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Circular Concrete Slab Calculator

Calculate exact cubic yards, cubic feet, and premix bags for circular slabs and round pads.

How Much Concrete Do I Need for a Round Slab?

A standard 8-foot diameter round concrete slab poured 4 inches thick requires 16.76 cubic feet (0.621 cubic yards net), or 18.43 cubic feet (0.683 cubic yards) with a standard 10% waste buffer. This equals 31 eighty-pound bags, 41 sixty-pound bags, or 62 forty-pound bags of premixed concrete. For a 10-foot round patio pad at 4 inches thick, you will need approximately 0.97 cubic yards net and about 1.07 cubic yards with 10% waste (around 48 eighty-pound bags).

Interactive Circular Concrete Slab Calculator

Interactive Calculator

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Currently measured in ft. Full circular span across the center of the pad.

Radius = half the diameter: 4 ft

Currently measured in in. 4 inches is standard for patios and fire pits; 6 inches for heavy loads.
Advanced settings(Extra to order: 10%, 80-lb yield: 0.6 cu ft/bag, 60-lb yield: 0.45 cu ft/bag, 40-lb yield: 0.3 cu ft/bag)▼
Default: 10% waste bufferCurrently measured in %. Allowance for circular form flexing, subgrade dips, and spillage.
typical value, change to match your productCurrently measured in cu ft / bag. Volume produced by one standard 80-lb premix bag.
typical value, change to match your productCurrently measured in cu ft / bag. Volume produced by one standard 60-lb premix bag.
typical value, change to match your productCurrently measured in cu ft / bag. Volume produced by one lightweight 40-lb premix bag.
What to buy / main result

Buy 0.68 cu yd of concrete (18.4 cu ft with waste) — approx. 31 eighty-pound bags (or 41 sixty-pound bags).

How the Circular Slab Volume is Calculated:

The volume of a circular slab equals the circular face area (π × radius²) multiplied by thickness, converted to cubic yards by dividing cubic feet by 27.

Net Volume

0.621 cu yd net

0.474 m³ net16.76 cu ft net (before waste)
Concrete to Order

0.683 cu yd with waste

0.522 m³ with waste18.43 cu ft with waste (+10%)
80-lb Bags (Premix)

31 bags

At 0.6 cu ft/bag with waste
60-lb Bags (Premix)

41 bags

At 0.45 cu ft/bag with waste
40-lb Bags (Premix)

62 bags

Lightweight retail bags (at $0.3 cu ft/bag with waste)
Suitable for Bagged PremixAt under 1.5 cubic yards, this volume can be mixed on site using a portable electric mixer or wheelbarrow.
Important Installation & Material Notes
  • This estimates a SOLID round pad.
  • Pads usually sit on a compacted gravel base (typically 4 inches of crushed stone).
  • Heavy loads such as hot tubs, sheds or vehicles may need thicker concrete or reinforcement; ask a local pro.
  • Check your local code for sub-base and thickness requirements before forming your slab.

Pouring a round tube or column footing? Use the Concrete Footing Calculator. Rectangular pad? Use the Concrete Slab Calculator.

How the Circular Slab Formula Works

A round concrete pad is geometrically a short cylinder. Calculating its volume requires finding the circular surface area and multiplying by slab thickness:

1. Radius (ft) = Diameter (ft) ÷ 2

2. Thickness (ft) = Thickness (inches) ÷ 12

3. Net Volume (cu ft) = π × (Radius)² × Thickness (ft)

4. Net Volume (cu yd) = Net Volume (cu ft) ÷ 27

5. Order Volume with Waste = Net Volume × (1 + Waste % ÷ 100)

6. Premix Bags = ⌈Order Volume (cu ft) ÷ Bag Yield (cu ft)⌉

80-lb Premix BagYields ~0.60 cu ft. Takes 45 bags to equal 1 cubic yard.
60-lb Premix BagYields ~0.45 cu ft. Takes 60 bags to equal 1 cubic yard.
40-lb Premix BagYields ~0.30 cu ft. Takes 90 bags to equal 1 cubic yard.

Worked Calculation Examples

Example 1: 8 ft Diameter Round Pad at 4 in Thickness (10% Waste)

Scenario: Pouring a circular backyard fire pit pad measuring 8 feet across and 4 inches deep.

