How Much Concrete for an 18x24x36 Footing?
How an 18" x 24" x 36" footing translates to 9 cu ft, 0.3333 yd³, and bag counts for a single post or column.
Get the exact concrete volume and bag counts for an 18" x 24" x 36" rectangular footing: 9 cubic feet (0.3333 yd³) and planning amounts including 10% waste for a smooth pour.
For a single support post or a column base, an 18" x 24" x 36" footing gives a solid footprint with deep bearing. This footing size suits deck posts, small porch columns, and freestanding supports where a deeper embedment helps resist uplift and frost heave.
Because this is a relatively small, deep rectangular footing, planning for mixing or ordering is straightforward: the pour volume is 9 cubic feet (0.3333 cubic yards). Add about 10% for waste and spillage, reinforcement take-up, and minor overexcavation when ordering materials.
Quick Answer
Exact volume: 9.0 cubic feet, which is 0.3333 cubic yards. With a 10% allowance for waste, plan on 9.9 cubic feet (0.3666 cubic yards).
For bagged concrete, plan quantities with waste: about 33 40-lb bags, 22 60-lb bags, or 17 80-lb bags. The exact counts (without waste) are 30, 20, and 15 bags respectively.
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How to Use This Footing Estimate
Quick steps to pour this footing for a post or column.
- Mark the footing location and excavate to a consistent 36" depth and the full 18" × 24" plan area.
- Check and compact the bearing soil, adding a gravel pad if needed for drainage and uniform support.
- Install forms and place reinforcement per plans—usually a small rebar cage or chairs for one footing.
- Mix or order concrete to the planned volume and pour, working the concrete into corners and around rebar.
- Strike off the top, finish lightly for form removal, and keep the concrete moist while it cures for at least several days.
Formula
The math for a rectangular footing is length × width × depth. For this footing that is 18 in × 24 in × 36 in = 15,552 cubic inches. Convert cubic inches to cubic feet by dividing by 1,728.
15,552 in³ ÷ 1,728 = 9.0 cubic feet. Convert cubic feet to cubic yards by dividing by 27: 9.0 ÷ 27 = 0.3333 cubic yards.
Include a planning allowance for waste. At 10% waste: 9.0 cu ft × 1.10 = 9.9 cu ft, or 0.3666 cubic yards. Bag counts come from dividing the total volume by the volume per bag: with this footing a 40-lb bag works out to ~0.30 cu ft (9 ÷ 30), a 60-lb bag ~0.45 cu ft (9 ÷ 20), and an 80-lb bag ~0.60 cu ft (9 ÷ 15). Use the planning bag counts when buying: 33 (40-lb), 22 (60-lb), or 17 (80-lb) including waste.
Worked Example
Single footing example: one 18" × 24" × 36" footing uses 9.0 cubic feet of concrete, which is 0.3333 cubic yards. With the 10% allowance plan for 9.9 cubic feet (0.3666 yd³) and about 33 40-lb bags or 17 80-lb bags for a single pour.
Multiple-footing example: three identical footings total 27.0 cubic feet of concrete (3 × 9.0 cu ft). That converts to 1.0 cubic yard. If you include the 10% planning allowance before multiplying, three footings would be 3 × 9.9 = 29.7 cubic feet (about 1.1 cubic yards) and you would buy roughly 99 40-lb bags, 66 60-lb bags, or 51 80-lb bags based on the per-footing planning counts.
Bag Count Estimate
- 17 bags of 80 lb concrete mix
- 22 bags of 60 lb concrete mix
- 33 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 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 | 9 | 15 | 20 | 30 |
| +5% typical DIY | 9.45 | 16 | 22 | 32 |
| +10% recommended | 9.9 | 17 | 22 | 33 |
| +15% rough conditions | 10.35 | 18 | 23 | 35 |
When Bagged Concrete vs Ready-Mix Makes Sense
For a single 18" × 24" × 36" footing the total concrete needed is small—0.3333 cubic yards—so bagged concrete is often the most practical option. Bagged concrete lets you match volume closely, avoid large minimum truckloads, and pace the pour if you’re mixing by hand or with a small mixer.
Ready-mix trucks usually have minimum orders that far exceed this footing’s volume and may add waiting time and higher cost for a single small pour. If you have multiple footings (enough to reach a partial yard), ready-mix becomes more attractive for consistent slump and less on-site labor. Consider site access, how many footings you’re pouring, and whether you need the speed and consistency of ready-mix versus the flexibility of bagged concrete.
Common Uses for This Footing Size
An 18x24x36 footing is commonly used for supporting deck posts, pergola posts, fence posts, and small porch columns. Its rectangular shape and size make it suitable for single-point supports where the load is relatively light. This footing can help keep wooden posts stable and anchored in place for outdoor structures that don’t carry heavy weight or large spans.
Whether this footing size is right for your project depends on several factors like the weight of what you’re supporting, the soil type and how well it holds up, local frost depth, and if reinforcement is needed. Building codes in your area may also affect footing dimensions and installation requirements. It’s important to consider these details before deciding on footing size.
Common Mistakes to Avoid
Common mistakes to avoid with this footing size.
- Mis-measuring depth or plan area and under-ordering concrete.
- Failing to add the 10% waste allowance and running short mid-pour.
- Skipping soil-bearing checks or not compacting the base before placing forms.
- Leaving out specified reinforcement or using undersized rebar for the applied loads.
- Overwatering mixes on-site, which weakens the cured concrete.
- Not protecting the pour from rapid drying or freezing during the cure.
Frequently Asked Questions
How much concrete do I need in cubic feet and cubic yards for a single 18x24x36 footing?
You need 9.0 cubic feet, which equals 0.3333 cubic yards for a single footing.
How many bags of concrete should I buy for this footing?
Plan for the waste-adjusted counts: about 33 40-lb bags, 22 60-lb bags, or 17 80-lb bags. The exact (no-waste) counts are 30, 20, and 15 respectively.
Why should I add 10% waste to my order?
Adding 10% covers spillage, slight overexcavation, reinforcement displacement, and small measurement errors so you don’t run short during the pour.
Is this footing size adequate for a deck post?
It depends on the post load, soil-bearing capacity, local frost depth, and code requirements. A structural check is recommended and the footing may be suitable where a 36-inch depth aligns with local frost-depth and bearing needs.
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.