How to Use This Footing Estimate

Use this quick sequence to plan and place concrete for the footing.

1. Confirm footing dimensions on your plans: 24" wide, 24" long, 36" deep.
2. Compute volume in feet (2 ft × 2 ft × 3 ft = 12 cu ft).
3. Add a waste allowance (recommended 10% → 13.2 cu ft) and convert to cubic yards if ordering ready-mix.
4. Choose between bags of concrete mix or ready-mix based on quantity, labor, and site access.
5. Prepare the excavation, compact the base, and place reinforcement if required by plans or local code.
6. Pour, consolidate, screed, and cure per material instructions and inspection requirements.

Formula

The basic formula for a rectangular or square footing is length × width × depth. Use feet for all dimensions to get cubic feet, then divide by 27 to convert to cubic yards.

For this footing convert the inches to feet: 24" = 2 ft, 24" = 2 ft, 36" = 3 ft. Multiply: 2 ft × 2 ft × 3 ft = 12.0 cubic feet.

Convert cubic feet to cubic yards by dividing by 27: 12.0 ÷ 27 = 0.4444 cubic yards (rounded to four decimals as shown).

To plan for waste and minor variations add 10%: 12.0 × 1.10 = 13.2 cubic feet, which is 13.2 ÷ 27 = 0.4888 cubic yards for ordering or bag estimates.

Worked Example

Single footing: one 24" x 24" x 36" footing needs 12.0 cubic feet of concrete, which is 0.4444 cubic yards. With 10% waste plan on 13.2 cubic feet (0.4888 cubic yards). That planning amount equals about 44 40-lb bags, 30 60-lb bags, or 22 80-lb bags.

Multiple footings: for three identical footings total volume is 3 × 12.0 = 36.0 cubic feet, which is 1.3333 cubic yards. With 10% waste plan on 3 × 13.2 = 39.6 cubic feet, or 1.4667 cubic yards. In bags that means roughly 132 40-lb bags, 90 60-lb bags, or 66 80-lb bags for all three footings.

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.

When Bagged Concrete vs Ready-Mix Makes Sense

For a single 24" × 24" × 36" footing the total volume is small enough that buying bags of concrete mix can be the most practical option—you can match the planning bag counts and mix on-site as needed. Bagged concrete gives flexibility if you have limited access or only one small pour.

Ready-mix offers labor savings and consistent mixes but often comes with a minimum delivery size and additional charges. If you're pouring multiple footings or larger concentrated pours, ready-mix becomes more economical. For this single footing plan on 0.4888 yd³ with waste; if ordering ready-mix verify the plant’s minimum load and consider combining pours to meet that minimum.

Common Uses for This Footing Size

A 24x24x36 footing is commonly used for supporting posts in residential projects. You might see it under deck posts, pergola posts, or fence posts where the load isn't very heavy. It can also support small porch columns or similar structures that rest on a single point. This size gives enough concrete volume to hold these types of posts steady in typical yard or garden settings.

Whether this footing size works depends on several factors. The weight of what’s being supported, the type of soil, how deep the frost line is in your area, and if any reinforcement is needed all matter. Local building codes may have specific rules about footing sizes and depths, so it’s important to check those before deciding to use this footing size for your project.

Common Mistakes to Avoid

Avoid these common planning and placement errors to save time and material.

• Underordering concrete by not including a waste allowance.
• Mixing up inches and feet when converting dimensions.
• Assuming bag yields without checking manufacturer yield per bag.
• Skipping consultation with the local building department about depth or reinforcement.
• Pouring without proper base compaction or formwork alignment.
• Neglecting to account for access, mixing space, or curing needs on site.