How to Use This Footing Estimate

Quick steps to get this footing poured right:

1. Measure and mark the footing excavation to the specified 12" × 18" × 48" dimensions and compact the base.
2. Place any required formwork and reinforcement per the plans or engineer’s instructions.
3. Order concrete or stock the correct number of bags based on the planning volume (6.6 cu ft with waste).
4. Mix and place the concrete, working it around the reinforcement and eliminating voids.
5. Float, finish, and cure the concrete following recommended curing time and protection.

Formula

Volume is length × width × depth. Convert the inch dimensions to feet first: 12" = 1.0 ft, 18" = 1.5 ft, 48" = 4.0 ft. Multiply those to get cubic feet: 1.0 × 1.5 × 4.0 = 6.0 cubic feet.

Convert cubic feet to cubic yards by dividing by 27: 6.0 ÷ 27 = 0.2222 cubic yards for the exact footing volume.

Apply a default 10% waste factor to account for spillage and minor measurement variation: 6.0 × 1.10 = 6.6 cubic feet, which converts to 0.2444 cubic yards. Bag counts and ready-mix orders should be based on the waste-adjusted volume when planning.

Worked Example

Single-footing example: one 12" × 18" × 48" footing needs 6.0 cu ft (0.2222 yd³) of concrete. Plan for 6.6 cu ft (0.2444 yd³) with waste — that translates to about 22 40‑lb bags, 15 60‑lb bags, or 11 80‑lb bags for a bagged-concrete approach. If using ready-mix, ordering a 0.25 yd³ truck delivery covers the planned volume.

Multiple-footing example: for three identical footings multiply the planning volume first: 3 × 6.6 cu ft = 19.8 cu ft total, which equals 0.7332 cubic yards. Using bags, plan on 66 40‑lb bags, 45 60‑lb bags, or 33 80‑lb bags total. For ready-mix, round the required 0.7332 yd³ up to a practical delivery size (for example, 0.75 yd³).

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

Because a single 12" × 18" × 48" footing only needs about 0.2444 cubic yards with waste, bagged concrete is often convenient: you can buy exactly the number of 40‑, 60‑, or 80‑lb bags and mix on site. Bagged concrete keeps truck scheduling simple and can be cost-effective for one or two footings.

If you have multiple footings or limited time, ready-mix simplifies placement and consistency. For three or more footings you’ll likely prefer a small ready-mix delivery; it reduces mixing labor and gives a more uniform mix, though you should round up the order to the supplier’s minimum batch size.

Common Uses for This Footing Size

A 12x18x48 footing is often used for supporting deck posts, pergola posts, fence posts, or small porch columns. Its rectangular shape and size make it suitable for single-point supports where the load is relatively light and the footing needs to fit within limited space. This footing can be placed below grade to help distribute the weight of wooden posts and prevent settling in typical residential settings.

Whether this footing size will work depends on several factors like the weight it needs to carry, the type of soil, how deep the frost line is in your area, and if reinforcement like rebar is necessary. Local building codes also influence what footing sizes and designs are acceptable, so it’s important to consider these details before deciding on this size for your project.

Common Mistakes to Avoid

Common mistakes to avoid when ordering and placing this footing:

• Ordering too little concrete and having to stop the pour to get more.
• Forgetting to add a waste allowance — order the planning volume, not just the exact volume.
• Skipping proper reinforcement or not following engineered details for rebar/placement.
• Failing to compact or prepare the soil beneath the footing before pouring.
• Using the wrong concrete strength or mix for the intended load and exposure.
• Not protecting the fresh concrete during curing, which weakens the finished footing.