How to Use This Footing Estimate

Basic steps to get the footing poured correctly.

1. Measure and mark the location and verify dimensions are 18" × 18" and 48" deep.
2. Excavate to the required depth and remove loose material; compact the bottom of the hole.
3. Install forms or use the excavation itself, then add compacted gravel if required for drainage and bearing.
4. Place reinforcement if required by design (rebar or wire), tying it so it stays in position during the pour.
5. Mix bags or schedule ready‑mix; account for the planning volume of 9.9 cu ft per footing.
6. Pour and consolidate the concrete, screed smooth, and allow proper curing time before applying load.

Formula

Calculate footing volume by multiplying footprint area by depth. Convert inches to feet first: 18" = 1.5 ft and 48" = 4 ft. Footing area is 1.5 ft × 1.5 ft = 2.25 sq ft.

Volume in cubic feet = area × depth = 2.25 sq ft × 4 ft = 9.0 cu ft. To get cubic yards, divide cubic feet by 27: 9.0 cu ft ÷ 27 = 0.3333 cu yd.

Include waste by multiplying by the waste percentage: 9.0 cu ft × 1.10 = 9.9 cu ft (0.3333 yd³ × 1.10 = 0.3666 yd³). Use those planning figures when ordering or buying bags of concrete mix.

Worked Example

Single footing example: one 18" × 18" × 48" footing is 9.0 cubic feet (0.3333 yd³). With 10% waste you’ll need 9.9 cu ft (0.3666 yd³), which works out to about 33 40‑lb bags, 22 60‑lb bags, or 17 80‑lb bags for that single hole.

Multiple‑footing example: three identical footings equal 3 × 9.0 = 27.0 cubic feet total, which converts to 1.0 cubic yard. With waste the total is 3 × 9.9 = 29.7 cu ft (about 1.1 yd³). For bags, plan on roughly 99 40‑lb bags, 66 60‑lb bags, or 51 80‑lb bags for the three footings combined.

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.

When Bagged Concrete vs Ready-Mix Makes Sense

For a single 18" × 18" × 48" footing the quantity is small — bagged concrete is often the most convenient option if you have time and labor to mix. Buying bags lets you match the exact planning quantity of 9.9 cu ft and avoid ordering more concrete than you need.

Ready‑mix saves mixing time and gives consistent concrete, but suppliers commonly set a minimum delivery size. If you’re pouring multiple footings (for example three or more), a ready‑mix truck becomes more attractive because the total volume approaches a typical minimum and you get continuous placement and better consistency.

Decide based on crew size, access to the site, and whether you value speed and consistency over smaller material costs.

Common Uses for This Footing Size

An 18x18x48 footing is commonly used for supporting deck posts, pergola posts, fence posts, and small porch columns. Its size fits well for single-point supports where the load isn’t too heavy. This footing can be poured to hold up wooden or metal posts in residential settings, providing a solid base for structures like garden trellises or light outdoor coverings.

Whether this footing size works for your project depends on several factors. The weight of the structure, soil type, frost line depth, and whether reinforcement like rebar is needed all play a role. Local building codes may also have specific requirements that affect footing dimensions and installation. It’s important to consider these details before deciding on this footing size.

Common Mistakes to Avoid

Watch for these common pitfalls when preparing and pouring an 18" × 18" × 48" footing.

• Underordering material by forgetting the 10% waste allowance.
• Failing to compact or level the bottom of the excavation before placing concrete.
• Skipping required reinforcement or placing it incorrectly so it shifts during the pour.
• Mixing bags too wet or too dry, which weakens the finished concrete.
• Not checking soil bearing capacity and site conditions before sizing the footing.
• Pouring in very cold or hot conditions without proper curing measures.