How Much Concrete for an 18x24x48 Footing?
Plan concrete for an 18" x 24" x 48" footing — cu ft, cu yd, and bag counts for ordering and budgeting.
Find exact quantities for an 18" x 24" x 48" concrete footing: 12 cu ft (0.4444 yd³), planning 13.2 cu ft (0.4888 yd³), plus bag counts for 40-, 60-, and 80-lb mixes.
Before you order anything, plan the job around the actual volumes and how you want to place the concrete. An 18" x 24" x 48" rectangular footing gives you 12 cubic feet of concrete (0.4444 cubic yards) per footing, which makes bagged mixes a practical option on small jobs.
This size works well for isolated column footings, small pier supports, or retrofit posts where a deep anchor is required. Check with your local building department for permit and reinforcement requirements before you buy materials or start digging.
Quick Answer
Exact volume for one 18" x 24" x 48" footing is 12.0 cu ft, which equals 0.4444 cubic yards. With a 10% waste allowance, plan on 13.2 cu ft or about 0.4888 cubic yards.
For ordering, expect to use roughly 44 40-lb bags, 30 60-lb bags, or 22 80-lb bags after adding the 10% waste allowance. Exact bag counts (no waste) are 40, 27, and 20 respectively.
Need a custom estimate? Use the Footing Calculator. Browse footing size pages. All project pages.
How to Use This Footing Estimate
Steps to plan, buy, and place concrete for this footing:
- Confirm footing dimensions and local code requirements before you dig.
- Calculate volume in cubic feet (1.5 ft × 2.0 ft × 4.0 ft = 12 cu ft) and add 10% for waste (13.2 cu ft).
- Decide between bags or ready-mix based on quantity and site access.
- Buy the planned number of bags or order the required cubic yards; stage bags near the pour.
- Mix, place, consolidate, and finish the concrete; follow manufacturer directions for bag mixes.
- Cure the concrete and protect it from rapid drying and freezing for the first several days.
Formula
Use the rectangular volume formula: volume = length × width × depth. Convert all dimensions to feet first. For this footing that is 1.5 ft × 2.0 ft × 4.0 ft.
Multiply the footprint area by depth: 1.5 ft × 2.0 ft = 3.0 sq ft; 3.0 sq ft × 4.0 ft = 12.0 cubic feet. To get cubic yards divide cubic feet by 27: 12.0 ÷ 27 = 0.4444 yd³.
Include a waste factor to cover spillage, overfill, and small measurement errors. At 10% waste: 12.0 cu ft × 1.10 = 13.2 cu ft, and 0.4444 yd³ × 1.10 ≈ 0.4888 yd³. Bag counts are derived from those planning volumes.
Worked Example
Single-footing example: One footing uses 12.0 cubic feet (0.4444 yd³) of concrete. With 10% waste plan on 13.2 cu ft (0.4888 yd³). That translates to about 44 40-lb bags, 30 60-lb bags, or 22 80-lb bags for the job.
Multiple-footing example: If you need three identical footings, total volume is 36.0 cu ft (36.0 ÷ 27 = 1.3333 yd³). With 10% waste plan on 39.6 cu ft, which is about 1.4667 yd³. For bags, multiply the per-footing planning counts: 132 40-lb bags, 90 60-lb bags, or 66 80-lb bags.
Bag Count Estimate
- 23 bags of 80 lb concrete mix
- 30 bags of 60 lb concrete mix
- 45 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 48"
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 | 12 | 20 | 27 | 40 |
| +5% typical DIY | 12.6 | 22 | 29 | 43 |
| +10% recommended | 13.2 | 23 | 30 | 45 |
| +15% rough conditions | 13.8 | 23 | 31 | 46 |
When Bagged Concrete vs Ready-Mix Makes Sense
For this footing the planned volume with waste is only 0.4888 cubic yards, which sits near the lower end of what many ready-mix suppliers will deliver. Because the quantity is under one cubic yard, bags of concrete mix are often more convenient and avoid minimum delivery fees.
Bagged concrete lets you match the exact number of bags to the project, store leftovers for small repairs, and avoid coordinating a truck. Ready-mix saves mixing time and labor for larger totals; if you need multiple footings raising total volume past a truck minimum, ready-mix may become practical.
Common Uses for This Footing Size
An 18x24x48 footing is often used for supporting deck posts, pergola posts, fence posts, or small porch columns. Its rectangular shape and dimensions make it suitable for single-point supports where the load is not too heavy. This size can help keep wooden posts stable in typical residential settings, especially when the structure is relatively light and the soil conditions are average.
Whether this footing size is right depends on several factors like the weight it needs to hold, the type of soil, how deep the frost line is in your area, and if any reinforcement is necessary. Local building codes might also have specific rules about footing sizes, so it’s important to consider those before deciding on this footing dimension.
Common Mistakes to Avoid
A few common mistakes that increase cost or create rework:
- Ordering exactly 12 cu ft and skipping any waste allowance — always add a margin.
- Confusing per-footing bag counts with the total needed for multiple footings.
- Not checking local building department rules for depth, reinforcement, or inspections.
- Neglecting reinforcement or incorrectly placing rebar where required.
- Poor mixing or overwatering bag mixes, which weakens the finished concrete.
- Failing to cure properly, leading to premature cracking and reduced strength.
Frequently Asked Questions
How much concrete does an 18x24x48 footing need in cubic feet and cubic yards?
One 18" x 24" x 48" footing requires 12.0 cubic feet, which equals 0.4444 cubic yards.
How many bags of concrete mix do I need for this footing?
Exact bag counts (no waste) are 40 40-lb bags, 27 60-lb bags, or 20 80-lb bags. With a 10% waste allowance, plan on 44 40-lb bags, 30 60-lb bags, or 22 80-lb bags.
Why should I add 10% waste and how does that change ordering?
A 10% waste allowance covers spillage, over-excavation, and measurement variance. It raises the footing from 12.0 cu ft to 13.2 cu ft (0.4888 yd³), which is what you should order or use to calculate bag counts.
Is this footing size adequate for a deck post?
It depends on the deck loads, local soil bearing capacity, frost depth, and local code requirements. This 18" x 24" x 48" footing may be suitable where a 48-inch depth meets frost-depth rules, but you should confirm sizing and reinforcement with your local building department or a structural professional.
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.