Concrete Column Calculator

What This Calculator Covers

Volume for round (cylindrical) concrete columns and pillars — the type you'd pour in a Sonotube form for porch columns, structural piers, deck supports, or decorative pillars. The math is straightforward: volume equals π × radius² × height. A 12-inch diameter column 8 feet tall takes 0.23 cubic yards (about 12 bags of 80-lb mix) per column.

For square or rectangular columns, use the slab volume formula (length × width × height) — or simpler, the concrete slab calculator with the column dimensions plugged in.

Column Volume by Size

The table below shows volume per column at 8 feet tall — a common porch and deck-pier height. For shorter or taller columns, scale linearly with height.

Multiply by your column count to get total project volume. Four 12-inch × 8-ft columns = 0.92 cubic yards (about 46 bags), which is right at the bagged-vs-ready-mix breakpoint.

Sonotube Form Sizing

Sonotube and equivalent fiber form tubes are stocked in standard diameters: 8, 10, 12, 14, 16, 18, and 24 inches. Specialty sizes (20, 30, 36 in) are available by special order.

• Buy 6–12 inches longer than the finished column height. You'll cut the top to grade after positioning, and the extra length gives you something to brace against during the pour.
• Brace tubes against blow-out. A 12-inch tube full of fresh concrete has roughly 50 lbs of lateral pressure per linear foot at the bottom — enough to bow or burst an unbraced tube, especially in tall columns. Use 2x4 strongbacks on opposite sides plus diagonal kickers.
• Decorative options. Square, octagonal, and fluted form tubes exist for porch columns where the round look isn't wanted.

Rebar Cage Standards

Structural columns need vertical rebar plus horizontal ties:

• Vertical bars — 4 pieces of #4 (1/2-inch) or #5 (5/8-inch) rebar, evenly spaced around the perimeter, set on a chair to keep them off the bottom.
• Horizontal ties — #3 (3/8-inch) bar bent into a closed loop, spaced every 12–18 inches along the column height. Ties keep the vertical bars in place during the pour and provide shear resistance.
• Cover — 1.5 inches of concrete between any rebar and the form face. Use plastic chairs or concrete dobies to maintain spacing.
• Engineer review. Anything carrying real structural load — a porch beam, a deck girder, an outdoor kitchen roof — should be sized by a structural engineer. The rebar pattern above is a typical starting point, not a substitute for design.

Pour Technique

• One continuous pour per column. Stopping mid-column creates a cold joint that becomes a structural weak point. Mix or order enough to finish each tube without breaks.
• Vibrate as you go. Use a small concrete vibrator or rod aggressively with a length of #4 rebar to release trapped air and consolidate the concrete around the rebar cage. Voids in a column dramatically reduce its load capacity.
• Tap the form sides. A rubber mallet on the outside of the tube helps small voids migrate to the surface as you fill.
• Strip the tube after the column has cured at least 48 hours. Sonotube peels off in strips from a vertical razor cut.

For sonotube-only volume (the void inside the form, with no rebar adjustment), the sonotube calculator handles standard tube sizes by length.