Concrete Footing Calculator: Volume, Bags, and Depth
A concrete footing calculator uses length × width × depth ÷ 27 to estimate cubic yards; if the footing is especially wide or deep, or supports an unknown load, it may need engineer review. Wrong sizing wastes money on extra bags or short pours that weaken the footing. This guide shows the formula, bag and ready-mix conversions, worked examples, and the DIY-safe limits for decks, walls, piers, and small foundations.
How much concrete do I need for a footing?
Use the footing shape, measure in feet, then turn cubic feet into cubic yards by dividing by 27, since 1 cubic yard equals 27 cubic feet. Footings may be used for pads, foundations, and piers, and this calculator returns results in cubic yards or bags. For most small projects, volume math works only after width, depth, and soil support are checked.
Footing shapes this calculator handles
This page covers three common footing types: isolated pad footing under a pier or column, continuous strip footing under a wall or several supports, and pier footing for deck posts or fence posts. The shape matters because the formula changes, and so does the risk if the footing is too small for the load or soil.
The volume formula for each shape
- Rectangular pad footing: length × width × depth ÷ 27
- Circular pad footing: π × radius² × depth ÷ 27
- Strip footing: total linear length × width × depth ÷ 27
These are the standard footing volume formulas. Circular pads use radius, not diameter, so divide the diameter by 2 before squaring. Strip footings run continuously under walls or supports, so the length is the total run, not the footing width.
Convert cubic feet to cubic yards
One cubic yard equals 27 cubic feet. If the volume is already in cubic feet, divide by 27 to get cubic yards. For ordering, round up to the next usable amount and add a waste factor, because excavation irregularity and placement loss can leave a short pour.
Rectangular pad footing volume uses length × width × depth divided by 27.
How do I calculate a strip footing?
A strip footing is calculated as total linear length × width × depth ÷ 27. Use it for a wall base or multiple supports in a line. If a stem wall sits on top, calculate the footing and stem wall separately, then add the volumes before converting or ordering.
Length × width × depth formula
Measure the full footing run in feet. Multiply that length by the footing width in feet and the footing depth in feet. Divide by 27 for cubic yards. This is the same base math used in foundation takeoffs, and it works best when trench width stays consistent from end to end.
How to include a stem wall if present
If the footing supports a stem wall, add the stem wall volume to the footing volume. A simple way is to calculate the footing as one block and the wall above it as a second block. That matters because strip footings often form part of a foundation base rather than a standalone pour.
Worked example with units
Example: a strip footing with the same general dimensions.
30 × 1.5 × 0.75 = 33.75 cubic feet.
33.75 ÷ 27 = 1.25 cubic yards.
If the trench is uneven, add some waste on top of that number before ordering. For small pours, a modest allowance is often used, and irregular excavation can justify a higher allowance.
How do I calculate a pier or pad footing?
A pier footing uses diameter and depth, while a pad footing uses length, width, and depth. Round piers and sonotube footings are common under deck posts and fence posts, and the bottom must clear frost depth. Pad footings are common under columns and isolated supports.
Round pier footing formula using diameter
For a round footing, convert diameter to radius by dividing by 2. Then use π × radius² × depth ÷ 27. Common sonotube sizes include 8, 10, 12, 16, 18, and 24 inches, so diameter choice changes both volume and the load path to the soil.
Rectangular pad footing formula
For a rectangular pad, use length × width × depth ÷ 27. This works for isolated pad footing under a pier or column, and it is often easier to estimate than a round pier when the form is square or rectangular. Keep all dimensions in feet for cubic yard output.
Worked example for deck posts or columns
Example: a round pier footing 18 in in diameter and 4 ft deep.
18 in = 1.5 ft, so radius = 0.75 ft.
π × 0.75² × 4 = 7.07 cubic feet.
7.07 ÷ 27 = 0.26 cubic yards.
That is a small pour, so bag count matters more than yard pricing unless several piers are being poured together.
Footing takeoff worksheet: formula, bag count, and order rule
This worksheet turns a footing takeoff into an order decision. First calculate exact volume. Then convert to bags or cubic yards. Finally, choose bags for very small pours and ready-mix for larger or more irregular pours. The goal is to avoid buying too much concrete or stopping short of the full pour.
Exact volume math
| Footing type | Formula | Example dimensions | Volume in cubic feet | Volume in cubic yards |
|---|---|---|---|---|
| Rectangular pad | l × w × d | 4 ft × 4 ft × 1 ft | 16 | 0.59 |
| Round pier | π × r² × d | 18 in dia. × 4 ft deep | 7.07 | 0.26 |
| Strip footing | length × width × depth | 30 ft × 1.5 ft × 0.75 ft | 33.75 | 1.25 |
Bag counts for 50-lb, 60-lb, and 80-lb bags
Bag yields vary by manufacturer and mix design, so the order count should be based on the bag yield listed on the package. For small pours, a practical approach is to divide the needed cubic feet by the bag yield, then round up to the next full bag. Some concrete calculators list 25 kg bags at roughly 0.01 m³ each and around 80 bags per cubic meter, which shows how quickly bag counts rise on larger footings.
| Bag size | Use case | Order note |
|---|---|---|
| 50-lb bag | Very small pier or patch pours | Best when the total volume is low and access is easy |
| 60-lb bag | Small footings and deck piers | Common mid-size bag choice for hand mixing |
| 80-lb bag | Fewer bags for larger hand-mixed pours | Heavier to handle, but fewer bags to open and mix |
When to order ready-mix instead of bags
Ready-mix becomes the cleaner choice when the footing total grows beyond a few small piers or when the pour has multiple shapes. ConcreteCalculate lists 2026 delivered concrete pricing at $125 to $195 per cubic yard, which is useful for comparing yard delivery against the labor of many bag mixes. Once the pour is large enough that mix timing matters, ready-mix usually cuts the chance of a bad pour.
