Measure The Circle Before You Order
Estimate concrete for round slabs, circular pads, tank bases, circular walls, annular rings, manhole surrounds and partial-circle pours. Enter diameter, thickness, wall height or sector angle to calculate cubic feet, cubic yards, metric volume, concrete bags and optional ready-mix cost.
Choose the closest geometry, enter finished dimensions, then calculate exact and order-adjusted concrete quantity.
Calculate a solid round slab, circular equipment pad, tank base or other full-circle concrete pour.
Estimate a circular wall from outside diameter, wall thickness and wall height.
Calculate concrete between an outer circle and an inner opening.
Estimate a sector such as a half-circle, quarter-circle or custom-angle circular concrete slab.
Round slabs, pads, tank bases and circular equipment foundations.
Hollow cylindrical walls using outside diameter, thickness and height.
Concrete between an outside circle and a central opening.
Half circles, quarter circles and custom-angle circular sectors.
A Circular Concrete Calculator is a volume-estimating tool for concrete projects based on circular geometry. Instead of multiplying length by width as you would for a rectangular slab, circular projects begin with radius or diameter. The calculator converts the circular area into a three-dimensional concrete volume by multiplying that area by slab thickness, wall height or footing depth.
This page supports several common circular concrete shapes in one place. The first mode handles a full solid circle such as a round patio, silo pad, tank base, equipment pad or circular slab. The second estimates a hollow circular wall. The third handles an annular ring with an opening through the center. The fourth calculates a sector, which is useful for a half-circle, quarter-circle or custom-angle portion of a round slab.
After the geometry is calculated, the tool converts cubic feet into cubic yards and cubic meters, estimates approximate 60 lb and 80 lb concrete bag quantities, applies an optional ordering allowance and can estimate ready-mix material cost when you enter a price per cubic yard.
For a full circular slab or pad, calculate circle area first, then multiply by concrete thickness. Radius is one-half of diameter.
Use feet for radius and feet for thickness to obtain cubic feet.
If your project is measured by diameter, the same formula can be written using diameter directly:
This is mathematically equivalent to πr² × thickness.
For a 12 ft diameter slab that is 6 inches thick, radius is 6 ft and thickness is 0.5 ft. Circle area is approximately 113.10 square feet. Multiplying by 0.5 ft gives approximately 56.55 cubic feet, or about 2.09 cubic yards before allowance.
A round concrete slab is a cylinder with a relatively small height. The same cylinder formula used for piers applies, but the slab thickness is usually measured in inches rather than feet.
Suppose a circular patio is 14 ft in diameter and 4 inches thick. Radius is 7 ft, so the area is π × 7², or about 153.94 square feet. Four inches equals 0.3333 ft. Multiplying gives roughly 51.31 cubic feet of concrete, which is approximately 1.90 cubic yards.
If the circular slab includes a thickened perimeter, center footing, embedded pedestal or other deeper section, calculate those additional shapes separately and add them to the basic slab volume. Do not assume the whole slab has the deeper thickness unless that is actually the case.
Round tank bases are a common circular-concrete application. For a simple solid base slab, the full-circle mode can estimate material from diameter and thickness. If the tank foundation is a ring rather than a solid disk, use the annular mode instead.
A tank foundation may also include a thickened ring, central pad, pedestal or embedded anchorage. Those details can materially change volume. Structural tank foundations require engineering because settlement, bearing, uplift, anchorage and liquid loads are outside the scope of geometric volume calculation.
For a ring-style tank foundation, measure outside diameter, inside diameter and footing depth. The Concrete Ring Calculator provides an even more focused annular-concrete workflow for that geometry.
A circular wall is a hollow cylinder. The calculator derives the inside diameter by subtracting twice the wall thickness from the outside diameter. It then subtracts the inner circular area from the outer circular area and multiplies the remaining cross-sectional area by wall height.
Inside diameter = outside diameter − 2 × wall thickness.
This method is useful for circular tank walls, shafts, pits, silos, wells and other hollow cylindrical concrete walls when the dimensions can be represented by concentric circles.
