Concrete Footing Calculator

What the Concrete Footing Calculator Does (and Why It Matters)

A concrete footing spreads the load of a wall, deck, or structure into the soil. If you underestimate concrete, you risk cold joints (stopping mid-pour) and weaker support. If you overestimate, you pay for excess material and disposal. ProcalcAI’s Concrete Footing Calculator helps you estimate how much concrete you need for continuous footings (strips) and gives results in cubic yards, cubic feet, and an approximate count of 80 lb bags.

You’ll enter three inputs:

- Footing Width (in) - Footing Depth (in) - Total Length (ft)

The calculator then converts inches to feet, computes volume, converts to cubic yards, and estimates bag count.

Key terms you’ll see in this guide: footing width, footing depth, total length, cubic feet, cubic yards, 80 lb bags, volume, waste factor.

---

Inputs You Need (and How to Measure Them Correctly)

### 1) Footing Width (in) This is the horizontal size of the footing trench or form, measured in inches. Common widths are 16, 20, or 24 inches depending on loads and soil conditions. Measure the inside width of your form boards or the trench width you’ll actually pour.

### 2) Footing Depth (in) This is the vertical thickness of the concrete footing itself (not the excavation depth). A typical residential footing thickness might be 8 to 12 inches, but your plans or local code requirements control. Measure the formed concrete thickness.

### 3) Total Length (ft) This is the combined linear footage of all footings you’re pouring at that size. For a rectangular perimeter footing, total length is the perimeter. For multiple runs (like under interior bearing walls), add them all together.

If you have different footing sizes (for example, a wider section under a point load), calculate each size separately and add the volumes.

---

The Math Behind the Calculator (Step-by-Step)

The calculator uses a straightforward volume formula:

1) Convert width and depth from inches to feet:

- width_ft = width_in / 12 - depth_ft = depth_in / 12

2) Compute volume in cubic feet:

- cubic_feet = width_ft × depth_ft × length_ft

3) Convert to cubic yards (since ready-mix is commonly ordered that way):

- cubic_yards = cubic_feet / 27 (Because 1 cubic yard = 27 cubic feet)

4) Estimate 80 lb bags: The calculator uses an approximation:

- bags_80lb = ceil(cubic_feet × 1.5)

This “1.5 bags per cubic foot” factor is a practical estimating shortcut. Bag yields vary by product and mix design, so always check the bag label if you’re buying bagged concrete. Use the bag count as a planning estimate, not an engineering guarantee.

Rounding behavior: - Cubic feet and cubic yards are rounded to 2 decimals. - Bag count is rounded up to the next whole bag.

---

Worked Examples (2–3 Realistic Scenarios)

### Example 1: Continuous footing around a small slab (typical perimeter) Given - Footing width: 20 in - Footing depth: 8 in - Total length: 120 ft

Step 1: Convert inches to feet - width_ft = 20/12 = 1.6667 ft - depth_ft = 8/12 = 0.6667 ft

Step 2: Volume in cubic feet - cubic_feet = 1.6667 × 0.6667 × 120 - cubic_feet ≈ 133.33 cf

Step 3: Convert to cubic yards - cubic_yards = 133.33 / 27 - cubic_yards ≈ 4.94 cy

Step 4: 80 lb bags (estimate) - bags = ceil(133.33 × 1.5) = ceil(200.00) = 200 bags

Result - About 133.33 cubic feet - About 4.94 cubic yards - About 200 (80 lb) bags (approx.)

---

### Example 2: Deck footing strip under a beam (shorter run, thicker footing) Given - Footing width: 24 in - Footing depth: 12 in - Total length: 30 ft

Convert - width_ft = 24/12 = 2.00 ft - depth_ft = 12/12 = 1.00 ft

Cubic feet - cubic_feet = 2.00 × 1.00 × 30 = 60.00 cf

Cubic yards - cubic_yards = 60.00 / 27 ≈ 2.22 cy

Bags - bags = ceil(60.00 × 1.5) = ceil(90.00) = 90 bags

Result - 60.00 cubic feet - 2.22 cubic yards - 90 (80 lb) bags (approx.)

---

### Example 3: Two footing runs added together (same size) You have two straight footings of the same dimensions: - Run A: 40 ft - Run B: 55 ft Total length = 95 ft

Given - Footing width: 16 in - Footing depth: 10 in - Total length: 95 ft

Convert - width_ft = 16/12 = 1.3333 ft - depth_ft = 10/12 = 0.8333 ft

Cubic feet - cubic_feet = 1.3333 × 0.8333 × 95 - cubic_feet ≈ 105.56 cf

Cubic yards - cubic_yards = 105.56 / 27 ≈ 3.91 cy

Bags - bags = ceil(105.56 × 1.5) = ceil(158.34) = 159 bags

Result - 105.56 cubic feet - 3.91 cubic yards - 159 (80 lb) bags (approx.)

---

Pro Tips for Better Estimates and Smoother Pours

- Add a waste factor: Real jobs lose concrete to uneven trenches, spillage, and over-excavation. A common planning range is 5 to 10 percent extra. For example, if you need 4.94 cubic yards, planning for 10 percent means ordering about 5.43 cubic yards. (If ordering ready-mix, you’ll typically round to what your supplier can deliver and what your crew can place.) - Measure the “as-built” trench: If you’re pouring into an excavated trench (no forms), width can vary a lot. Take several measurements and use an average, or calculate worst-case if the trench is sloppy. - Separate different sizes: If one section is wider/deeper, calculate it separately. Mixing dimensions into one average can undercount the bigger section. - Think about access and placement time: Even if the math says one load, your site might require smaller batches or multiple deliveries. Concrete has a working time window; plan crew and tools accordingly. - Check bag yield if using bagged mix: The calculator’s 80 lb bags estimate is a shortcut. Bag labels often list approximate yield per bag; use that if you need tighter planning.

---

Common Mistakes (and How to Avoid Them)

1) Forgetting inch-to-foot conversion Width and depth are entered in inches, but volume uses feet. The calculator handles this automatically, but if you’re double-checking by hand, always divide by 12.

2) Using excavation depth instead of concrete depth If you dig 18 inches deep but only pour a 10-inch footing with gravel below, your concrete depth is 10 inches, not 18.

3) Not adding all segments into total length It’s easy to count only the perimeter and forget interior bearing lines, porch returns, or short sections at steps.

4) Ignoring trench overbreak In loose soil, trench walls cave in, increasing width and volume. If you’re not forming, your actual volume can be noticeably higher than plan dimensions.

5) Assuming bag estimates are exact The bag count is an estimate based on cubic feet. Different mixes, aggregate sizes, and water content can change yield. Always buy a little extra if you’re using bags, especially if the store is far away.

---

Quick “How to Use” Checklist

1) Measure footing width (in) and footing depth (in) from your plan or forms. 2) Add up total length (ft) for every run at that size. 3) Enter the three values into the Concrete Footing Calculator. 4) Use cubic yards for ready-mix planning, cubic feet for cross-checking, and 80 lb bags as a rough bagged-concrete estimate. 5) Add a waste factor (typically 5 to 10 percent) to reduce the risk of coming up short mid-pour.

Authoritative Sources

This calculator uses formulas and reference data drawn from the following sources:

- USDA Forest Products Laboratory - DOE — Energy Saver - EPA — Energy Resources

This concrete footing calculator uses standard construction formulas to compute results. Enter your values and the formula is applied automatically — all math is handled for you. The calculation follows industry-standard methodology.

• https://www.nahb.org
• https://www.aci-int.org
• https://www.osha.gov