Concrete Footing Calculator

Exact geometry, then bags

A footing is a rectangular prism, so its volume is length ft × (width in ÷ 12) × (depth in ÷ 12) cubic feet, times the number of identical footings, then ÷ 27 for cubic yards. The worked example: each 20 ft × 16 in × 8 in footing is 20 × 1.3333 × 0.6667 = 17.78 cu ft. Two of them total 35.56 cu ft = 1.32 cu yd, or 1.01 m³. Adding the 10% pour allowance gives 39.11 cu ft to mix — 66 bags of 80 lb mix at the manufacturer's 0.6 cu ft yield, with 0.49 cu ft left in the last bag, or 131 bags of 40 lb mix at 0.3 cu ft each. Every figure above is computed at build time by the same engine wired to the form.

The units underneath that chain — why a cubic yard holds exactly 27 cubic feet, how cubic meters relate to both, and when a volume is better ordered from a truck than mixed from 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 footing tool runs all five, and because bagged mix comes in several package sizes, its coverage and rounding stages run once per bag size rather than once.

1. Geometry. The length is taken in feet; the width and depth are taken in inches and each divided by 12 to turn them into feet. The product of the three is the volume of one footing in cubic feet, and multiplying by the count gives the total. From that one unrounded cubic-feet figure the engine also reports cubic yards (divided by 27) and cubic meters (multiplied by 0.028316846592, the international foot cubed). In metric mode the meters and centimeters you type are converted to feet and inches first, so the arithmetic downstream is identical in both systems.
2. Coverage. The total volume is divided by what one bag yields. The divisors are manufacturer figures: 0.3 cu ft per 40 lb bag, 0.375 cu ft per 50 lb bag, 0.45 cu ft per 60 lb bag, 0.6 cu ft per 80 lb bag, from the QUIKRETE Concrete Mix (Product No. 1101) product data sheet, which states them as approximate; and, when the metric bag table is selected, 0.43 cu ft per 25 kg bag and 0.52 cu ft per 30 kg bag, from Target Products, the licensed QUIKRETE manufacturer in western Canada. This stage runs once for every size in the table, which is why the result lists a bag count for each.
3. Waste. The engine applies volume × (1 + waste ÷ 100) to the total cubic feet before anything is rounded. The default of 10% is a trade convention recorded in the site's materials data — the common allowance for spillage and an uneven subgrade — not a manufacturer figure, and the form lets you replace it. The result is the adjusted volume: what the pour is expected to consume, shown on the result as the amount to mix.
4. Package rounding. For each bag size the engine computes ceil(adjusted ÷ yield per bag). A bag is its own package, so this is a straight round-up to whole bags, done separately for every size; the leftover for a size is the concrete those whole bags yield beyond the adjusted volume. This is the only rounding in the chain and it always rounds up. When the division lands on a whole number within floating-point noise, the engine treats it as an exact fit rather than adding a phantom bag.
5. User-priced cost. If you type a price per bag, the cost is that price times the whole-bag count for the size you selected in the form — the rounded figure, not the adjusted one, because whole bags are what you buy. The other sizes are still listed, but only the selected one is priced. The site never supplies or averages a price; leave the field blank and the line does not appear.

Why each input matters

Length is the long run of the footing and goes in as decimal feet. The volume scales with it directly: doubling the length doubles the bag count before rounding. This form has no separate inches box for length, so a tape reading of 24 ft 6 in is entered as 24.5 — the mixed measurement converted to decimal feet, which is the site's convention for every dimension before any arithmetic runs. The feet & inches calculator does that conversion for awkward readings.

Width and depth are the two inputs to watch. They are small, they are entered in inches while the length is in feet, and they come from your plans rather than from this calculator. The tool computes quantities only: it totals the concrete for whatever section you give it and makes no judgment about the section itself — footing sizing is engineering governed by your building code, as the note under the form says, and the dimensions you enter should already carry that review. Arithmetically each is a straight multiplier, and the two multiply together, which is why a unit slip on either is so costly and why swapping them changes nothing.

Number of footings multiplies the single-footing volume before waste and rounding, and it must be a whole number of at least one; leave it blank and the engine uses one. Because the count is applied to the volume rather than to the bag count, the round-up happens once on the total, not once per footing — eight pads that each need a fraction of a bag are not each charged a whole bag. The examples below show the difference.

Waste multiplies the volume, so it moves the adjusted 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 count by one bag. That step behaviour is why the examples below show the adjusted volume and the bag count side by side. Zero is a legitimate entry when your dimensions already carry a margin.