Wall openings, doors, penetrations and large sleeves can be subtracted separately when they materially affect volume. Small embeds are often left in the ordering allowance rather than deducted individually.
An annulus is the area between two concentric circles. Concrete examples include manhole surrounds, tank ring foundations, circular collars, equipment rings and pads that intentionally leave an opening in the center.
The inner opening contains no concrete and must be subtracted.
One of the most common mistakes is calculating the full outside circle without subtracting the opening. For wide openings, that can substantially overestimate concrete. The annular mode handles the subtraction automatically.
If the opening is off-center, the outer-minus-inner area still gives the same theoretical area when the full inner circle remains completely inside the outer circle. However, form geometry and varying ring width can make field estimating and reinforcement design more complicated.
A circular sector is a fraction of a full circle defined by an angle. A half-circle is 180 degrees, a quarter-circle is 90 degrees, and a three-quarter circle is 270 degrees.
For 180°, use one-half of the full circular volume. For 90°, use one-quarter.
This mode is useful for semicircular patios, curved landscape pads, fan-shaped placements and other circular pours whose straight edges meet at the circle center. If the curved shape is a segment cut by a chord rather than a true sector, different geometry is required.
Diameter is measured through the center from one outside edge to the opposite outside edge. Radius runs from the center to the edge. Circumference is the distance around the circle.
For concrete volume, area is more important than circumference. For formwork, edge forms, expansion material or curved reinforcement length, circumference may also be useful.
The table below shows approximate exact volume for several full circular slabs before any extra allowance.
| Diameter | Thickness | Cubic Feet | Cubic Yards |
|---|---|---|---|
| 6 ft | 4 in | 9.42 ft³ | 0.349 yd³ |
| 8 ft | 4 in | 16.76 ft³ | 0.621 yd³ |
| 10 ft | 4 in | 26.18 ft³ | 0.970 yd³ |
| 12 ft | 4 in | 37.70 ft³ | 1.396 yd³ |
| 12 ft | 6 in | 56.55 ft³ | 2.094 yd³ |
| 16 ft | 6 in | 100.53 ft³ | 3.723 yd³ |
| 20 ft | 6 in | 157.08 ft³ | 5.818 yd³ |
For small circular pours, bagged concrete can be estimated from cubic feet. Approximate planning yields often used are about 0.45 cubic foot for a 60 lb bag and about 0.60 cubic foot for an 80 lb bag. The actual yield printed by the manufacturer should always control purchasing.
A 10 ft diameter slab at 4 inches thick contains about 26.18 cubic feet before allowance. Using a planning yield of 0.60 ft³ per 80 lb bag, exact volume alone would represent about 44 bags. An ordering allowance increases that count.
Once circular volume reaches several cubic yards, compare the labor and consistency of bag mixing with local ready-mix delivery. The Concrete Truck Count Calculator can help plan loads after cubic-yard quantity is known.
Exact geometry assumes the forms are perfectly circular, the subgrade is at the intended elevation and the thickness is uniform. Real projects can have excavation irregularity, uneven base, bowed flexible forms, over-depth areas, spills and pump or chute loss.
The calculator therefore separates exact volume from order-adjusted volume. You can select no extra material, 5%, 10% or 15%. The correct allowance depends on site conditions, measurement confidence and supplier practices rather than one universal rule.
Circle area is proportional to diameter squared. That means a 5% error in diameter can create roughly a 10% difference in area before thickness is even considered. This makes careful diameter measurement especially important for large circular foundations.
Measure in at least two directions through the center when forms are intended to be circular. If the measurements differ significantly, the shape may be elliptical or irregular rather than a true circle.
For flexible formwork, check radius from a fixed center point at several locations around the perimeter. This can identify bulges before concrete is placed.
Use layout points, intersecting diameter lines or survey control to identify the intended center of the circle.
Measure from the center to the form at multiple points. Repeated radius checks are more reliable than assuming one diameter measurement proves the whole form is round.
For slabs and footings, check compacted base elevation in several locations. A low center or low edge can increase concrete volume.
Use the final installed dimensions rather than preliminary layout values for the final concrete order.
