How Much Concrete Do You Need? Slabs, Footings, Curbs, and the Math Behind Each

Concrete is sold by the cubic yard in the US and the cubic meter elsewhere. Both come in trucks that hold a fixed amount (10 cubic yards is a typical full truck). Order too little and you're in a panic mid-pour. Order too much and you've paid for concrete you can't use. The math is straightforward but the project-specific adjustments (waste factor, depth variations, footing keyways) catch most DIYers. Here's how to calculate volume for the most common project types and what waste factor to use.

Last updated: May 2026

The Base Formula

Volume equals length times width times depth. The trick is keeping units consistent.

For US units (cubic yards):

Cubic yards = (length in feet x width in feet x depth in inches divided by 12) divided by 27

The 27 comes from 27 cubic feet in 1 cubic yard. The depth divided by 12 converts inches to feet so all dimensions are in the same unit.

For metric units (cubic meters):

Cubic meters = length in meters x width in meters x depth in meters

The concrete calculator handles both unit systems and the conversion automatically; the manual math above is for understanding what's happening.

Slabs (Patios, Driveways, Garage Floors)

The simplest case. Length times width times depth.

Standard depths:

• Patio: 4 inches (100 mm)
• Sidewalk: 4 inches
• Driveway (passenger vehicles): 4 inches over compacted base
• Driveway (heavy vehicles): 5 to 6 inches
• Garage floor: 4 inches
• Shed floor: 4 inches

Worked example: A 12 foot by 16 foot patio at 4 inches deep.

• Volume = (12 x 16 x 4 / 12) / 27 = 64 / 27 = 2.37 cubic yards
• Add 10% waste factor = 2.6 cubic yards
• Order: 3 cubic yards (round up to the nearest half-yard for delivery; you can't order partial yards)

Footings (Foundations, Posts, Footers)

Footings are typically rectangular trenches deeper than they are wide. The math is the same as slabs (length x width x depth) but applied per footing run.

Standard footing dimensions:

• Residential foundation: 16 inches wide by 8 inches deep (varies by load)
• Deck post footing: 12 to 16 inches diameter by 36 to 48 inches deep (frost depth dependent)
• Fence post footing: 8 to 12 inches diameter by 24 to 36 inches deep

Worked example: A 40 linear foot foundation footing at 16 inches wide and 8 inches deep.

• Width 16 inches = 1.33 feet, depth 8 inches = 0.67 feet
• Volume = (40 x 1.33 x 0.67) / 27 = 35.6 / 27 = 1.32 cubic yards
• Add 10% waste = 1.5 cubic yards

For circular post footings (cylindrical holes), the formula changes:

Cubic yards = (pi x radius squared in feet x depth in feet) divided by 27

For a 12 inch diameter (6 inch radius = 0.5 feet) by 36 inch deep (3 feet) footing:

• Volume = (3.14 x 0.25 x 3) / 27 = 2.36 / 27 = 0.087 cubic yards per footing
• Multiply by number of footings to get the total

Curbs and Edging

Curbs have a more complex cross-section (often L-shaped or trapezoidal) but the math is still volume = cross-sectional area x length.

Standard residential curb: 6 inches wide top by 8 inches tall, with a 4 inch base extending under adjacent paving.

For a simple rectangular curb (6 inches by 8 inches):

• Cross-section = 6 x 8 = 48 square inches = 0.33 square feet
• For 100 linear feet: volume = 0.33 x 100 = 33.3 cubic feet = 1.23 cubic yards
• Add 15% waste (curbs need higher waste because of formwork complications) = 1.4 cubic yards

Stairs

Stairs are the trickiest geometry because each tread requires a different volume calculation. The shortcut: treat the whole staircase as a triangular prism plus the volume of the treads.

For a 4 foot wide stair with 4 risers (each 7 inches tall, 11 inch tread depth):

• Total rise = 4 x 7 = 28 inches = 2.33 feet
• Total run = 4 x 11 = 44 inches = 3.67 feet
• Triangular cross-section area = (2.33 x 3.67) / 2 = 4.27 square feet
• Volume = 4.27 x 4 (width) = 17.1 cubic feet = 0.63 cubic yards
• Add 15% waste (formwork is complex) = 0.73 cubic yards

The Waste Factor

Always add a waste factor to your calculated volume. Common waste percentages:

• Slabs (rectangular, simple form): 5 to 10%
• Footings (in trench, simple form): 10%
• Curbs (formed cross-section): 15%
• Stairs (complex formwork): 15 to 20%
• Round columns (cylindrical forms): 10%

The waste factor accounts for: spillage during pour, slight over-excavation of the form, settling, and the inevitable "oh we need to top off this corner" reality of pouring concrete.

Ordering Considerations

Most ready-mix concrete suppliers have minimum order sizes (usually 1 cubic yard) and round up to the nearest half-yard. There's typically a "short load fee" for orders under 5 cubic yards because the truck is underutilized.

If you need more than 1 truck (over 10 cubic yards), the trucks need to arrive close enough together that the first pour doesn't set before the second arrives. Coordinate with your supplier for back-to-back delivery.

For very small jobs (under 1 cubic yard), bagged concrete from a hardware store is often more economical than a short-load delivery. A standard 80 pound bag yields about 0.6 cubic feet (so about 45 bags per cubic yard).

The Quick Calculator Workflow

For real projects, use the concrete calculator:

1. Pick the project type (slab, footing, column, stairs, curb).
2. Enter dimensions in your preferred units (feet/inches or meters).
3. The calculator outputs volume in cubic yards AND cubic meters AND number of 80 pound bags.
4. It includes the appropriate waste factor for the project type so you don't forget.

For multi-element projects (a slab plus footings plus stairs), calculate each separately, then sum the volumes and add a single project-level waste factor of 5%.

What to Do With Leftover Concrete

Concrete is often delivered in slight excess (you ordered for waste factor and didn't use it all). Don't throw it away into a random hole; cured concrete is hard to remove later. Better options: stepping stones (use a leftover form), small flat repair patches elsewhere on the property, fence post setting, or a planned-leftover purpose like a small pad for a propane tank or AC unit.