Check Bearing Area. Size The Pour. Plan The Pier.
Use this Concrete Pier Size Calculator to estimate a preliminary soil-bearing area from a known vertical load and allowable soil pressure, or calculate concrete volume for round piers, square piers and belled pier foundations. Get cubic feet, cubic yards, approximate bag quantities and optional ready-mix material cost.
Choose preliminary bearing size, round-pier volume, square-pier volume or belled-pier volume. Results stay closed until you press Calculate.
Estimate the minimum bearing area implied by a known vertical load and a user-entered allowable soil-bearing pressure. This is a bearing-only screening calculation, not a complete foundation design.
Use the project load value provided by your design basis.
Use a geotechnical/code-approved value, not a guess.
Optional comparison against the entered soil pressure.
Calculate concrete for cylindrical piers, drilled shafts, deck piers and round footing forms after diameter and concrete-filled depth are known.
Bag yields are approximate planning values. Verify the yield printed by the concrete-mix manufacturer.
Estimate concrete quantity for square piers, rectangular piers, pier pads and formed vertical foundation elements.
Estimate concrete for a cylindrical shaft plus an enlarged circular base modeled as a frustum. Use only dimensions specified for your project.
A Concrete Pier Size Calculator is a planning tool for two different but related questions: how much bearing area is implied by a known vertical load and allowable soil pressure, and how much concrete is required after a pier geometry has been selected. Those questions should not be confused. The bearing-area equation is a simple pressure relationship, while real foundation design also considers soil behavior, settlement, frost, lateral loading, uplift, eccentricity, structural strength, reinforcement, anchorage and locally adopted building requirements.
That is why this page separates preliminary size screening from concrete quantity calculations. The first mode accepts a vertical load and an allowable soil-bearing pressure supplied by your project basis. It returns the mathematical area required to keep average vertical bearing pressure at or below that entered value, then converts the area into an equivalent round diameter and square width. The remaining modes do not size the foundation structurally. They simply calculate concrete volume for dimensions you already know.
This distinction is especially useful when planning deck piers, porch supports, small accessory structures, isolated column supports, drilled pier foundations or formed concrete piers. The calculator can help you compare dimensions and order concrete, but the finished pier must still follow the project design and the requirements that apply at your site.
Converts vertical load and user-entered allowable bearing pressure into required area.
Calculates cylinder volume for drilled or formed circular concrete piers.
Handles square and rectangular pier dimensions with multiple identical units.
Adds a frustum-shaped enlarged base to the straight cylindrical shaft volume.
Start by choosing the calculator mode that matches the information you actually have. If an engineer, plan set, geotechnical report or applicable code basis gives you both a vertical load and an allowable soil-bearing pressure, the Bearing Size mode can perform the basic pressure calculation. If the pier dimensions have already been selected, skip the bearing mode and use the matching volume calculator.
Select bearing size, round pier, square pier or belled pier.
Enter project dimensions, load values and soil pressure from reliable project information.
Keep diameter and width units correct and measure the concrete-filled depth.
Review the geometric result, concrete volume, bags or cost estimate.
Confirm pier dimensions, depth and reinforcement against project and local requirements.
For a simple concentric vertical load acting on a pier base, average bearing pressure is load divided by bearing area. Rearranging that relationship gives a preliminary required area:
For a round pier: diameter = √(4A ÷ π). For a square pier: side = √A.
For example, a 12,000 lb vertical load divided by an entered allowable soil pressure of 1,500 psf produces 8.00 square feet of bearing area. A circle with 8.00 square feet of area has an equivalent diameter of about 3.19 feet, or roughly 38.3 inches. A square with the same area has a side length of about 2.83 feet, or roughly 33.9 inches.
Those numbers are only geometric equivalents for average bearing pressure. A real pier may need to be larger because of structural requirements, load eccentricity, lateral forces, settlement limits, construction tolerances, reinforcement clearances, minimum dimensions, frost or other site conditions. It can also be controlled by a different foundation system entirely.
