How to Use This Footing Estimate

Simple steps to go from calculation to pour for this footing:

1. Confirm dimensions and depth on site and locate utilities before digging.
2. Excavate to the full 36" depth and form the 12" x 18" footprint, keeping the sides square and level.
3. Compact the bottom and add a thin gravel pad if required by local code or soil conditions.
4. Mix bagged concrete or order the planned volume of concrete, allowing the 10% waste margin.
5. Place concrete, consolidate around any reinforcement, screed flush, and finish the top to the desired texture.
6. Protect the footing from premature freezing or drying during the initial cure period.

Formula

The volume formula for a rectangular footing is length × width × depth. Use consistent units — here we'll use feet so the result is in cubic feet.

For this footing: 1 ft (length) × 1.5 ft (width) × 3 ft (depth) = 4.5 cubic feet. To convert cubic feet to cubic yards divide by 27: 4.5 / 27 = 0.1667 cubic yards.

To include a planning allowance for waste, multiply by 1.10 (10%): 4.5 × 1.10 = 4.95 cubic feet, which is 4.95 / 27 = 0.1834 cubic yards. Use those waste-adjusted numbers when ordering or buying bags.

Worked Example

Single footing: the footing is 4.5 cubic feet (0.1667 yd³). With a 10% waste allowance plan on 4.95 cubic feet (0.1834 yd³). That means about 17 40-lb bags, 11 60-lb bags, or 9 80-lb bags for a single poured footing.

Three footings for a small deck (three posts): total volume is 3 × 4.5 = 13.5 cubic feet, which converts to 13.5 / 27 = 0.5 cubic yards. With 10% waste the total is 13.5 × 1.10 = 14.85 cubic feet, or 14.85 / 27 = 0.55 cubic yards. For bagged mix plan on 3 × 17 = 51 40-lb bags, 3 × 11 = 33 60-lb bags, or 3 × 9 = 27 80-lb bags.

Bag Count Estimate

• 9 bags of 80 lb concrete mix
• 11 bags of 60 lb concrete mix
• 17 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 12" x 18" x 36" footing the waste-adjusted volume is only about 0.1834 cubic yards, so buying bagged concrete is often the most practical option. Bagged concrete lets you buy exactly the small amount you need and store unused bags for future touch-ups.

If you have multiple footings or other pours that push the total above a few tenths of a cubic yard, ready-mix becomes more attractive because it saves mixing time and delivers consistent slump and strength. Keep in mind many ready-mix plants set a minimum delivery of around 1 cubic yard, so compare the total project volume before choosing.

Common Uses for This Footing Size

A 12x18x36 footing is commonly used for supporting deck posts, pergola posts, fence posts, and small porch columns. Its rectangular shape and size make it suitable for single-point supports that don’t carry heavy loads. This footing can help keep posts stable in typical residential settings where the structure is light and the ground is fairly even.

Whether this footing size will work depends on factors like the weight it needs to hold, the type of soil, how deep the frost line is in your area, and if reinforcement is needed. Local building codes also play a role in determining if this footing meets the requirements for your project. It’s important to consider these aspects before deciding on footing size.

Common Mistakes to Avoid

Avoid these common mistakes when working with this size footing:

• Not allowing for the 10% waste — underordering causes delays.
• Poorly compacted or unconsolidated footing bottoms before pouring.
• Skipping required reinforcement or using the wrong size/spacing of rebar.
• Ordering ready-mix for a single small footing without checking minimum delivery amounts.
• Pouring when the weather risks freezing or rapid drying without protection.
• Mixing bagged concrete too wet or too dry, which weakens the cured concrete.