Round patios are straightforward when slab thickness is uniform. Enter the finished patio diameter and concrete thickness. If the patio includes a center fire-pit opening, planter opening or other void, use the annular mode instead of the full-circle mode.
If only part of the patio is circular and another part is rectangular, calculate the two shapes separately and add them. Avoid placing a large rectangle around the whole shape because that includes empty corners and overestimates volume.
Decorative borders may be thicker or use a separate concrete mix. Treat those borders as their own ring-shaped section when appropriate.
Round agricultural and industrial foundations may support bins, silos, tanks or process equipment. Volume geometry can be estimated with a circular or ring calculator, but structural design is more demanding because loads may include stored material, overturning, wind, seismic actions, anchor forces and soil interaction.
Use the calculator only to estimate the concrete quantity from approved dimensions. If the foundation includes a ring footing under a thinner interior slab, calculate the slab and ring footing separately and avoid counting overlapping volume twice.
A concrete collar around a manhole or utility structure can often be represented as an annulus. Enter outside diameter, inside opening diameter and collar thickness. If the collar is only a partial circle because it intersects a curb, pavement edge or other feature, divide the geometry into separate measurable sections.
Actual utility details can include reinforcing steel, dowels, pavement transitions and minimum thickness requirements. Those construction details must come from the applicable drawings or agency standards.
The geometry may look similar, but a slab and footing serve different structural purposes. A slab distributes loads over an area and provides a surface. A footing transfers loads to supporting soil. The volume formula can be identical when both are solid cylinders, but the required depth, reinforcement and concrete strength may be very different.
This is a useful reminder that geometric similarity does not make structural requirements interchangeable. The calculator only answers “how much concrete fits these dimensions?”
Concrete volume does not tell you how much formwork is needed. For a round slab edge, the vertical form area can be estimated from circumference multiplied by slab thickness. For circular walls, the inside and outside vertical surfaces can create a much larger contact area.
Use the Concrete Form Area Calculator for form-contact estimates. For a simple circular slab, circumference is π × diameter, which gives the total curved edge length.
Round slabs and foundations may use radial bars, circumferential bars, welded reinforcement, conventional grids or other reinforcement layouts depending on design. Concrete volume does not determine bar size or spacing.
Once reinforcement spacing and geometry are specified, the Concrete Rebar Length Calculator can assist with general bar-length estimating. Circular reinforcement often requires lap lengths, hooks, cover and development details that must follow project requirements.
The calculator can multiply adjusted cubic yards by an optional ready-mix price. That figure represents a simple material subtotal only. Concrete project cost can also include short-load fees, delivery, pumping, placing labor, finishing, forms, reinforcement, base preparation, excavation, curing materials and testing.
For a small round pad, bagged concrete may be priced per bag. Compare the whole-bag count with ready-mix minimums rather than comparing only nominal unit cost.
Radius is half of diameter. Using the full diameter in the πr² formula multiplies area by four.
A 6-inch slab is 0.5 ft thick, not 6 ft. Always convert before multiplying when using cubic-foot formulas.
Ring foundations and collars need inner-circle area removed from the outer-circle area.
A sector is bounded by two radii and an arc. A segment is bounded by a chord and an arc and uses different geometry.
Multiple pads, columns or collars should be multiplied by quantity when they are identical.
If a ring footing overlaps a slab area already counted, calculate only the additional depth or use separate non-overlapping shapes.
The same formulas work in metric units. If diameter and thickness are entered in meters, the result is cubic meters directly. When dimensions are in millimeters, convert them to meters before multiplying.
The calculator displays cubic meters automatically after calculating in U.S. construction units. One cubic foot is approximately 0.0283168 cubic meter, and one cubic yard is approximately 0.764555 cubic meter.
For projects measured fully in metric dimensions, keep all geometry in meters to reduce conversion errors.
Radius = 5 ft. Area = π × 25 = 78.54 ft². Thickness = 0.3333 ft. Volume ≈ 26.18 ft³ = 0.97 yd³.
Radius = 8 ft. Area ≈ 201.06 ft². Thickness = 0.5 ft. Volume ≈ 100.53 ft³ = 3.72 yd³.
