How Much Concrete for an 18x18x36 Footing?
18" x 18" x 36" footing — 6.75 cu ft (0.25 yd³); plan for 7.425 cu ft and 25 40-lb bags
Calculate concrete for an 18" x 18" x 36" square footing: exact 6.75 cu ft (0.25 yd³), planning 7.425 cu ft, plus bag counts for 40-, 60-, and 80-lb mixes.
At 18" x 18" x 36" this square footing measures 1.5 ft by 1.5 ft by 3 ft, giving 6.75 cubic feet of concrete. That size is commonly used for single post supports, small porch columns, and other residential point loads where a deeper, narrow footing is required.
Because this footing is 36" deep you should check local frost-depth rules and permit requirements before you pour. For many homeowners this footprint is a practical DIY pour, but site conditions, soil bearing capacity, and reinforcement needs will affect whether you should call a pro.
Quick Answer
Exact volume: 6.75 cubic feet, which converts to 0.25 cubic yards. If you prefer bagged concrete the raw bag counts are 23 40-lb bags, 15 60-lb bags, or 12 80-lb bags for a single footing.
Plan for waste: add about 10% for spillage and slight measurement error, bringing the planning volume to 7.425 cu ft (0.275 yd³). For ordering, use the planning bag counts: 25 40-lb bags, 17 60-lb bags, or 13 80-lb bags.
Need a custom estimate? Use the Footing Calculator. Browse footing size pages. All project pages.
How to Use This Footing Estimate
Quick steps to measure, order, and place concrete for this footing:
- Confirm dimensions in the field: 18" x 18" x 36" (1.5 ft × 1.5 ft × 3 ft).
- Multiply length × width × depth to get volume in cubic feet (1.5 × 1.5 × 3 = 6.75 cu ft).
- Add 10% for waste to get planning volume (6.75 × 1.10 = 7.425 cu ft) and decide between bagged concrete or ready-mix.
- If using bagged concrete, buy the planning number of bags and mix on-site per manufacturer directions; if using ready-mix, order the planning cubic yards.
- Prepare the subgrade, set any required reinforcement or anchors, pour, consolidate, and follow curing recommendations for at least several days.
- Keep a small extra supply on hand in case you need to touch up edges or adjust embedments.
Formula
The basic formula for a rectangular footing is length × width × depth when each dimension is in feet. For an 18" × 18" × 36" footing that means 1.5 ft × 1.5 ft × 3 ft.
The math: 1.5 × 1.5 = 2.25 square feet of area, and 2.25 × 3 = 6.75 cubic feet. To convert cubic feet to cubic yards divide by 27: 6.75 ÷ 27 = 0.25 cubic yards.
To allow for waste multiply the exact volume by 1.10 (10% waste). So 6.75 cu ft × 1.10 = 7.425 cu ft, which is 0.275 cu yd. Bag counts are typically increased the same way: planning bags = exact bags × 1.10, rounded up (23 × 1.10 ≈ 25 for 40-lb, 15 × 1.10 ≈ 17 for 60-lb, 12 × 1.10 ≈ 13 for 80-lb).
Worked Example
Single-footing example: one 18"×18"×36" footing needs 6.75 cubic feet total, which is 0.25 cubic yards. With a 10% allowance you ll plan for 7.425 cu ft (0.275 cu yd) and buy 25 40-lb bags, 17 60-lb bags, or 13 80-lb bags.
Multiple-footing example: three identical footings need 3 × 6.75 = 20.25 cubic feet total, which is 0.75 cubic yards. Using the planning volume that becomes 3 × 7.425 = 22.275 cu ft (0.825 cu yd). For bagged concrete plan to buy 75 40-lb bags, 51 60-lb bags, or 39 80-lb bags.
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 18" x 18" 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 | 6.75 | 12 | 15 | 23 |
| +5% typical DIY | 7.09 | 12 | 16 | 24 |
| +10% recommended | 7.43 | 13 | 17 | 25 |
| +15% rough conditions | 7.76 | 13 | 18 | 26 |
When Bagged Concrete vs Ready-Mix Makes Sense
For a single 0.25 yd³ footing bagged concrete is often the most practical choice for DIYers: it s inexpensive to buy locally and you can mix small batches as needed. Bagged concrete also avoids minimum-order charges and waiting for a truck if you only have one or two footings.
Ready-mix is worth considering when you have multiple footings, tight timing, or prefer a consistently mixed product. Even though this footing is only 0.25 yd³, multiple footings quickly add up: three footings demand about 0.75 yd³ (0.825 yd³ with waste), which is where ready-mix becomes more convenient. Factor in delivery minimums, pump access, and labor when deciding.
Common Uses for This Footing Size
An 18x18x36 footing is commonly used for supporting posts in residential projects. You might see it under deck posts, pergola posts, or fence posts where the load isn’t very heavy. It can also be used for small porch columns or similar single-point supports that don’t carry a lot of weight. The square shape and these dimensions fit well for posts that need a solid base but don’t require a large footing.
Whether this footing size is right depends on several factors like how much weight the post will carry, the type of soil, how deep the frost line is in your area, and if any reinforcement is needed. Local building codes also play a big role in deciding if this footing meets the requirements for your specific project.
Common Mistakes to Avoid
Common mistakes to avoid when pouring this footing:
- Ordering exactly the computed volume and not adding waste—always include about 10% extra.
- Failing to check local frost-depth and permit requirements before digging and pouring.
- Ignoring poor subgrade preparation or inadequate compaction under the footing.
- Skipping required reinforcement or embed plates when plans call for them.
- Mixing excess water into bagged concrete, which weakens the finished footing.
- Not protecting the footing during initial cure from rapid drying or freezing conditions.
Frequently Asked Questions
How much concrete does an 18x18x36 footing require?
An 18" x 18" x 36" footing requires 6.75 cubic feet of concrete, which is 0.25 cubic yards.
How many bags of concrete do I need for this footing?
The raw bag counts are 23 40-lb bags, 15 60-lb bags, or 12 80-lb bags. For ordering, plan for 25 40-lb, 17 60-lb, or 13 80-lb bags to allow roughly 10% waste.
How much extra should I order for waste and spillage?
Use about 10% extra for waste and small errors. That raises the volume from 6.75 cu ft to 7.425 cu ft (0.25 yd³ to 0.275 yd³) and is reflected in the planning bag counts.
Is this footing adequate for a deck post?
It depends on the deck s loads, soil bearing capacity, frost depth at your site, and local building code. A 36" deep, 18"×18" footing may be acceptable in many residential situations, but check the local building department and consult a structural engineer if the post carries heavy loads or if soil and frost conditions are uncertain.
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.