cubic feet, which converts to 0.1111 cubic yards. Exact bag counts for the complete poured footing are 10 of the 40-lb bags, 7 of the 60-lb bags, or 5 of the 80-lb bags.
With a standard 10% waste allowance the planning totals are 3.3 cubic feet (0.1222 cubic yards). For purchasing, plan on about 11 40-lb bags, 8 60-lb bags, or 6 80-lb bags to leave a margin for spillage, variations in forms, and minor over-excavation.
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 a 12" x 12" x 36" footing correctly:
- Confirm footing location, depth, and permit requirements with your local building department.
- Excavate to the specified depth and compact the bottom of the hole.
- Set straight, secure forms for a 12" square and check that the forms are plumb and level.
- Place reinforcement if required—rebar or wire—as designed by the engineer or code.
- Mix bagged concrete or pour ready-mix to the footing; consolidate with a rod or vibrator to remove voids.
- Screed or float the top, let the concrete set, and cure per manufacturer recommendations before backfilling.
Formula
The basic formula for a rectangular footing is length × width × depth to get cubic feet, then divide by 27 to convert cubic feet to cubic yards. Use feet for all three dimensions when using this formula.
Applying the numbers: length = 1 ft, width = 1 ft, depth = 3 ft. Multiply 1 × 1 × 3 = 3.0 cubic feet. To get cubic yards divide 3.0 by 27, which equals 0.1111 cubic yards.
To allow for small errors, waste, and handling losses add 10%. Multiply the exact volume by 1.10: 3.0 × 1.10 = 3.3 cubic feet. In cubic yards that is 0.1111 × 1.10 = 0.1222 cubic yards. Use these planning numbers when ordering or buying bags.
Worked Example
Single footing: a lone deck post footing at 12" x 12" x 36" uses 3.0 cubic feet of concrete (0.1111 cubic yards) exactly. With 10% waste plan on 3.3 cubic feet (0.1222 cubic yards), which is roughly 11 40-lb bags, 8 60-lb bags, or 6 80-lb bags.
Three footings for a small pergola: total exact volume is 3 × 3.0 = 9.0 cubic feet, which is 0.3333 cubic yards. With 10% waste plan on 3 × 3.3 = 9.9 cubic feet total, or 0.3666 cubic yards. For bags that means about 33 40-lb bags, 24 60-lb bags, or 18 80-lb bags for the three-footing job.
Bag Count Estimate
- 6 bags of 80 lb concrete mix
- 8 bags of 60 lb concrete mix
- 12 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 a single 12" x 12" x 36" footing the volume is small—3.3 cubic feet with waste—so bagged concrete is often the most convenient option. Buying bagged concrete allows you to match the job size precisely, avoid minimum delivery charges, and manage several small pours without renting a pump or truck.
Ready-mix is an efficient choice when pouring many footings at once or when you need uniform batching and faster placement. For this exact footing the waste-adjusted volume is only 0.1222 cubic yards, which is usually well below common minimum delivery amounts, so compare costs and minimums before ordering ready-mix. If using bags, follow the planning bag counts (11 40-lb, 8 60-lb, or 6 80-lb) to ensure adequate material.
Common Uses for This Footing Size
A 12x12x36 footing is commonly used for supporting posts in residential projects. You might see it under deck posts, pergola posts, fence posts, or small porch columns. Its size fits well for single-point supports that don’t carry heavy loads. This footing can help keep posts stable and anchored in place for lighter structures around the yard or home.
Whether this footing size works depends on several factors. The weight of what’s being supported, the type of soil, how deep the frost line is in your area, and if any reinforcement is needed all play a role. Local building codes also influence what footing dimensions are acceptable. It’s important to consider these details before deciding if this footing fits your project.
Common Mistakes to Avoid
Common mistakes to avoid with this footing size:
- Underestimating material needs by skipping the 10% waste allowance.
- Not checking local frost-depth or permit requirements before digging.
- Skipping required reinforcement or using improperly placed rebar.
- Overwatering mixes or inconsistent mixing that weakens the concrete.
- Poor compaction of the hole bottom leading to future settlement.
- Not curing the concrete properly before backfilling or loading the footing.
Frequently Asked Questions
How much concrete does a 12x12x36 footing require?
The exact volume is 3.0 cubic feet, which is 0.1111 cubic yards. With a 10% waste allowance plan on 3.3 cubic feet or 0.1222 cubic yards.
How many bags of concrete do I need for this footing?
Exact bag counts for the poured footing are 10 40-lb bags, 7 60-lb bags, or 5 80-lb bags. With a 10% waste margin plan on about 11 40-lb bags, 8 60-lb bags, or 6 80-lb bags.
Why should I add a 10% waste allowance?
A 10% waste allowance covers spillage, slight over-excavation, irregular form shapes, and minor measurement differences. It provides a safety margin so you don’t run short mid-pour.
Is a 12x12x36 footing big enough for a deck post?
It depends on the applied load, soil bearing capacity, frost depth where you’re building, and local code or engineering requirements. This size may be suitable where a 36-inch depth meets local frost-depth requirements, but always confirm loads, reinforcement, and permit needs with your 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.