Bigfoot Concrete Calculator: How It Works
The calculator for this page is switched off while it is checked. The formula is below, and the worked example in the article shows how to use it by hand.
The Bigfoot concrete calculator estimates the volume of concrete needed for a Bigfoot-style footing system, which uses a flared, bell-shaped base to spread the load of a deck or post. You enter the post size, footing depth, and bell diameter, and it returns the required concrete volume in cubic feet or cubic yards, plus the number of bags you’ll need. This page explains the formula, how to use it, and what to watch out for when ordering concrete.
Formula
The Bigfoot footing is essentially a cone with a flared base. The volume of a cone is given by:
V = (1/3) × π × r² × h
Where:
- V = volume of concrete (cubic feet)
- π ≈ 3.14159
- r = radius of the bell (half the bell diameter) in feet
- h = depth of the footing in feet
In the calculator, you enter the bell diameter in inches and the depth in inches, so the formula converts them to feet by dividing by 12. The post width and length are not used in the volume calculation because the bell is the widest part; the post simply sits on top. The calculator assumes a perfectly smooth cone, which is a good approximation for most Bigfoot forms.
Worked Example
Using the default values: post width = 6 in, post length = 6 in, bell diameter = 20 in, footing depth = 36 in.
- Convert bell diameter to feet: 20 in ÷ 12 = 1.6667 ft
- Radius = 1.6667 / 2 = 0.8333 ft
- Convert depth to feet: 36 in ÷ 12 = 3 ft
- Volume = (1/3) × π × (0.8333)² × 3 = (1/3) × 3.14159 × 0.6944 × 3 = 2.1817 ft³
- Convert to cubic yards: 2.1817 / 27 = 0.0808 yd³
- Number of 60-lb bags (each yields about 0.45 ft³): 2.1817 / 0.45 = 4.85, so round up to 5 bags.
The calculator shows 2.18 ft³, 0.08 yd³, and 5 bags—matching the manual calculation.
When to Use This Calculator
Use this calculator when you are planning a deck, pergola, or other structure that uses Bigfoot-style footings. These footings are popular because they spread the load over a larger area, reducing the risk of settling in softer soils. You would typically use this calculator:
- To estimate how much concrete to order for a single footing.
- To compare the cost of different bell diameters (a larger bell uses more concrete).
- To plan the number of bags for a DIY project, especially when mixing by hand.
- To double-check a contractor’s estimate or a ready-mix order.
Remember that this calculator gives the volume of the bell only. If you also pour a cylindrical column above the bell, you need to add that volume separately.
How to Read the Results
The first output is the volume in cubic feet, which is the most direct measure. The second output converts that to cubic yards, which is how ready-mix concrete is ordered (e.g., 0.08 yd³). The third output tells you how many 60-lb bags to buy, assuming each bag yields about 0.45 ft³ of concrete. If you use 80-lb bags (yield about 0.6 ft³), divide the cubic feet by 0.6 instead. Always round up to the nearest whole bag, because you cannot order a fraction of a bag, and it is better to have a little extra than to run short.
Common Mistakes and Misconceptions
One common mistake is using the post width as the bell diameter. The bell is much wider—typically 16 to 24 inches—so using the post size will drastically underestimate the concrete. Another mistake is forgetting to convert inches to feet before applying the formula. If you plug in inches directly, you get a volume in cubic inches, which must be divided by 1728 to get cubic feet. Also, some people think the footing is a cylinder, but the flared base makes it a cone, which has one-third the volume of a cylinder of the same height and base diameter. Finally, the depth is the total height of the bell, not the depth of the hole above the bell. If you dig a deeper hole, the extra volume is not part of the bell.
Limitations of the Formula
The cone formula assumes a perfectly smooth, straight-sided flare. In practice, the bell may be slightly irregular, and some concrete may be lost to spillage or absorption by the soil. Add 10-15% extra to your order to account for waste and unevenness. Also, the formula does not account for the post hole above the bell, which is usually filled with gravel or concrete. If you are using a Bigfoot form, the manufacturer provides a specific volume for each size; check their documentation for exact numbers, as the form may have a slightly different shape than a pure cone.
Frequently Asked Questions
How many 60-lb bags do I need for one Bigfoot footing?
For a typical 20-inch bell with a 36-inch depth, you need about 5 bags of 60-lb concrete mix. That is because the volume is roughly 2.18 cubic feet, and each 60-lb bag yields about 0.45 cubic feet. If you use 80-lb bags, you would need about 4 bags (since 2.18 / 0.6 ≈ 3.63, round up to 4).
Can I use this calculator for other types of footings?
Yes, but only if the footing is cone-shaped. For a cylindrical footing (like a sonotube), use the cylinder volume formula (πr²h) instead. For a rectangular pad, use length × width × depth. This calculator is specifically for the flared bell shape of a Bigfoot footing.
Does the post size affect the concrete volume?
No, the post size does not affect the volume of the bell. The bell is wider than the post, and the post sits on top. However, if you extend the post below the bell, that extra length would add volume, but that is not typical for Bigfoot footings.
Should I add extra concrete for waste?
Yes, it is recommended to add 10-15% extra to account for spillage, uneven excavation, or slight over-digging. For example, if the calculator says 2.18 ft³, you might order 2.5 ft³ (about 6 bags of 60-lb mix). This is especially important if you are mixing by hand and cannot easily get more concrete mid-pour.
How deep should a Bigfoot footing be?
The depth depends on your local building code and frost line. In cold climates, the footing must extend below the frost line to prevent heaving. Typical depths range from 36 to 48 inches. This calculator uses 36 inches as a default, but you should check with your local building department for the required depth in your area.