Question 1

How do you calculate square footage for a concrete slab or patio?

Accepted Answer

To calculate the square footage of a concrete slab, measure the outer formwork length and width in feet and multiply them together: Area (sq ft) = Length (ft) × Width (ft). For example, a patio measuring 20 ft by 12 ft is 240 square feet. If you have measurements with inches (such as 20 ft 6 in by 12 ft), convert 6 inches to 0.5 feet before multiplying (20.5 × 12 = 246 sq ft).

Question 2

How many square feet does 1 cubic yard of concrete cover?

Accepted Answer

The square footage covered by 1 cubic yard (27 cubic feet) depends on slab thickness: at a standard 4-inch thickness, 1 cubic yard covers exactly 81 square feet (27 ÷ (4/12) = 81 sq ft). At a 6-inch thickness (common for heavy driveways), 1 cubic yard covers 54 square feet. At a 3-inch sidewalk thickness, it covers 108 square feet.

Question 3

How do you convert concrete square footage to cubic yards?

Accepted Answer

To convert square footage to cubic yards, multiply your total square footage by the slab thickness in feet (thickness in inches divided by 12), then divide the result by 27: Cubic Yards = (Square Footage × [Thickness in inches ÷ 12]) ÷ 27. For a 300 sq ft patio at 4 inches thick: (300 × 0.333) ÷ 27 = 3.70 cubic yards.

Question 4

How many 80lb or 60lb bags of concrete do I need per 100 square feet?

Accepted Answer

For a 4-inch thick pour over 100 square feet, you will need approximately 33.33 cubic feet of concrete. Since one 80lb bag yields 0.60 cu ft, you need about 56 bags (80lb) per 100 sq ft. For 60lb bags (which yield 0.45 cu ft each), you need approximately 74 bags per 100 sq ft. Always add a 10% safety margin for ground unevenness.

Question 5

Should I measure concrete square footage from inside or outside the forms?

Accepted Answer

Always measure from the inside face of your wooden 2x4 or 2x6 form boards, as this represents the actual volume that wet concrete will fill. If measuring before digging, measure the outer perimeter and add 6 to 12 inches on all sides for working room and gravel sub-base grading.

Square Footage Calculator for Concrete — Slabs, Patios & Pours

Project Inputs & Specs

Visual Blueprint & Diagram

Now calculate how much concrete you need

Concrete Footprint & 4-Inch Slab Equations

Step-by-Step Calculation Rule:

How to Measure & Calculate Concrete Square Footage (Worked Example)

Frequently Asked Questions

Related Construction & Material Calculators

Concrete Calculator — Slabs, Footings, Columns & Steps

Concrete Slab Calculator — Patio, Driveway & Shed Slabs

Square Footage Calculator — Rooms, Shapes & Multiple Rooms (2026)