Outer Circle Minus Inner Circle — Instant Concrete Volume
Estimate annulus area, ring-shaped concrete volume, cubic yards, concrete bags, weight and cost for circular pads, tank rings, planters, manholes, monuments, pits and other concrete rings.
Calculate a ring slab, hollow circular wall, multiple annuli or concrete bags, weight and cost.
Calculate ring-shaped concrete area and volume from outer diameter, inner diameter and thickness.
Estimate concrete volume for a hollow cylindrical ring wall using outer radius, inner radius and height.
Combine up to three ring-shaped concrete sections with different diameters and thicknesses.
Convert a known ring-shaped concrete volume into bag quantity, concrete weight and optional ready-mix cost.
Calculate concrete between two concentric circular boundaries.
Estimate a hollow cylindrical concrete ring using height.
Add several annular sections into one material total.
Convert ring volume into bags, weight and ready-mix budget.
The Concrete Annulus Calculator is built for ring-shaped concrete. A true annulus is the area between two concentric circles. Measure the finished outer diameter and the diameter of the circular opening, then enter concrete thickness. The calculator converts both diameters to radii, subtracts the inner circle area from the outer circle area and multiplies by thickness.
Use the Ring Wall panel when the annulus continues vertically as a hollow cylinder. Use Multiple Annuli when a project contains several separate rings. The Bags, Weight & Cost panel is useful after the concrete volume is already known.
Find the finished outer diameter or radius.
Find the finished inner circular diameter.
Use slab thickness or ring-wall height.
Get area, ft³ and yd³.
Check forms, allowance and supplier increments.
An annulus is created by subtracting a smaller circle from a larger circle with the same center. If you know diameters, divide each by two to obtain radius.
R is the outside radius and r is the inside radius.
Area of the full outside circle.
Area removed from the center.
The remaining annulus area.
Once ring area is known, multiply by concrete thickness or height to calculate volume. If the annulus is a flat slab, thickness is typically entered in inches and converted to feet before multiplication.
For cubic yards, divide cubic feet by 27.
Suppose a circular ring slab has a 10-foot outside diameter, a 6-foot inside diameter and is 6 inches thick. The outer radius is 5 feet and the inner radius is 3 feet.
With a 10% planning allowance, the estimated order becomes approximately 1.02 cubic yards before supplier rounding.
Outer and inner diameters define the overall concrete ring. The radial ring width is the difference between the outer radius and inner radius. This is not the same as subtracting the diameters without dividing by two.
A 10 ft OD and 6 ft ID produces a 2 ft radial ring width.
| Outer Diameter | Inner Diameter | Ring Area | 6 in Thick Volume | Cubic Yards |
|---|---|---|---|---|
| 6 ft | 2 ft | 25.13 ft² | 12.57 ft³ | 0.47 yd³ |
| 8 ft | 4 ft | 37.70 ft² | 18.85 ft³ | 0.70 yd³ |
| 10 ft | 6 ft | 50.27 ft² | 25.13 ft³ | 0.93 yd³ |
| 12 ft | 8 ft | 62.83 ft² | 31.42 ft³ | 1.16 yd³ |
| 16 ft | 10 ft | 122.52 ft² | 61.26 ft³ | 2.27 yd³ |
A circular ring wall is mathematically a hollow cylinder. Instead of a thin slab thickness, use the full vertical wall height. The cross-sectional plan area is still the annulus area between outside and inside radii.
Multiply by quantity if the project contains more than one identical ring wall.
This geometry can be useful for circular tank bases, planter walls, pits, monument rings and similar concrete elements. It does not determine wall reinforcement, minimum thickness, lateral resistance or structural adequacy.
For solid round elements, use the Concrete Column Calculator or Concrete Pier Calculator.
Projects sometimes include several rings of different sizes—for example multiple planter bases, equipment pads or monument rings. Calculate each annulus independently, then add the exact volumes before applying the project-wide planning allowance.
The Multiple Annuli panel accepts three rings. If you have more, group identical rings or run the calculator in several passes and add the results.
For small concrete rings, bagged concrete may be practical. Once planned cubic feet are known, divide by the selected bag yield and round up.
| Bag Size | Approx. Yield Used | Bags per 10 ft³ | Bags per yd³ |
|---|---|---|---|
| 40 lb | 0.30 ft³ | About 34 | About 90 |
| 60 lb | 0.45 ft³ | About 23 | About 60 |
| 80 lb | 0.60 ft³ | About 17 | About 45 |
Verify actual manufacturer yield before buying or batching materials.
Concrete weight is useful for material handling, demolition estimates, lifting and dead-load planning. The calculator multiplies concrete volume by the selected density.
For additional conversions, use Concrete Volume to Weight.
The cost panel multiplies cubic yards by a local concrete price. This represents basic concrete material only and does not include delivery, short-load fees, pump service, reinforcement, formwork, labor or finishing.
For truck and delivery planning, use the Concrete Delivery Calculator.
Ring-shaped concrete requires both outside and inside form control. Even small dimensional errors can materially change concrete volume because both circular areas are squared in the formula. Measure finished inside and outside diameters at multiple locations if the forms are not perfectly rigid.
Reinforcement may follow circular, radial or project-specific detailing. Material quantity does not establish bar size or spacing. Use the Concrete Rebar Weight Calculator after reinforcement is specified.
For concrete design, construction and material guidance, consult the American Concrete Institute (ACI) and the National Ready Mixed Concrete Association (NRMCA). Verify finished dimensions, reinforcement, concrete strength, supplier increments and project requirements before placing concrete.
Visualize outer radius, inner radius, ring width and concrete thickness for an annular pour.
Answers about concrete ring area, annular volume, ring walls, bags, weight and cost.