Bag size you plan to buy selects which row the cost line prices. It changes nothing else: every size in the table is counted and listed whichever one you select. Price per bag touches only that last line and never feeds back into the quantities, so you can add or change it after the fact without disturbing the counts.

The units toggle changes what the fields expect, which bag table is compared, and how the result is presented — not what is computed. In metric mode, meters are divided by 0.3048 and centimeters by 2.54 — both exact definitions — before the engine sees them; the bag table switches to the 25 and 30 kg sizes with their own manufacturer yields; and the result panel leads with cubic meters and reports the amount to mix in litres.

What is sourced, what is yours

The only external constants on this page are the bag yields. Everything else — dimensions, count, waste, price — is yours, and the calculator never suggests a footing size, a mix strength, or a rebar layout: those come from plans and code. Each figure the page relies on, with its status as the site's materials data labels it:

• Yield per lb bag — 0.3 cu ft per 40 lb bag, 0.375 cu ft per 50 lb bag, 0.45 cu ft per 60 lb bag, 0.6 cu ft per 80 lb bag — manufacturer figures from the QUIKRETE Concrete Mix (Product No. 1101) product data sheet, "YIELD" section, which the manufacturer states as approximate; locked by automated tests. Other brands are batched similarly, but check your bag if you buy a different product.
• Yield per kg bag — 0.43 cu ft per 25 kg bag and 0.52 cu ft per 30 kg bag — manufacturer figures from Target Products Ltd., the licensed QUIKRETE manufacturer in western Canada, for the two metric sizes it sells. They are the manufacturer's published yields for those bags, not a pound-for-pound rescaling of the US figures, which is why the metric table carries its own constants. The bag-yields guide covers each size.
• 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. Adjustable in the form.
• 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 to report the m³ line and, in metric mode, the litres to mix, at 1,000 L per cubic meter — a definition.
• Meters per foot, 0.3048, and centimeters per inch, 2.54 — exact definitions, used only to convert metric entries into the engine's feet and inches.

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; the bag column shows one size per row, and the paragraphs below give the others. The metric row was entered as meters and centimeters, converted exactly, and compared against the metric bag table, which is what the toggle does.

The continuous footing, with and without an allowance. One 36 ft × 12 in × 12 in footing has a section of 12 ÷ 12 by 12 ÷ 12 — one square foot — so its cubic feet equal its length: 36 cu ft, 1.33 cu yd, 1.02 m³. With the default 10% there is 39.6 cu ft to mix: 66 bags of 80 lb or 88 of 60 lb. Set the waste field to 0 and the same footing needs 36 cu ft — 60 bags of 80 lb, with 0 cu ft left over, because 36 divided by 0.6 is a whole number. Binary arithmetic can leave a quotient like that a hair above the boundary; the engine's exact-fit tolerance is what keeps it from buying one bag more.

The pad footings. A 2 ft × 24 in × 12 in pad is 2 × 2 × 1 = 4 cu ft, and 8 of them are 32 cu ft, 1.19 cu yd. The allowance lifts that to 35.2 cu ft — 59 bags of 80 lb with 0.2 cu ft spare, or 94 bags of 50 lb. The count matters here. Run as 8 separate single-pad calculations, each pad is 4.4 cu ft to mix and rounds up to 8 bags of 80 lb, which is 64 bags in all — 5 more than the single run, because each run rounds up on its own. Enter the count and let the round-up happen once.

The priced footings. Four 12 ft × 18 in × 10 in footings are 15 cu ft each, 60 cu ft in all, 2.22 cu yd. With 10% waste there is 66 cu ft to mix, and with the 60 lb bag selected the priced row is 147 bags at 0.45 cu ft each. With $8.00 per bag typed into the price field, the cost line reads $1,176.00 — the whole-bag count times the price, nothing else. That price is a placeholder 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 footing measured in feet and inches. The tape says 24 ft 6 in, so the length is entered as 24.5 ft. At 20 in wide and 10 in deep that is 24.5 × 1.6667 × 0.8333 = 34.03 cu ft, 1.26 cu yd; the allowance makes it 37.43 cu ft and the 80 lb row reads 63 bags with 0.37 cu ft spare. The inches were folded into the length before the engine ran; they did not go into the width or depth fields, which take inches of a different dimension.

The metric footings. In metric mode, 3 footings of 6 m × 40 cm × 20 cm reach the engine as 19.69 ft × 15.75 in × 7.87 in. Each is 0.48 m³ (16.95 cu ft), and the three are 1.44 m³ — 50.85 cu ft, 1.88 cu yd. With 10% the amount to mix is 1,584 L (55.94 cu ft), and the metric bag table gives 108 bags of 30 kg or 131 bags of 25 kg at the Target Products yields. The result panel leads in cubic meters, but the volume underneath it is the same one the imperial path would produce from the converted dimensions.