Once a round pier diameter and concrete-filled depth have been selected, its concrete quantity is calculated as a cylinder. Convert diameter from inches to feet, divide by two for radius, square the radius, multiply by π and then multiply by depth.
Cubic yards = cubic feet ÷ 27. Multiply by the number of identical piers and then apply your selected ordering allowance.
A 24-inch-diameter pier has a radius of 1 foot. At 4 feet deep, one pier contains about 12.57 cubic feet of geometric concrete volume. Six identical piers contain about 75.40 cubic feet before any ordering allowance. The calculator performs these conversions automatically and also shows approximate 40 lb, 60 lb and 80 lb bag counts.
Square and rectangular piers use the basic rectangular-prism formula. Convert width and length from inches to feet, then multiply width by length by concrete-filled depth.
For a square pier, width and length are the same. Multiply by pier count for the total project volume.
This mode is useful for formed pier pads, square column supports and other vertical concrete elements where the excavation or form is not circular. Measure the actual concrete dimensions rather than a post or column sitting on top of the pier.
A belled pier has a straight shaft and an enlarged base. The calculator models the straight section as a cylinder and the enlarged base as a circular frustum transitioning from the shaft diameter to the larger bell diameter. This is a quantity calculation only; it does not determine whether a belled base is appropriate for your soil, loading or construction method.
R is the bell-base radius, r is the shaft radius and h is the bell height. The calculator adds this to the straight shaft cylinder.
Because actual excavated bells can be irregular, field volume may differ from ideal geometry. Verify excavation dimensions and construction requirements before ordering. Where a drilled shaft is excavated directly in soil, sloughing, overbreak, groundwater or construction method can materially change actual concrete consumption.
This diagram separates the variables used for bearing-area math from the dimensions used for concrete-volume calculations.
The soil-bearing input has a direct mathematical effect on the preliminary area result. If the same vertical load is divided by a lower allowable pressure, more bearing area is required. If the approved allowable pressure is higher, the bearing-only area becomes smaller. This does not mean a high soil value should be selected to reduce concrete. The input must represent the actual project basis.
American Concrete Institute terminology describes allowable bearing capacity as the maximum pressure to which soil or another material should be subjected to guard against shear failure or excessive settlement. That definition highlights why the value is not simply a material strength number: settlement behavior and site conditions matter too.
For projects where soil conditions are uncertain, disturbed, filled, wet, expansive, organic or otherwise questionable, professional evaluation can be more important than calculator precision. A mathematically exact diameter based on an unreliable soil value is not a reliable foundation design.
The bearing-area calculation on this page intentionally does not calculate pier depth. Depth can be controlled by frost protection, undisturbed bearing material, lateral stability, uplift, embedment, expansive soil conditions, project geometry and code requirements. In climates where frost is relevant, foundation requirements commonly address frost depth or an approved frost-protected design.
The International Code Council's residential foundation provisions include minimum-depth and frost-protection requirements, but the exact rules that apply depend on the locally adopted code, amendments, structure type and site. Use the current requirements for your jurisdiction rather than treating an online calculator as a permit or design document.
The table below shows ideal geometric volume for several common round pier dimensions. It is a quantity reference, not a recommended structural size chart. Required diameter and depth must be selected for the actual project.
| Round Pier | Depth | Volume / Pier | Cubic Yards | Approx. 80 lb Bags* |
|---|---|---|---|---|
| 12 in diameter | 3 ft | 2.36 ft³ | 0.087 yd³ | 4 |
| 16 in diameter | 3 ft | 4.19 ft³ | 0.155 yd³ | 7 |
| 18 in diameter | 4 ft | 7.07 ft³ | 0.262 yd³ | 12 |
| 24 in diameter | 4 ft | 12.57 ft³ | 0.465 yd³ | 21 |
| 30 in diameter | 4 ft | 19.63 ft³ | 0.727 yd³ | 33 |
| 36 in diameter | 4 ft | 28.27 ft³ | 1.047 yd³ | 48 |
*Bag count uses approximately 0.60 ft³ per 80 lb bag and rounds up. Manufacturer yields vary.
