Concrete Bell Pier Calculator

1. Enter the shaft diameter in inches and the shaft depth in feet, down to where the bell starts.
2. Enter the bell diameter and bell height in inches, and any vertical toe below the flare.
3. Set the number of piers and a waste allowance of 10 percent or more.
4. Pick a bag size for a bag count, then read the cubic yards to order.

Here is what each result line means:

A concrete bell pier calculator works out the concrete in a belled pier, also called a belled caisson or an underreamed drilled shaft. The hole is drilled as a straight shaft, then widened at the bottom into a bell that spreads the load over more soil.

Because the bell is a cone that flares outward, a plain cylinder formula misses much of the concrete. The tool adds the shaft, the bell as a cone frustum and a short toe at full bell width, then converts to cubic yards and bags. To get the mix right use the concrete mix calculator, and for the binder the cement calculator.

A belled pier is three simple solids stacked. The shaft is a cylinder. The bell is a frustum, a cone with its tip cut off, running from the shaft radius at the top to the bell radius at the bottom. The toe is a short cylinder at the full bell radius.

1. Halve the shaft and bell diameters for the radii and convert everything to feet.
2. Find the shaft as pi times the shaft radius squared times the shaft depth.
3. Find the bell as pi times the bell height over 3, times the bell radius squared plus both radii multiplied plus the shaft radius squared.
4. Add the toe as pi times the bell radius squared times the toe height, multiply by the number of piers and add waste.

For the placed weight of the piers, the concrete weight calculator gives the tonnage.

Suppose a pier has a 12 inch shaft 8 feet deep, a 30 inch bell 18 inches high and a 6 inch toe, with 10 percent waste.

Calculation: the shaft radius is 0.5 ft and the bell radius 1.25 ft. The shaft is pi x 0.25 x 8 = 6.28 cu ft. The bell is pi x 1.5 / 3 x (1.5625 + 0.625 + 0.25) = 3.83 cu ft. The toe is pi x 1.5625 x 0.5 = 2.45 cu ft. Together that is 12.57 cu ft, or 13.82 cu ft with waste, about 0.51 cu yd.

The lesson: the bell and toe hold half the concrete in this pier, so treating it as a plain shaft would order half of what you need.

Bell Diameter

The bell radius is squared in both the bell and toe, so a wider bell adds concrete fast. Going from a 30 to a 36 inch bell adds far more than six inches suggests.

Shaft Diameter and Depth

The shaft is a long cylinder, so depth adds concrete steadily and diameter adds it as a square.

Bell Height and Angle

A taller bell at the same width has a gentler flare and more volume. The bell angle is set by the drilling tool and the soil.

Oversized Holes

Augers wobble and soil caves, so drilled holes often run larger than the nominal size. A generous waste allowance covers it.

The bell is what separates the two, and it changes both the concrete and the load the pier can spread.

For a straight pier or post you can also use the column calculator in the related tools; for the delivered price, the concrete cost calculator prices the pour by the yard.

Ordering Concrete for Drilled Piers

Total all the piers in cubic yards with waste before you order, so the truck can fill every hole in one visit.

Comparing Bell Sizes

See how much concrete a wider or taller bell adds, which helps when the engineer gives a range.

Checking an Over-pour

Compare the planned volume with what the holes actually took to spot oversized or caved holes.

1. Treating the Pier as a Plain Cylinder

Ignoring the bell can leave out half the concrete. Always enter the bell diameter and height.

2. Using Radius Instead of Diameter

The fields ask for diameters; the tool halves them. Entering radii quarters the volume.

3. Counting the Bell Height in the Shaft Depth

The shaft depth stops where the flare begins. Counting the bell twice over-orders.

4. Skimping on Waste

Drilled holes almost always run a little large. Too small an allowance leaves the last hole short.

5. Treating the Estimate as a Design

Pier size, depth and bell come from a geotechnical report and an engineer, not from this tool.

The geometry is exact for a smooth shaft, frustum and toe; real holes are rougher.

Add three parts: the shaft as pi times the shaft radius squared times its depth, the bell as a cone frustum, pi times the bell height over 3 times (R squared plus R r plus r squared), and any toe at full bell width. A 12 inch shaft 8 feet deep on a 30 inch bell is about 12.6 cubic feet.

It is a drilled pier whose bottom is widened into a bell, so the load spreads over a larger area of soil. It is also called an underreamed pier or a belled drilled shaft.

It depends on the size, but it is often a large share. In a 12 inch pier 8 feet deep with a 30 inch bell and a 6 inch toe, the bell and toe hold about half the concrete. The tool shows the shaft and bell volumes separately.

The toe is the short vertical section at full bell diameter at the very bottom, below the flared cone. Many bells have a few inches of toe; enter 0 if yours ends at the flare.

Yes. Set the bell height to 0 and the toe to 0, or set the bell diameter equal to the shaft, and it calculates a plain cylinder.

At least 10 percent, and more in loose or caving soil. Drilled holes often run larger than the nominal size, and running short on the last pier is costly.

A 12 inch pier 8 feet deep with a 30 inch bell is about 24 eighty-pound bags with waste. Most pier jobs are poured from a truck, but the bag count helps for a single small pier.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• American Concrete Institute (ACI; reinforced concrete and foundation standards).
• American Cement Association (American Cement Association; concrete foundations basics).