A half-circle uses 180 ÷ 360 = 0.50 of full-circle volume. If the full slab would require 4 yd³, the same-diameter half-circle at the same thickness requires 2 yd³ before allowance.
For a 10 ft outer diameter and 6 ft inner diameter, annular area is π ÷ 4 × (100 − 36) = approximately 50.27 ft². At 6 inches thick, volume is about 25.13 ft³.
Decorative circular borders are often built as rings around a central slab, planter, fountain or landscape feature. If the center remains open or is filled with another material, do not calculate the border as a full solid circle. Use the annular mode so the center opening is removed from the concrete quantity.
For a circular patio with a colored or stamped outer band, calculate the center disk and the outer ring separately when the two areas use different concrete mixes, colors, thicknesses or placement schedules. This also makes material-cost comparisons easier because each mix can have its own price.
If the border is thicker than the center slab, first calculate the normal ring thickness and then add only the extra thickened volume. This prevents the deeper section from being counted twice.
Some circular foundations combine a broad round base with a smaller central pedestal. In that case, calculate the base as a circular slab and the pedestal as a second cylinder. Add the two volumes only where they do not overlap, or calculate the pedestal's additional height above the base when the pedestal is monolithic with the slab.
For example, if a 12 ft diameter base is 12 inches thick and a 3 ft diameter pedestal rises another 2 ft above the base, calculate the full 12 ft base first. Then calculate only the 3 ft diameter × 2 ft high pedestal above the base. Do not calculate the pedestal through the full base thickness again.
This same principle applies to circular equipment foundations, light-pole bases, monument bases and machinery pads.
A curved walkway is not always a simple sector. If the walkway follows two concentric arcs with a constant width, its plan area is an annular sector. The full annular-ring formula can be multiplied by the arc angle divided by 360. If your curved walkway is a full ring, use the annular mode; if it is only part of a ring, calculate the corresponding fraction.
Where walkway width changes, divide the path into several segments rather than forcing one average width across the whole curve. Small width changes can affect area noticeably on long arcs.
For a true curved path defined by inner and outer radii, the most exact quantity comes from plan geometry. Field measurements along only the centerline are not enough unless walkway width and curvature are also known.
Because circle area depends on radius squared, concrete volume rises faster than diameter. If slab thickness stays the same, doubling the diameter makes the circular area four times as large. This is why large-diameter pads can require much more concrete than they appear to when compared visually with smaller pads.
| Diameter Change | Relative Area | Effect at Same Thickness |
|---|---|---|
| 5 ft to 10 ft | 4× area | 4× concrete volume |
| 10 ft to 15 ft | 2.25× area | 2.25× concrete volume |
| 10 ft to 20 ft | 4× area | 4× concrete volume |
| 12 ft to 18 ft | 2.25× area | 2.25× concrete volume |
This squared relationship is also why careful form layout matters. A large circle that is only a few inches oversized around the perimeter can add meaningful volume.
After the circular geometry is converted to adjusted cubic yards, compare the quantity with ready-mix truck capacity and supplier minimums. A single circular foundation may be large enough for several truck loads, while a small round patio may fall into a short-load category.
For multi-truck pours, delivery timing matters as much as total volume. Crews must be able to place, consolidate and finish concrete at a rate that matches truck arrivals. Circular walls can be especially sensitive to placement planning because form pressure, lift sequence and access can influence production rate.
Use the Concrete Truck Count Calculator after calculating cubic yards. For pumped placements, coordinate pump capacity, hose reach and crew production separately rather than assuming truck volume alone determines the schedule.
Combine circular volume with related concrete tools for a more complete material estimate.
For concrete structural design, durability, reinforcement, joints and construction requirements, use the applicable project documents and qualified engineering guidance. The American Concrete Institute publishes concrete codes, guides and technical resources. The American Cement Association provides educational information about cement and concrete materials. These sources address topics that go beyond geometric quantity estimating.
Full-circle, annular and partial-circle concrete all begin with circular area and then multiply by concrete depth.
Answers to common questions about round slabs, circular walls, ring-shaped concrete and partial-circle pours.