Columns, piers, posts and tube forms (sonotubes), pick the shape, enter the size and height, and get the concrete per column and for the whole row.
Column Calculator
Total for all columns (with waste)
Tube forms are sold by their nominal diameter and can run slightly under. If your piers carry a deck or structure, rounding bags up costs little and protects the pour.
A concrete column calculator finds the volume of round and square columns, piers and tube forms. Round columns use π × radius² × height; square columns use side × side × height. Enter the size, height and how many columns you are pouring above to get the volume per column, the total yards and the bag count.
How to Calculate Concrete for Columns and Piers
Round columns, deck piers, fence and gate posts, sonotube footings, are cylinders, and square columns are boxes. Both are simple once you use the right cross-section area, but the round formula is the one people most often get wrong by using the diameter where the radius belongs.
The Column Formulas
SQUARE: Volume = Side × Side × Height
Total = per-column volume × number of columns
Worked Example: Six 12-inch Piers, 4 ft Deep
Radius = 6 in = 0.5 ft. Area = π × 0.5² = 0.785 ft². Per pier = 0.785 × 4 = 3.14 ft³. Six piers = 18.85 ft³ = 0.70 yd³ exact, 0.77 yd³ with 10% waste, about 35 × 80 lb bags, a classic bag job.
Common Tube Sizes and Yields
| Tube diameter | Volume per foot of height | 80 lb bags per foot |
|---|---|---|
| 8 in | 0.35 ft³ | 0.6 bags |
| 10 in | 0.55 ft³ | 0.9 bags |
| 12 in | 0.79 ft³ | 1.3 bags |
| 16 in | 1.40 ft³ | 2.3 bags |
Round vs Square: Which Holds More?
A square column always holds more than a round one of the same nominal size: a 12 in square has an area of 1.00 ft² against the 12 in circle’s 0.785 ft², about 27% more concrete. When you swap a planned square pier for a tube (or the reverse), re-run the numbers instead of reusing the old order quantity.
How to Measure and Calculate: Step by Step
For round work, measure the diameter straight across the centre of the tube or hole, and measure the inside of the form, since tube forms are sold by nominal size and can run slightly under. Holes dug in soil are rarely the auger’s nominal diameter at the top, so measure the dug hole at top and mid-depth and average. Height is measured from the bottom of the hole or flare to the top of the pour, not to the top of a tube you plan to trim later.
- Choose Round or Square at the top of the calculator.
- Enter the diameter (or side) in inches, radius is calculated for you.
- Enter the height or depth of one column in feet.
- Enter the total number of identical columns or piers.
- Use the per-column bag figure to stage bags at each hole before you start mixing.
Worked Example 2: Eight Square Columns, 10 in Side, 3 ft High
Area = (10 ÷ 12)² = 0.694 ft². Per column = 0.694 × 3 = 2.08 ft³. Eight columns = 16.7 ft³ = 0.62 yd³ exact, 0.68 yd³ with 10% waste, about 31 × 80 lb bags. Compare a 10-inch round tube at the same height: 0.545 ft² area and 1.64 ft³ per column. The square holds about 27% more, exactly the π/4 relationship explained below.
Common Quantities at a Glance: Cheat Sheet
| Job / quantity | Concrete needed |
|---|---|
| 8 in tube, per ft of height | 0.35 ft³ · 0.6 bags |
| 12 in tube, per ft of height | 0.79 ft³ · 1.3 bags |
| 12 in tube, 4 ft deep | 3.14 ft³ · 6 bags with waste |
| 16 in tube, per ft of height | 1.40 ft³ · 2.3 bags |
| 12 in square vs 12 in round | square holds ~27% more concrete |
Practical Tips From the Pour
Stage the bags at each hole before the first mix: piers are usually spread across a site, and carrying bags between holes while concrete waits in the mixer wastes its working life. Pour each tube in lifts and rod or vibrate gently to remove air pockets around the post hardware. Set posts, brackets or anchor bolts immediately while the concrete is workable, then check alignment once more before it stiffens, a pier that is 1 inch off at the base is a problem no bracket fully hides.
Common Mistakes to Avoid
- Using the diameter as the radius, which quadruples the calculated volume.
- Measuring the outside of a tube form instead of the inside bore.
- Forgetting the flared footing at the base of deck piers in the total.
- Pricing one column and forgetting to multiply by the full count.
- Swapping square piers for tubes without re-running the 27% volume difference.
