Concrete cylinder volume calculator

Round piers and columns, one or a dozen at a time. Cubic yards, cubic feet, or cubic metres, with the bag count and — for drilled piers — the belled base that a plain cylinder formula leaves out.

01 How the math works

A straight pier or column is a cylinder. Its volume is π times the radius squared times the height, with the radius and height in the same units. Diameter is what you measure, so halve it first. A 12 in (305 mm) pier holds 0.785 ft³ for every foot of depth (0.0222 m³ per 0.3048 m); that per-foot figure is the number to keep in your head when the depth changes.

A belled pier adds a cone-shaped base — a frustum — that widens from the shaft radius r to the bell radius R over the bell height h. The frustum identity above is standard geometry, not a rule of thumb. The calculator adds the shaft cylinder and the bell frustum, multiplies by the number of piers, then applies the waste factor and rounds up to what a supplier will actually sell.

02 Worked example

Take a deck on four round piers: 12 in diameter, poured 36 in deep (305 mm × 914 mm). Each pier is 2.36 ft³ (0.067 m³). Four of them come to 9.4 ft³, which is 0.35 yd³ (0.265 m³).

At the standard 0.60 ft³ yield per 80 lb bag, that raw volume is 16 bags before waste. Add the 10% waste factor and it rounds to 18 bags for the set. Under a cubic yard total, this is a bagged pour — no reason to call for a truck.

Now give each pier a belled base: keep the 12 in shaft 36 in deep, then flare it to a 24 in bell over the bottom 12 in. The bell frustum adds 1.83 ft³ to each pier, so one belled pier holds 4.19 ft³ instead of 2.36 ft³ — the base nearly doubles the concrete. A cylinder calculator would have you order for the shaft alone and come up short at the bottom of every hole.

03 When this calculator is wrong

The volume is exact geometry. What it can't know is the hole you actually dig and the depth code requires. Both move the number that matters — what to order.

• The depth of a below-grade pier is not yours to pick. An exterior footing has to reach the greater of 12 in (305 mm) below undisturbed grade and the local frost line (IRC R403.1.4 and R403.1.4.1). Frost lines run from 0 in in the frost-free south to more than 60 in in the coldest northern states. Guess a 24 in pier where code wants 48 in and a 12 in pier is short by 1.57 ft³ — two extra feet at 0.785 ft³ each — before it heaves out of the ground the first winter.
• A cast-against-earth pier takes more than the nominal diameter. A fiber tube form gives you exactly the cylinder above. An augered or hand-dug hole runs oversize and the walls slough, so the concrete placed exceeds the design diameter. Order to the hole you actually dig, not the number on the drawing — the manufacturer's own post-setting guidance notes that wider holes simply hold more concrete.
• The belled base is engineered, not estimated. The bell diameter and height come from a soils report or an engineer sizing the bearing area — this calculator prices the concrete for the bell you enter, it doesn't tell you how big the bell should be. If you're guessing bell dimensions, that's the number to go verify first.
• Bag yield is a spec-sheet figure. The 0.60 ft³ per 80 lb bag assumes ideal mixing water; real yield runs 0.55–0.60 ft³. On a tight bag count, buy one extra rather than trust the round number.

04 What to do with the result

Under about 1 yd³ (0.75 m³) total — which most residential pier sets are — pour it from bags and buy the bag count above. Above that, ready-mix wins on cost per yard and on time, and it's ordered in half-yard steps, so use the rounded order figure. Either way, confirm your pour depth against the local frost requirement before you buy anything: the authority having jurisdiction sets that number, and it's the one input that turns a correct volume into the wrong order.

05 Common questions

How do you calculate the volume of a concrete cylinder?

Volume is π × (diameter ÷ 2)² × height, everything in the same units. For a 12 in pier that's 0.785 ft³ per foot of depth. Divide cubic feet by 27 for cubic yards.

How many 80 lb bags of concrete for a pier?

Divide the pier volume by the 0.60 ft³ yield of an 80 lb bag and round up. A single 12 in × 36 in pier at 2.36 ft³ is 4 bags; four of them come to 16 bags before waste, 18 with 10% added.

How deep should a concrete pier be?

The bottom must reach the greater of 12 in (305 mm) below undisturbed grade and the local frost line (IRC R403.1.4/.1). In a frost-prone area the frost line governs and is often deeper than any rule of thumb. Confirm the figure with your building department.

What is a belled pier?

A drilled pier with a flared, cone-shaped base wider than the shaft, used to spread the load over more soil. The extra concrete is a frustum below the shaft — this calculator's belled mode adds it. The bell size itself is set by an engineer, not a rule of thumb.

Does the calculator subtract for a steel column or rebar?

No. Rebar displaces under 0.5% of the volume and is ignored. If a pier encases a steel column or pipe, subtract that object's own volume from the result — the same way a fence-post hole is the hole less the post below grade.