Concrete Column Calculator

Cylinders and prisms, exactly

The tool computes the volume of one column from the shape you pick, multiplies by the count, and hands the total to the same bag engine every concrete tool on this site uses. The worked example under the form: one 10 in tube form has a radius of 5 in, which the engine converts to 0.4167 ft by dividing by 12, so 8 ft of it holds π × 0.4167² × 8 = 4.36 cu ft. 4 forms total 17.45 cu ft — 0.65 cu yd once divided by 27, or 0.49 m³. With the 10% pour allowance that is 19.2 cu ft to mix; divided by the manufacturer's 0.6 cu ft per 80 lb bag it is 31.9977 bags, rounded up to 32 bags, with only 0.0014 cu ft bought beyond the allowance because the division landed just under a whole bag. The other sizes come out at 64 × 40 lb, 52 × 50 lb, 43 × 60 lb. Every one of those figures was computed when this page was built by the engine wired to the form, not typed in.

The units underneath that chain — why a cubic yard holds exactly 27 cubic feet, how cubic meters relate to both, and at what volume a truckload starts to make more sense than bags — are the subject of the guide Cubic feet, cubic yards, and ordering concrete, which runs the same chain for slabs, footings, and columns. Read it if the conversions are new to you. The rest of this page explains what this calculator does with the dimensions you give it, stage by stage, and how to read what comes back.

What the calculator computes, stage by stage

Every estimator on this site follows the five-stage pipeline documented on the methodology page. The column tool's geometry stage has two branches, one per shape; everything after it is shared with the concrete footing calculator and the concrete bag calculator.

1. Geometry. For a round form the diameter in inches is halved and divided by 12 to give the radius in feet, and the volume is π × radius² × height. For a rectangular pier both sides are divided by 12 and the volume is side A × side B × height. Either result is the volume of one column in cubic feet; the count multiplies it. From that one unrounded total the engine also reports cubic yards (divided by 27) and cubic meters (multiplied by 0.028316846592, the international foot cubed). In metric mode the centimeters and meters you type are converted to inches and feet first, so the arithmetic downstream is identical in both systems.
2. Coverage. The total is divided by how much concrete one bag yields. The engine does this for every bag size at once — the four pound sizes, or the two kilogram sizes in metric mode — so each row of the result is a separate answer to "how many if I bought only this size?". The yields are the one sourced input on this page and are listed with their status below.
3. Waste. Before any division is rounded, the engine applies net × (1 + waste ÷ 100) to the volume. The default of 10% is a trade convention recorded in the site's materials data, not a manufacturer figure, and the form lets you replace it. The result is the to mix line: what the pour is expected to consume.
4. Package rounding. Each bag row is ceil(adjusted ÷ yield per bag). This is the only rounding in the chain and it always rounds up, because a count rounded down leaves the last form short. The spare in a row is the concrete those whole bags provide beyond the adjusted volume. When the division lands on a whole number within floating-point noise, the engine treats it as an exact fit rather than buying a phantom bag — the pair of square piers below shows the case.
5. User-priced cost. If you type a price per bag, the cost is that price times the whole-bag count of the size you said you will buy — the rounded figure, not the adjusted one, because the rounded figure is what you pay for. The site never supplies or averages a price; leave the field blank and the line does not appear.

Why each input matters

Shape chooses the formula and nothing else. Round forms use the cylinder, formed piers the rectangular prism; the count, allowance, and bag stages are the same for both.

Diameter, or the two sides, go in as inches and come from your plans — the tool sizes the pour for whatever section you give it and makes no judgment about whether that section suits the load or the code, as the note under the form says. Arithmetically the diameter is the input with the most leverage: the volume goes with its square, so a form twice the diameter holds four times the concrete at the same height. The sides of a rectangular pier each scale the volume linearly.

Height is a straight multiplier entered in decimal feet. A tape reading of 3 ft 6 in is entered as 3.5 — the mixed measurement converted to decimal feet, which is the site's convention for every length before any arithmetic runs; the feet & inches calculator does that conversion for awkward readings. Height is also where the inches-versus-feet slip described below usually happens, because it sits beside a field that does take inches.

Count must be a whole number of at least one. It multiplies the volume before the allowance and the round-up, which matters: the round-up to a whole bag then happens once for the whole order instead of once per column. The deck piers below show the difference between pooling eight piers and rounding each on its own.

Waste multiplies the volume, so it moves the to-mix figure proportionally and the bag counts in whole-bag steps. Small changes often change nothing, because the round-up absorbs them; then a further change tips a row by a bag. The same four forms at three allowances below show that step behaviour.

Bag size and price per bag touch only the last line. The rows always show every size; the size selector says which one the cost line should multiply, and the price never feeds back into the quantity, so you can add or change it after the fact without disturbing the counts.