What It Costs and How to Order Smart
Column work is bag territory: the six-pier example on this page totals 0.77 yd³, which in bags is about $210 at $6 per 80 lb bag, against a ready-mix ticket carrying delivery and almost certainly a short-load fee. The economics flip only when the count or size grows, a long row of big piers can pass 2 yd³, where a truck starts to compete. Stage the decision on the total on this page, not the per-column number, because fees are charged per delivery, not per pier.
Who Uses This Calculator?
Builders laying out deck and porch foundations use this calculator pier by pier, because post layouts change late and the bag count has to change with them. Contractors use the per-column figure to stage material at each hole, the workflow detail that decides whether a pier day runs smoothly. Engineers checking pier volumes for a permit set compare against the same πr²h arithmetic. Architects detailing landscape structures, pergolas, sign posts, fence lines, use it for the footing schedule. Homeowners building a deck get the answer to the classic hardware-store question: how many bags per hole, times how many holes?
Column Calculator FAQs
How many bags for a 12-inch sonotube 4 ft deep?
One tube needs about 3.14 ft³, which is 5–6 × 80 lb bags. Multiply by the number of tubes and add the waste allowance.
Should the pier be wider at the bottom?
Many deck piers use a flared footing at the base for extra bearing and frost resistance. Calculate the flare separately as a short, wider cylinder and add it to the tube volume.
Does this work for fence posts?
Yes, treat each post hole as a short round column. A 10 in diameter hole, 3 ft deep, is about 1.6 ft³ or 3 × 80 lb bags per post.
How many bags do I need for a 12-inch sonotube 4 ft deep?
The tube’s volume is π × 0.5² × 4 = 3.14 cubic feet. Divided by an 80 lb bag’s 0.60 ft³ yield, that is 5.2 bags, so buy 6 per tube to cover rounding and waste. In 60 lb bags (0.45 ft³ yield) it is 7 bags per tube. Multiply by your tube count and stage that many bags at each hole before mixing starts.
How much concrete does a 10-inch tube hold per foot?
A 10-inch tube has a radius of 5 inches (0.417 ft), giving an area of π × 0.417² = 0.545 ft², so each foot of height holds 0.545 cubic feet, just under one 80 lb bag (0.9 bags) per foot. A 4 ft pier therefore needs about 2.18 ft³, or 4 bags. The full tube-size table on this page lists 8, 10, 12 and 16 inch yields for quick checks.
Why does a square column hold more than a round one?
Because a circle inscribed in a square covers only π/4, about 78.5%, of the square’s area. At the same nominal 12-inch size, the square’s area is 1.00 ft² and the circle’s is 0.785 ft², so the square column takes roughly 27% more concrete at any height. It also carries load differently, which is an engineering question; for ordering, just never reuse a round figure for a square form.
How deep should deck piers be?
The base of the pier must reach below your local frost line and rest on undisturbed soil, the depth is a code figure for your area, not a preference. The tube above that depth is simply forming. If your holes end up deeper than planned after digging, re-run this calculator with the real depth: an extra 6 inches on six 12-inch piers adds about 4.7 ft³, roughly 8 more bags.
Should I add a footing flare at the bottom of a pier?
A flared or belled base spreads the load over more soil and resists frost uplift, and many deck codes expect one. Calculate it as a separate short cylinder: for example a 16-inch-diameter, 8-inch-deep flare under a 12-inch tube adds π × 0.667² × 0.667 ≈ 0.93 ft³ per pier. Add the flare total to the tube total in this calculator by increasing the bag count, or run the flare through as extra columns of the wider size.
How many bags for fence post holes?
Treat each hole as a short round column. A typical 10-inch-diameter hole, 3 ft deep, holds 1.64 ft³, which is 3 × 80 lb bags per post with waste. Twenty posts therefore need about 55–60 bags, nearly a pallet. Fast-setting post mix follows the same yield maths; the difference is working time, not volume, so the counts from this calculator still apply.
Related Calculators
Sources used for standards and conversions: National Ready Mixed Concrete Association (NRMCA) guidance on ordering and yield; American Concrete Institute (ACI) practice for mix and placement; Portland Cement Association (PCA) design and control guidance; US bagged-concrete manufacturers’ printed yields (80 lb bag = 0.60 ft³). Your local building code sets frost depths, footing sizes and step dimensions. Wherever the code differs from the general figures on this page, follow the code.
Disclaimer: Results are estimates for planning and ordering. Before the pour, confirm the quantities with your ready-mix supplier. Structural work (footings, walls, columns and anything load-bearing) should be checked by a licensed engineer.