1. Measure the full diameter through the center
Diameter is the straight-line distance from one edge of the circle to the opposite edge through its center. The volume formula needs radius, which is exactly half of that diameter.
r = diameter ÷ 2If the pour is not a full circle, do not force it into this formula. Split the project into geometry that matches the actual shape or use a tool designed for that geometry.
2. Calculate circular plan area
Once radius is known, the full circular plan area is pi times radius squared.
A = π × r²This is the horizontal plan area only. Concrete volume also needs the actual project depth.
3. Multiply the circle area by concrete depth
Use compatible units before multiplication. BuildNumbers normalizes Imperial and Metric inputs internally before calculating the volume.
V = A × depthThe entered depth is an input, not a recommendation. Structural slab design and site preparation remain separate decisions.
4. Combine identical pours before rounding
If several pads or slabs have the same diameter and depth, multiply the unrounded volume of one by the quantity. This avoids accumulating package-rounding error at every individual pour.
Vnet = V × quantity5. Apply a visible allowance
BuildNumbers keeps extra material explicit instead of hiding a fixed waste assumption inside the geometry.
Vorder = Vnet × (1 + allowance ÷ 100)6. Convert to ready-mix volume or complete bags
For bagged concrete, divide the final allowance-adjusted volume by the selected product yield and round upward once. Published yields are approximate, so verify the exact product before purchase.
A controlled 12 ft diameter × 4 in example has a 6 ft radius, a 113.097 ft² circle area, and 37.699 ft³ of concrete before allowance. That is 1.396 yd³. At 0.60 ft³ per 80 lb bag, 37.699 ÷ 0.60 = 62.832, so the purchase quantity is 63 complete bags. The dimensions are an arithmetic example, not a design recommendation.
7. Verify what the quantity estimate does not cover
The estimate does not include reinforcement, forms, subbase, excavation, thickened edges, footings, joints, finishing, delivery constraints, drainage, slope, or structural requirements. Check the actual project plan, site conditions, product data, and supplier ordering rules before purchase.
Run the measured dimensions
Calculate the circular concrete quantity.
Circular slab concrete FAQ
What measurements do I need for a circular concrete slab?
You need the full diameter measured through the center and the actual concrete depth from your project information. If several pours are identical, also record the quantity.
Why does the formula divide diameter by two?
Circle area uses radius. Radius is half of the full diameter, so the calculator converts diameter to radius before applying pi times radius squared.
Should I round bags for every circular pad separately?
No. For identical pours, combine the unrounded concrete volume first, apply the chosen allowance to the project total, then round the final bag quantity upward once.
Does this guide recommend slab thickness?
No. Thickness, reinforcement, subbase, concrete strength, edge details, joints, loads, frost protection, drainage, and code requirements are outside this quantity guide.