The units toggle changes what the fields expect and how the result is presented, not what is computed. In metric mode, centimeters are divided by 2.54 and meters by 0.3048 — both exact definitions — before the engine sees them, the result panel leads with cubic meters, and the bag rows use the kilogram yields instead of the pound ones.

The figures this page relies on

• Concrete mix yield per pound bag — 40 lb → 0.3 cu ft; 50 lb → 0.375 cu ft; 60 lb → 0.45 cu ft; 80 lb → 0.6 cu ft. A manufacturer figure from the QUIKRETE Concrete Mix (Product No. 1101) product data sheet, "YIELD" section, which states the yields as approximate. The bag calculator's guide, concrete bag yields by size, covers each size.
• Concrete mix yield per kilogram bag — 25 kg → 0.43 cu ft; 30 kg → 0.52 cu ft. A manufacturer figure published by Target Products Ltd., the licensed manufacturer of QUIKRETE in western Canada. These are the manufacturer's own yields for its metric bags, not the pound figures scaled by weight.
• Default waste, 10% — a common trade convention recorded in the site's materials data, where it is labelled a convention because no primary source publishes a universal figure; it is the common allowance for spillage and an uneven subgrade. Adjustable in the form.
• π — mathematics; the engine uses the closest double-precision value, the one non-rational constant in the geometry module.
• Inches per foot, 12, and cubic feet per cubic yard, 27 — definitions, exact.
• Cubic meters per cubic foot, 0.028316846592 — the international foot of 0.3048 m, cubed; exact. Used for the m³ line in both modes.
• Centimeters per inch, 2.54, and meters per foot, 0.3048 — exact definitions, used only to convert metric entries into the engine's inches and feet.
• Column dimensions — none. The page holds no default diameter, section, or height and suggests none; those three figures are entirely yours.

Worked examples

Every figure in this table was produced at build time by the same engine that runs the form above. The inputs are chosen to show different behaviours of the pipeline: a pooled count, an exact bag fit, a pour large enough to compare with a truck, a priced order, a metric entry, and the allowance set to zero. The bags column shows the 80 lb row, or the 30 kg row for the metric entry; the result panel lists every size.

The deck piers, pooled and one at a time. Suppose the plans call for 8 tube forms of 8 in × 4 ft. Each is π × (8 ÷ 2 ÷ 12)² × 4 = 1.4 cu ft, and the 8 together are 11.17 cu ft — 0.41 cu yd. With 10% that is 12.29 cu ft to mix and 21 bags of 80 lb mix. Run one pier alone and it needs 1.54 cu ft, which rounds up to 3 bags; multiply that by hand and you would buy 24, 3 more than the pooled order, because each pier's fraction of a bag was rounded up separately. Enter the count and let the engine round once.

The pair of square piers — an exact fit. Two formed piers of 12 × 12 in by 3 ft have a section of exactly one square foot, so each is 3 cu ft and the pair 6 cu ft. The 10% allowance makes that 6.6 cu ft, which divides into 80 lb bags exactly 11 times and into 40 lb bags exactly 22 times, with 0 cu ft spare on both rows; the 60 lb row does not divide evenly and rounds up to 15 with 0.15 cu ft spare. Binary floating point leaves the adjusted volume a hair above the exact figure — the engine holds it as 6.6000000000000005 — and a naive ceiling on that value ÷ 0.6 would buy 12 bags. The engine treats a quantity within one part in a billion of a bag boundary as an exact fit, so the count is 11.

The porch columns — hundreds of bags. Twelve round forms of 12 in × 10 ft are 7.85 cu ft each and 94.25 cu ft in all: 3.49 cu yd, or 2.67 m³. After the allowance there is 103.67 cu ft to mix — 173 bags of 80 lb mix, or 346 of 40 lb. The arithmetic is no different from the deck piers; what changes is which line of the result you act on. At this scale the cubic-yard figure is the one a ready-mix plant would quote against, and the bag rows are there for comparison. The guide linked above has a section on where bags stop being the practical route.

The priced piers, measured in feet and inches. Six 10 in forms are 3 ft 6 in tall, so the height is entered as 3.5 ft. Each holds 1.91 cu ft, the six 11.45 cu ft, and with 10% there is 12.6 cu ft to mix: 21 bags of 80 lb mix. With $10.00 typed into the price field and the 80 lb size selected, the cost line reads $210.00 — the whole-bag count times the price, nothing else. That price is a stand-in chosen only to show the arithmetic; the site holds no concrete prices, and the cost is computed on the rounded count because the rounded count is what you buy.

The metric forms. In metric mode, four 25 cm × 2.4 m forms reach the engine as 9.8425 in × 7.874 ft. Each is 4.16 cu ft, which the panel presents as 0.1178 m³, and the four are 0.47 m³ — 16.64 cu ft, 0.62 cu yd. Because the conversions are exact, computing the cylinder directly in meters, π × 0.125² × 2.4 × 4, gives 0.4712 m³, and this page's build checks that the two agree. With 10% the to-mix volume is 0.5184 m³ (18.31 cu ft), and against the manufacturer's metric yields that is 36 bags of 30 kg or 43 bags of 25 kg mix.

