Concrete tools

Circular Slab Concrete Calculator — Volume & Bags

Enter the actual diameter and concrete depth for one or more identical circular slabs or pads to estimate volume and complete bag quantities.

Diameter-based circle geometry
Metric & imperial inputs
Final-project bag rounding

Circular slab geometry

Enter the actual circular pour dimensions

Measurement system

Slab diameterftMeasure straight across the full circle through its center.Concrete depthinEnter the actual project thickness. BuildNumbers does not select slab thickness.Identical circular slabsqty

Quantity only — BuildNumbers does not choose structural thickness, reinforcement, concrete strength, subbase, edge thickening, frost protection, or load capacity.

Extra allowance%Adjust for field variation, placement loss, and form tolerances.Bag sizeApproximate manufacturer-published yields; verify your product.

Optional price per 80 lb bag$Uses the supplier price you enter for the current 80 lb bag. No live prices are fetched.Currency labellabelDisplay only. BuildNumbers does not convert currencies or exchange rates.

This device

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Transparent by design

Circular slab concrete formula

BuildNumbers converts the entered diameter to radius, calculates the full circle area, then multiplies by the entered concrete depth. Identical pours are combined before allowance and package rounding.

Radiusr = diameter ÷ 2Plan areaA = π × r²One slabV = A × depthOrder volumeV_order = (V × quantity) × (1 + allowance ÷ 100)

This is a material quantity calculation only. It does not determine structural slab thickness, reinforcement, subbase, edge conditions, or load capacity.

Worked example

One 12 ft diameter × 4 in circular slab

This controlled arithmetic example demonstrates circle geometry and final bag rounding. It is not a recommended slab thickness or design.

01
Radius12 ÷ 2 = 6 ft
02
Circle areaπ × 6² = 113.097 ft²
03
Concrete volume113.097 × (4 ÷ 12) = 37.699 ft³
04
80 lb bags37.699 ÷ 0.60 = 62.832; round up once

Verified result1.396 yd³ and 63 × 80 lb bags at 0% allowance

Confirm the actual project diameter, depth, structural requirements, and selected product yield before ordering.

Common questions

Circular slab concrete calculator FAQ

Circle quantity guidance without hidden structural assumptions.

How do I calculate concrete for a circular slab?

Divide the diameter by two to get radius, calculate circle area as pi times radius squared, then multiply by the concrete depth. For identical slabs, multiply the unrounded volume by the quantity before allowance and package rounding.

Does this calculator choose the slab thickness?

No. BuildNumbers calculates quantity from dimensions you provide. Thickness, reinforcement, concrete strength, subbase, edge details, joints, loads, frost protection, drainage, and code requirements must come from project information and applicable requirements.

Can I calculate several identical circular pads at once?

Yes. Enter the number of identical circular slabs or pads. BuildNumbers combines their unrounded concrete volume first and rounds the final bag quantity only once.

Can I use metric dimensions?

Yes. Metric mode uses meters for diameter and centimeters for concrete depth. Switching systems converts the values already entered.

How are concrete bags calculated?

The allowance-adjusted total is divided by the selected approximate manufacturer-published bag yield, then rounded upward to a complete bag. Verify the exact product yield before purchase.

Continue planning

Use the geometry that matches the pour.

GuideHow Much Concrete for Circular Slabs?Diameter, depth & bags LiveRectangular Slab ConcreteLength × width × depth