  • Radius: 8 ft ÷ 2 = 4 ft.
  • Thickness in feet: 4 in ÷ 12 = 0.3333 ft.
  • Net volume: π × (4)² × (4 ÷ 12) = 16π ÷ 3 = 16.76 ft³ net (0.621 yd³ net).
  • Order volume with 10% waste: 16.755 × 1.10 = 18.43 ft³ with waste (0.683 yd³ with waste).
  • 80-lb bags: ⌈18.431 ÷ 0.60⌉ = 31 bags.
  • 60-lb bags: ⌈18.431 ÷ 0.45⌉ = 41 bags.
  • 40-lb bags: ⌈18.431 ÷ 0.30⌉ = 62 bags.

Example 2: 12 ft Diameter Round Gazebo Slab at 6 in Thickness (10% Waste)

Scenario: Pouring a heavy-duty 12-foot round base for a cedar gazebo at 6 inches thick.

  • Radius: 12 ft ÷ 2 = 6 ft.
  • Thickness in feet: 6 in ÷ 12 = 0.50 ft.
  • Net volume: π × (6)² × 0.50 = 18π = 56.55 ft³ net (2.094 yd³ net).
  • Order volume with 10% waste: 56.549 × 1.10 = 62.20 ft³ with waste (2.304 yd³ with waste).
  • Ready-mix recommendation: Volume exceeds 1.5 yd³; ready-mix truck delivery is strongly advised.
  • If bagged: ⌈62.204 ÷ 0.60⌉ = 104 eighty-pound, 139 sixty-pound, or 208 forty-pound bags.

Example 3: Metric Round Pad (2.5 m Diameter × 100 mm Thickness, 10% Waste)

Scenario: Estimating concrete for a 2.5-meter round decorative garden feature at 100 mm thick.

  • Radius: 2.5 m ÷ 2 = 1.25 m.
  • Thickness in meters: 100 mm ÷ 1,000 = 0.10 m.
  • Net volume: π × (1.25)² × 0.10 = 0.491 m³ net (17.33 ft³ net).
  • Order volume with 10% waste: 0.491 × 1.10 = 0.540 m³ with waste (19.07 ft³ with waste).
  • Premix bags: 32 eighty-pound bags, 43 sixty-pound bags, or 64 forty-pound bags.

Common Mistakes When Measuring Circular Slabs

  • ✕
    Entering Diameter as Radius: The radius is exactly half the diameter. If you plug an 8-foot diameter into a radius field, the calculated volume will be four times larger than needed because radius is squared in the formula.
  • ✕
    Mixing Inches and Feet: Always divide thickness in inches by 12 before multiplying by square footage. Multiplying square feet directly by inches results in a 12-fold over-estimate.
  • ✕
    Omitting Waste on Uneven Ground: Excavated dirt subgrades are never perfectly flat. Circular forms (hardboard, Bender Board, or flexible steel) also bow outward under wet concrete pressure. A 10% waste buffer prevents running short before finishing the circle.
  • ✕
    Measuring Across the Wrong Chord: A circle's diameter must pass directly through the absolute center point. Measuring off-center measures a shorter chord and underestimates your total concrete volume.

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Installation Guide · 8 min readHow to Calculate Concrete Volume: Slabs, Footings & Bag Counts
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Frequently Asked Questions

How to calculate concrete for a circular slab?
To calculate concrete for a round pad: 1) Divide diameter by 2 to find radius; 2) Square the radius (radius × radius); 3) Multiply by 3.14159 (pi); 4) Multiply by thickness in feet (thickness in inches ÷ 12) to get cubic feet; 5) Divide cubic feet by 27 to get cubic yards. Finally, add 10% for waste and compaction.
How much concrete do I need for a 10 ft round pad?
A 10-foot round pad poured 4 inches thick requires about 0.97 cubic yards net (26.18 cu ft). With a standard 10% waste allowance, order about 1.07 cubic yards (28.80 cu ft with waste), which requires approximately 48 eighty-pound bags (or 64 sixty-pound bags).
How many bags of concrete do I need for a circular slab?
Determine the order volume in cubic feet including 10% waste, then divide by the bag yield: 0.60 cu ft for an 80-lb bag, 0.45 cu ft for a 60-lb bag, or 0.30 cu ft for a 40-lb bag. Always round up to the next whole bag.
What is the difference between diameter and radius when ordering concrete?
Diameter is the full distance across the circle passing through the center point. Radius is the distance from the center point to the outer edge (half the diameter). The volume formula requires the radius; entering the diameter in place of radius results in 4 times too much concrete.