Deck and porch foundations are common reasons people search for a concrete pier calculator. The geometry calculator can be useful once the required pier size is known because it quickly converts diameter, depth and pier count into a concrete order. For example, if a project drawing calls for six 24-inch-diameter piers with 4 feet of concrete depth, the round-pier mode can total all six holes and apply an ordering allowance.
Do not assume that one generic pier diameter is suitable for every deck. Tributary area, beam layout, span, snow or live load, soil conditions, post reactions, height above grade and local requirements can change foundation demands. Some systems use concrete piers with post bases above grade, while others use different approved footing or helical systems. Follow the project-specific design.
For concrete quantities around embedded posts rather than full structural piers, use the Concrete Post Calculator. It can subtract the volume occupied by a post when appropriate. The pier calculator on this page assumes the entire entered pier geometry is filled with concrete.
Small pier projects are often mixed from bags. This calculator estimates 40 lb, 60 lb and 80 lb bag quantities using approximate planning yields of 0.30, 0.45 and 0.60 cubic foot per bag. The result always rounds upward because a fraction of a bag generally means another full bag must be purchased.
Actual bag yield can vary by product, mix design, water content and manufacturer. Check the package before purchase. For many large piers or a larger group of foundations, ready-mix delivery may be more practical than hand mixing. Use the Concrete Truck Capacity Calculator to compare total cubic yards with an actual supplier truck capacity.
When ordering ready-mix, the material cost is not necessarily the total delivered project cost. Delivery, short-load charges, pumping, site access, overtime, waiting time, reinforcement, forming, excavation and labor may be separate.
The round, square and belled pier calculators let you enter a local ready-mix price per cubic yard. The tool multiplies your final order quantity—including the selected allowance—by that entered price. This produces a simple concrete-material estimate.
It does not include rebar, anchor hardware, post bases, excavation, drilling, spoil disposal, form tubes, temporary casing, pumps, labor or inspections. If your piers require reinforcing steel, estimate bar quantity separately with the Concrete Rebar Calculator.
Ideal geometry assumes every hole or form is exactly the intended size. Field conditions are rarely perfect. Drilled holes can widen in loose soil, excavation can be uneven, forms can vary, and concrete can be lost during handling and placement. That is why the volume modes include optional 5%, 10% and 15% ordering allowances.
There is no universal percentage that is correct for every pier project. A clean form tube in controlled conditions may be close to geometric volume, while irregular drilled holes can consume noticeably more. Use the allowance as a planning tool and consider actual excavation quality, supplier minimums and the consequences of running short during a continuous placement.
Round piers are common when holes are drilled with an auger or formed with cylindrical tubes. The bearing area is πr² and the concrete volume is πr²h. Round forms can be efficient for drilled construction and are easy to calculate when diameter is known.
Square piers and pier pads are common where conventional forms are used. Bearing area is width × length and concrete volume is width × length × depth. Rectangular geometry can also match specific column, wall or grade-beam layouts.
Neither shape is automatically “better.” Geometry should follow structural, geotechnical, construction and project requirements. The calculator simply converts the chosen shape into area and volume.
Pier work often connects to other concrete quantity and reinforcement tasks. Use the Concrete Calculator for general slabs, walls and footings; the Concrete Post Calculator for concrete around embedded posts; the Concrete Rebar Calculator for reinforcement quantity; the Concrete Retaining Wall Calculator for wall and footing volume; and the Concrete Truck Capacity Calculator when planning ready-mix deliveries.
For technical background, see the American Concrete Institute foundation resources, ACI Concrete Terminology, and the ICC residential foundation provisions. Always use the edition and amendments adopted for your project location.
Common questions about pier bearing area, diameter, depth, concrete volume and bags.