How to read the result

The headline line is the pour: Pour N cu yd of concrete, or cubic meters in metric mode. Beneath it, a line labelled with the count and the single-footing volume — for the worked example, 2 footings × 17.78 cu ft — gives the total in cubic feet, cubic yards, and cubic meters, so a typo in a dimension is visible before it is padded. With N% extra shows the waste-adjusted volume to mix, in cubic feet or litres. Then one line per bag size — 40 lb bags: 131 bags and so on down to 80 lb — each a complete alternative: buy the count on one row, not the sum of the rows. A cost line appears only when a price is entered, and it names the size it priced. Read the second line to check your geometry, the adjusted line to check your allowance, and the row for the bag you will buy to place the order. The copy button places the pour and every bag count on the clipboard as plain text.

Common misreadings

Depth in feet. Width and depth go in as inches; length as feet. Enter the worked example's 8 in depth as 8 feet and the engine sees 96 in: the same two footings become 426.67 cu ft, 15.8 cu yd, and 783 bags of 80 lb instead of 66 — 12 times the correct volume, the classic estimating error on this form. If the cubic-yard figure looks wildly high, check the depth and width fields first. The guide linked above shows the slip in the other direction too.

Metric figures in the imperial fields. The metric footings above are 6 m × 40 cm × 20 cm. Type those same numbers with the toggle still on Imperial and the engine sees 3 footings of 6 ft × 40 in × 20 in: 100 cu ft and 3.7 cu yd, against the correct 1.88. The error is not a simple factor, because the length shrank while the width and depth grew; if the pour looks implausible for the footing, check the toggle.

The count left blank. An empty count field means one footing. The worked pair at 20 ft × 16 in × 8 in needs 66 bags of 80 lb; with the count blank the result is for a single footing, 17.78 cu ft and 33 bags. The second line of the result names the count it used — read it before the bag rows.

Width and depth swapped — not an error. Enter the worked footing as 8 in wide and 16 in deep and the volume is 35.56 cu ft, the same 35.56: the engine multiplies the two, so their order does not matter. What matters is that both are inches and both come from the plans.

The bag rows as a shopping list. The result lists a count for every bag size because the coverage stage runs once per size. The rows are alternatives, not components: the worked example needs 66 bags of 80 lb or 131 bags of 40 lb, never both. Pick the size you will buy and read that row.

Waste as a fraction. The waste field takes a percentage. The pads at 10% need 59 bags of 80 lb; typed as 0.1 it is read as 0.1%, the amount to mix falls to 32.03 cu ft, and the count drops to 54 — 5 bags short of the intended allowance, with no warning, because a fraction of a percent is a valid input.

Padding twice. Rounding a dimension up before entering it, or adding a bag or two to the count afterwards, repeats work the pipeline already does: the allowance is applied to the volume and the round-up to the allowance, exactly once each. The leftover on a bag row is the round-up doing its job, not an error to correct.

Where the model stops

The engine models identical rectangular prisms of uniform section and assumes that shape is what gets poured. It does not model a stepped footing, a footing whose width or depth changes along its run, or the pier that may stand on a pad — run the footing and the pier separately, the pier in the concrete column calculator, and add the volumes. An uneven trench bottom and over-excavation are among the reasons real consumption differs from the geometry, and the waste allowance is the single flat percentage you are given to fold them into; the model does not estimate them. It counts concrete and nothing else: subgrade preparation, formwork, reinforcement layout, delivery minimums, and moisture and temperature effects are outside it, as they are for every estimator on the site. It also stops at bags — a footing large enough for a ready-mix truck takes its cubic-yard figure to the concrete slab calculator's quarter-yard ordering or straight to the supplier. And, as the note under the form says, footing sizing is engineering governed by your building code: this tool only totals the concrete the dimensions require. The methodology page lists the simplifications every estimator shares.

The rest of the pour

Round piers or formed columns on top of the footing? The concrete column calculator handles cylinders and rectangular forms. Setting posts instead of pouring strip footings is the post hole calculator's job, slab-on-grade volume is the concrete slab calculator's, and to compare every bag size for an arbitrary volume there is the concrete bag calculator. Reinforcement counts for the slab above the footing live in the rebar calculator. The feet & inches calculator turns tape readings into decimal feet. For the units and the reasoning behind the chain, see cubic feet, cubic yards, and ordering concrete, concrete bag yields by size, and waste factors, explained, or browse all guides.

Frequently asked questions

Dimensions never leave your browser. Bag-yield sources and the estimating pipeline are documented on the methodology page.