- Calculate total volume in cubic yards.
- Add 5% to 10% for waste and overdig.
- Compare the total against bag yields and delivery minimums.
- Use bags for very small pours with simple access.
- Use ready-mix when the footing is larger, deeper, or split into multiple sections.
How big should a concrete footing be?
Footing size depends on soil bearing, supported load, and project type. ConcreteCalculate says IRC R403.1 minimum footing width values are shown by soil class and stories, and it also lists 1,500 psf as a presumptive soil value for clay, sandy clay, and silt. Width cannot be chosen from volume alone.
Width checks tied to soil bearing and load
The footing must spread the load enough for the soil underneath. A wider footing lowers pressure on weaker soil, while a narrow footing may be acceptable only when the load is light and the code table allows it. That is why width should be checked against the soil class and the number of stories supported.
Why frost depth controls footing depth
The bottom of the footing should be below the frost line. Frost depth varies by region, so a shallow pier footing that looks fine on paper can still be wrong if it sits above local frost depth. This matters most for deck posts, fence posts, and foundation footings in cold climates.
When IRC Table R403.1 is not enough on its own
Minimum widths from the code table do not solve every case. Sloped sites, unusual loads, retaining conditions, or mixed foundation details can push the design beyond a simple lookup. If the footing is wider than 24 in, deeper than 24 in, or carrying an unknown load, engineer review is the safer path.
When should a footing go to engineer review?
A footing should go to engineer review when the shape, soil, or loading makes the takeoff uncertain. Simple volume math still works, but it does not verify capacity. Stepped footings, monolithic stem walls, and irregular trenches can change the load path and the amount of concrete enough to matter.
Stepped footings and sloped sites
Stepped footings break the pour into multiple height segments. On sloped sites, that can change both volume and bearing conditions from one section to the next. A single average depth may undercount concrete and miss a design issue at the same time.
Monolithic stem walls and mixed foundation details
A monolithic stem wall combines footing and wall geometry in one system. That is harder to estimate than a simple strip footing because the footing base and wall height both affect total volume. Mixed details, such as pads tied into walls, should be reviewed before ordering concrete.
Irregular trenches, overdig, and uncertain soil conditions
Real trenches are rarely perfect rectangles. Overdig, loose edges, form thickness, and uneven bottoms all add volume. If the excavation is irregular, the waste factor should move toward the higher end, and uncertain soil conditions should trigger a design check rather than a guess.
What this calculator assumes
This calculator assumes measurements are taken in feet, inches are converted before volume math, and the final result is rounded up. It also assumes a waste factor of 5% to 10%, with more allowance for rough excavation. The output is a takeoff number, not a structural approval.
Waste factor and rounding up
Round up the final order. Even a small shortfall can stop a pour and leave a cold joint or patch. A 5% allowance fits cleaner layouts, while 10% is safer for hand-dug or irregular footings. For odd shapes and rough trench bottoms, choose the higher number.
Measurement in feet, inches, cubic feet, and cubic yards
Keep the math in one unit system. If the footing is measured in inches, convert to feet before multiplying. Then turn cubic feet into cubic yards by dividing by 27. That keeps the takeoff consistent across pads, strips, and piers.
What to do with form thickness and trench irregularity
Form thickness and trench irregularity can change the real pour size. If forms are used inside an oversized excavation, measure the formed concrete shape, not the hole. If the trench width varies, use the widest practical average or move the project to an engineered takeoff.
Frequently asked questions
how big should a concrete footing be
Footing size depends on the supported load, soil bearing, number of stories, and frost depth. IRC Table R403.1 gives minimum widths by soil class and stories, but that table is only a starting point. If the footing is wider than 24 in or deeper than 24 in, it deserves review.
how do you calculate concrete for a footing
Use the footing shape formula, then turn cubic feet into cubic yards by dividing by 27. Rectangular pads use length × width × depth, circular pads use π × radius² × depth, and strip footings use length × width × depth. Add waste before ordering to avoid a short pour.
how many bags of concrete do I need for a footing
Divide the total volume by the bag yield, then round up to the next full bag. Small piers may only need a few bags, while strip footings can move quickly into ready-mix territory. Bag count depends on the bag size and the actual yield listed on the package, so always check that value first.
what depth should a concrete footing be
The bottom of the footing should be below the frost line. Depth also has to match load and soil conditions, not just the desired form height. A shallow footing may look acceptable in dry weather and still fail the frost-depth requirement in colder regions.
what width should a concrete footing be
Width depends on soil bearing and the supported load. ConcreteCalculate notes that IRC R403.1 minimum footing widths are shown by soil class and stories, and 1,500 psf is a presumptive soil value for clay, sandy clay, and silt. If the loading is unclear, engineer review is the safer call.
how do I calculate concrete for a strip footing
Multiply the total linear length by the footing width and depth, then divide by 27 for cubic yards. If there is a stem wall, calculate that volume separately and add it to the footing. This method works best when the trench width stays consistent along the run.
how do I calculate concrete for a pier footing
Measure the diameter, convert to radius, and use π × radius² × depth ÷ 27. Pier footings are common under deck posts and fence posts, and they must clear frost depth. For sonotube pours, common sizes include 8, 10, 12, 16, 18, and 24 inches.
what is the formula for concrete footing calculation
The core formulas are simple: rectangular pad footing equals length × width × depth ÷ 27, circular pad footing equals π × radius² × depth ÷ 27, and strip footing equals length × width × depth ÷ 27. Those formulas give volume, but they do not replace width, load, or frost-depth checks.