The same four forms at three allowances. The worked example's 17.45 cu ft is 29.0888 bags of 80 lb mix before any allowance, so with the waste field set to 0 the order is 30 bags. The default 10% lifts the quantity to 31.9977 and the order to 32; 15% makes it 20.07 cu ft and 34 bags. The percentage moves the quantity smoothly and the count in steps — and setting it to zero removes the allowance, not the round-up, which is why the no-waste row of the table still shows a spare.

How to read the result

The headline line is the volume to pour: Pour N cu yd of concrete, or cubic meters in metric mode. Beneath it, a line labelled with the count, shape, and per-column volume shows the total in cubic feet with its cubic-yard and cubic-meter equivalents, so a typo in a dimension is visible before it is padded. With N% extra shows the to-mix volume with the percentage that produced it. Then one line per bag size — four in imperial mode, two in metric — each giving the whole bags that size alone would take. Read the per-column figure to check your geometry, the to-mix line to check your allowance, and the row for the size you intend to carry to place the order. A cost line appears only when a price is entered, and only for the size you selected. The copy button places a short plain-text summary of the result on the clipboard.

Common misreadings

Height in inches. The height field takes feet; the diameter and sides take inches. Enter the worked example's 8 ft forms as 96 — their height in inches — and the engine sees 96 ft: 209.44 cu ft, 7.76 cu yd, and 384 bags of 80 lb mix instead of 32 — exactly 12 times the correct volume, because there are 12 inches to the foot. A bag count that looks wildly high for a few forms usually means the height field.

Diameter in feet. The slip runs the other way on the diameter. Type the porch columns' 12 in diameter as 1, a foot, and the engine sees a 1 in form: the twelve columns collapse to 0.65 cu ft and 2 bags of 80 lb mix instead of 173. Because the volume goes with the square of the diameter, the error is a factor of 144 — 12 squared — not 12. A count that looks absurdly small for the number of columns means the diameter or side field.

Metric figures in the imperial fields. The metric forms above are 25 cm × 2.4 m. Type those same numbers with the toggle still on Imperial and the engine sees 25 in × 2.4 ft forms: 32.72 cu ft against the correct 16.64. The error is not a simple factor, because the diameter grew while the height shrank; if the result bears no resemblance to the forms you are filling, check the toggle.

Adding the bag rows together. Each row answers "how many if I bought only this size?", so the rows are alternatives — adding them would buy the job four times over. The tool does not plan a mix of sizes; if you intend to combine, say, 80 lb bags with a few smaller ones to trim the spare, that split is your own arithmetic on the to-mix volume.

Rounding per column by hand. Running a single column and multiplying its bag count by the number of columns repeats the round-up once per column, as the deck piers show. Enter the count instead; the engine pools the volume and rounds once.

Treating the spare as an error. The spare in a bag row is the round-up rule doing its job: the whole bags provide a little more than the adjusted volume, and the panel reports the difference as information, not as a suggestion to buy fewer. A large spare on a small pour simply means the volume landed just past a whole-bag boundary.

Where the model stops

The engine models a uniform section from top to bottom — one cylinder or one prism per column — and assumes that shape is what gets filled. It does not model a flare, bell, or taper, and it does not include the pad a pier bears on: a rectangular pad is the concrete footing calculator's job, and a proprietary flared form has a volume its manufacturer publishes, to be added as its own quantity. Nor does it model formwork, reinforcement layout, subgrade preparation, delivery minimums, or moisture and temperature effects; those are outside every estimator on the site, and the waste allowance is the single flat percentage you are given to fold real-world variation into.

This tool estimates material quantities only. It does not determine the diameter, section, or height a column needs, its reinforcement, the footing beneath it, structural adequacy, code compliance, or permit requirements — those come from your approved plans, a licensed engineer, or your building department, and the dimensions you enter should already carry that review. The tool will compute a volume for any dimensions you give it, which is why the plans, not the calculator, are the authority. The methodology page lists the simplifications every estimator shares.

Above and below the column

The pad the pier bears on is the concrete footing calculator's job; fence and deck posts set directly in holes belong to the post hole calculator. For slabs there is the concrete slab calculator, the concrete bag calculator compares every bag size for any volume you bring it, and the steel your plan calls for is counted by the rebar calculator. The feet & inches calculator turns tape readings into decimal feet, and the construction unit converter moves any figure between cubic feet, cubic yards, and cubic meters. For the units and the reasoning behind the chain, see cubic feet, cubic yards, and ordering concrete and concrete bag yields by size, or browse all guides.

Frequently asked questions

Dimensions never leave your browser. Sources and the estimating pipeline are documented on the methodology page.