How to Measure for Concrete (and Not Run Short)

Running out halfway through a pour is the expensive mistake. Measure inside the forms, convert everything to feet, split odd shapes into simple ones, and add a waste allowance. Here's the method with worked examples.

The one formula

Cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27

Concrete is sold by the cubic yard, which is 27 cubic feet. Bags are sold by what they yield: an 80 lb bag of standard mix makes about 0.60 ft³ (QUIKRETE), so a cubic yard is 45 bags.

Measure in the right places

• Inside the forms, not outside: the concrete fills the inside.
• Thickness in several places. Ground is rarely flat. Take five or more readings across the area and use the average.
• The real board size. A 2×4 form is 3½ in tall, not 4.

Example: readings of 3.5, 4, 4.5, 5 and 4 in average out to 4.2 in. Using 4 in would leave you about 5% short.

Inches to feet

Divide inches by 12, or use this table:

So 10 ft 6 in is 10.5 ft, and a 4 in slab is 0.333 ft thick. pourplot's calculator takes feet and inches directly, so you can skip this step.

Odd shapes

• L-shapes and T-shapes: split them into rectangles and add them up. Example: a 16 × 10 ft patio with an 8 × 6 ft extension, 4 in thick, is 2.57 yd³, or 2.82 yd³ (128 bags of 80 lb) with 10% extra.
• Circles: π × radius² × thickness. A 10 ft round pad at 4 in is 26.2 ft³ (0.97 yd³).
• Steps: treat each step as a block from the ground up and add them. The calculator has a Steps shape that does it for you.
• Tubes and post holes: use the circle formula with the height, times the number of holes.
• Thickened edges or footings under a slab: add them as separate pieces.

How much extra to add

Some concrete is always lost to spillage, form bulges, uneven subgrade and what's left in the mixer. pourplot adds 10% by default. Raise it if the ground is rough or you're pouring by hand. Lower it only for small, carefully formed pours. Ordering a little over is much cheaper than a second delivery or a cold joint where a short pour stopped.

A quick checklist

1. Sketch the pour and split it into rectangles, circles and steps.
2. Measure inside the forms; average the thickness.
3. Convert to feet (or let the calculator do it).
4. Add the pieces and the waste allowance.
5. Turn cubic yards into bags or a ready-mix order, and print the ticket.

Sources

• QUIKRETE Concrete Mix 1101 data sheet (bag yields)
• Concrete Network: concrete driveway thickness

Estimates for planning. Follow your plans, your supplier's instructions and your local building code.