Concrete Volume Calculator Formula
Understand the math behind the concrete volume calculator. Each variable explained with a worked example.
Formulas Used
Volume (Cubic Feet)
cubic_feet = length * width * depthVolume (Cubic Yards)
cubic_yards = length * width * depth / 27Variables
| Variable | Description | Default |
|---|---|---|
length | Length(feet) | 20 |
width | Width(feet) | 10 |
depth | Depth(feet) | 0.5 |
How It Works
How to Calculate Concrete Volume
Formula
Cubic Feet = Length (ft) x Width (ft) x Depth (ft)
Cubic Yards = Cubic Feet / 27
There are 27 cubic feet in one cubic yard (3 x 3 x 3). Always order 5-10% extra concrete to account for spillage, uneven ground, and slight over-excavation.
Worked Example
A concrete pad 20 ft long, 10 ft wide, and 0.5 ft (6 inches) deep.
- 01Cubic feet = 20 × 10 × 0.5 = 100 cu ft
- 02Cubic yards = 100 / 27 = 3.70 cu yd
- 03Order approximately 4 cubic yards to allow for waste.
When to Use This Formula
- Ordering ready-mix concrete for a driveway, patio, or sidewalk where you need to tell the supplier exactly how many cubic yards to deliver.
- Estimating the number of bags of concrete mix needed for a small project like a fence post, mailbox base, or shed foundation.
- Calculating the volume for a concrete slab with non-uniform thickness, such as a garage floor that slopes toward a drain.
- Planning a footing or foundation pour where the depth, width, and length must be converted from feet and inches into cubic yards for the order.
- Budgeting a construction project where concrete cost per cubic yard is the largest material expense.
Common Mistakes to Avoid
- Forgetting to convert all measurements to the same unit before multiplying — mixing feet for length and inches for depth without converting will give an answer off by a factor of 12.
- Not adding a waste factor — concrete work typically requires 5-10% extra to account for spillage, uneven subgrade, and form irregularities. Ordering the exact calculated volume almost always leaves you short.
- Confusing cubic feet with cubic yards — there are 27 cubic feet in one cubic yard, and ready-mix concrete is sold by the cubic yard. Dividing by 3 instead of 27 is a common error that triples your order.
- Ignoring the shape of the pour — using length x width x depth only works for rectangular slabs. Cylindrical piers, tapered footings, and curved paths require different volume formulas.
Frequently Asked Questions
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. There are 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft = 27 cu ft).
How much extra concrete should I order?
Order 5-10% more than the calculated amount to account for spillage, uneven subgrade, and slight variations in formwork.
How do I convert depth from inches to feet?
Divide the depth in inches by 12. For example, 6 inches = 6/12 = 0.5 feet.
How much does a cubic yard of concrete weigh?
A cubic yard of standard concrete weighs approximately 4,000 lbs (about 2 tons). This is useful for planning equipment needs — a standard concrete truck holds 8-10 cubic yards, weighing 32,000-40,000 lbs when loaded.
Learn More
Guide
How to Calculate Concrete Needed for Any Project
Learn how to calculate the exact amount of concrete needed for slabs, footings, columns, and other projects. Covers volume formulas, waste factors, and ordering tips.
Ready to run the numbers?
Open Concrete Volume Calculator