How to Use This Footing Estimate

Quick steps to estimate and place concrete for this footing:

1. Confirm footing footprint and depth: 18" long × 24" wide × 24" deep.
2. Calculate volume in feet: 1.5 ft × 2 ft × 2 ft = 6.0 cu ft.
3. Add 10% waste for planning: 6.0 cu ft × 1.10 = 6.6 cu ft.
4. Decide supply: buy bagged concrete or order ready-mix based on total yardage and delivery minimums.
5. Mix, place, consolidate, and cure per mix instructions and any reinforcement details.
6. Verify frost depth and local code requirements before finalizing the pour.

Formula

The basic formula is length × width × depth (all in feet) to get cubic feet, then divide by 27 to convert cubic feet to cubic yards. For this footing use 1.5 ft × 2 ft × 2 ft.

Exact math: 1.5 × 2 × 2 = 6.0 cubic feet. Convert to cubic yards: 6.0 ÷ 27 = 0.2222 cubic yards (rounded to four decimals as shown).

Include a waste allowance: 6.0 cu ft × 1.10 = 6.6 cu ft planning volume. In yards that's 0.2222 × 1.10 = 0.2444 cubic yards. Based on common bag yields, that planning volume translates to about 22 40-lb bags, 15 60-lb bags, or 11 80-lb bags for the footing.

Worked Example

Single-footing example: one 18" × 24" × 24" footing uses 6.0 cubic feet (0.2222 yd³) of concrete. With 10% waste plan on 6.6 cu ft, or about 0.2444 yd³, which equates to roughly 22 40-lb bags (planning quantity).

Multiple-footing example: three identical footings total 18.0 cubic feet of concrete (18.0 ÷ 27 = 0.6666 yd³). With 10% waste the planning total is 19.8 cu ft, about 0.7333 yd³, so order roughly 66 40-lb bags, 45 60-lb bags, or 33 80-lb bags for the three footings.

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

At 0.2444 cubic yards per footing (with waste), a single 18" × 24" × 24" footing is small enough that bagged concrete is often the simplest option. Bagged concrete lets you buy only what you need and avoid minimum delivery charges.

Ready-mix trucks commonly expect larger orders and may have a one-yard minimum plus delivery fees, so they tend to be more practical when you have multiple footings or other concrete work that brings the total above that threshold. If you combine several footings and other pours and exceed the ready-mix minimum, compare the total cost and labor before deciding.

Common Uses for This Footing Size

An 18x24x24 footing is commonly used for supporting posts in residential projects where the loads are relatively light. You might see this size under deck posts, pergola supports, fence posts, or small porch columns. The rectangular shape and these dimensions offer enough footing area for single-point supports that don’t carry heavy weight. It’s a practical choice for many outdoor structures that need a stable base without a large footprint.

Whether this footing size is suitable depends on several factors like the weight of what it’s holding up, the type of soil, how deep the frost line goes in your area, and if any reinforcement is necessary. Local building codes also play a big role in determining what footing dimensions are acceptable for your project. It’s important to consider these details before deciding on footing size.

Common Mistakes to Avoid

Common mistakes to avoid when planning this footing:

• Skipping the waste allowance and running out of concrete mid-pour.
• Not checking frost-depth and local code for required footing depth or reinforcement.
• Underestimating loads and failing to provide proper reinforcement or a larger footing when required.
• Poor consolidation or finishing that leaves voids or weak surfaces.
• Ordering ready-mix without confirming minimum order size and delivery fees.
• Incorrect unit conversions when calculating cubic feet to cubic yards.