Concrete Wall Calculator Cubic Yards
- 80-lb Bags
- bags
- Result
- Wall Length
- 20 ft
- You entered
- Wall Height
- 8 ft
- You entered
A concrete wall calculator cubic yards tool works out the volume of concrete needed for a wall based on its length, height, and thickness, returning the result in cubic yards. You enter the wall’s dimensions in feet or inches, and the calculator gives you the exact cubic yards of concrete to order, plus the number of 80-pound bags if you’re mixing on site. This guide explains the formula, shows a worked example, and covers common mistakes so you order the right amount the first time.
Formula
The basic formula for concrete volume in cubic yards is:
Volume (cubic yards) = (Length × Height × Thickness) ÷ 27
Where:
- Length (L) = wall length in feet
- Height (H) = wall height in feet
- Thickness (T) = wall thickness in inches (converted to feet by dividing by 12)
- 27 = number of cubic feet in one cubic yard (3 ft × 3 ft × 3 ft)
Most contractors add a waste factor (usually 5–10%) to account for spillage, uneven subgrade, or slight over-excavation. The formula then becomes:
Volume with waste = (L × H × T/12) / 27 × (1 + waste%)
Worked Example
Using the default values from the calculator above: a wall 20 feet long, 8 feet high, 8 inches thick, with a 10% waste factor.
- Convert thickness to feet: 8 in ÷ 12 = 0.6667 ft
- Calculate volume in cubic feet: 20 ft × 8 ft × 0.6667 ft = 106.67 cu ft
- Convert to cubic yards: 106.67 ÷ 27 = 3.95 cu yd
- Add 10% waste: 3.95 × 1.10 = 4.35 cu yd
The calculator rounds to 4.35 cubic yards. For bag count, each 80-lb bag yields about 0.6 cubic feet of concrete. Total cubic feet needed = 4.35 × 27 = 117.45 cu ft. Bags = 117.45 ÷ 0.6 = 195.75, rounded up to 196 bags.
How to Use the Calculator
Enter your wall’s length and height in feet (or convert inches to feet first). For thickness, use inches because that’s how most walls are specified (e.g., 8-inch, 10-inch, 12-inch). The waste factor is optional but recommended—enter 0 if you want the exact volume without overage. The calculator returns two numbers: the cubic yards to order and the number of 80-lb bags if you’re mixing manually.
For a retaining wall, remember that the footing (the base that spreads the load) is separate. Measure the footing’s length, width, and depth, and calculate that volume separately, then add it to the wall volume. The calculator above handles only the vertical wall section.
Common Mistakes and How to Avoid Them
- Mixing units: Length and height are in feet, thickness in inches. Forgetting to convert inches to feet will overestimate volume by 12×.
- Forgetting the waste factor: Concrete shrinks slightly as it cures, and some is lost during placement. Always add 5–10% unless you’re ordering a precisely measured precast panel.
- Rounding too early: Round only at the final step. If you round the thickness conversion to 0.67 instead of 0.6667, a 100-foot wall will be off by about 0.2 cubic yards.
- Ignoring the footing: The wall’s footing often contains as much concrete as the wall itself. Measure it separately and add.
Understanding the Result
The output is in cubic yards because ready-mix concrete is sold by the cubic yard in the US and by the cubic meter in most other countries. One cubic yard equals 27 cubic feet or about 0.765 cubic meters. If you’re ordering from a ready-mix plant, you’ll typically order in whole or half-yard increments—round up to the nearest 0.5 yard to avoid a short load. For example, 4.35 cubic yards would become 4.5 yards.
If you’re mixing bags, each 80-lb bag yields roughly 0.6 cubic feet, a 60-lb bag yields about 0.45 cubic feet, and a 40-lb bag yields about 0.3 cubic feet. The calculator uses 80-lb bags, but you can adjust by dividing the total cubic feet by the yield of your chosen bag size.
Cost Considerations
While the calculator gives volume, cost depends on your local ready-mix price per cubic yard, which varies by region and quantity. A typical short load (under 5 yards) often carries a surcharge. For small walls, bagged concrete is usually more expensive per cubic yard but avoids the delivery fee and lets you mix only what you need. For large walls, ready-mix is more economical.
To estimate cost, multiply the cubic yards (including waste) by your local price per yard, then add delivery and any short-load fees. For bagged concrete, multiply the number of bags by the per-bag price.
Safety and Practical Tips
- Always wear gloves and eye protection when handling concrete—it’s caustic.
- For walls taller than 4 feet, consult a structural engineer. The wall must resist lateral earth pressure, and the footing size and rebar spacing are critical.
- Concrete should be placed within 90 minutes of mixing. Plan your crew and tools before the truck arrives.
- If using bags, mix in small batches—one 80-lb bag at a time—unless you have a mechanical mixer.
- Order a little extra (the waste factor) but not too much—unused ready-mix cannot be returned.
Alternatives to a Concrete Wall
If you’re building a non-structural garden wall or a planter, consider concrete block (CMU) filled with mortar, which uses far less concrete than a poured wall. For retaining walls, interlocking concrete blocks or segmental retaining wall systems are easier for DIY and don’t require forms. For very tall walls, reinforced concrete is typically the only option, but you’ll need engineering input.
Frequently Asked Questions
How many 80-lb bags of concrete do I need for a 10×8 wall that is 6 inches thick?
First calculate volume: 10 ft × 8 ft × (6/12) ft = 40 cubic feet. That’s 40 ÷ 27 = 1.48 cubic yards. Add 10% waste = 1.63 cubic yards, which is 44 cubic feet. Each 80-lb bag covers 0.6 cu ft, so you need 44 ÷ 0.6 = 73.3, rounded up to 74 bags.
Should I order ready-mix or use bags for a small wall?
For walls under about 1 cubic yard (27 cu ft), bags are often practical because ready-mix companies charge a short-load fee that can double your cost. For 1–2 cubic yards, compare the delivery fee against the bag price. Above 2 yards, ready-mix is almost always cheaper and faster.
Why does my calculator result differ from the ready-mix company’s estimate?
Most ready-mix companies add their own waste factor (often 10–15%) and round up to the next half-yard. Also, they may include the footing in their measurement. Make sure you’re comparing the same dimensions and waste percentage.
Can I use this calculator for a concrete block wall?
No—this calculator is for poured concrete (cast-in-place). A concrete block wall uses hollow blocks filled with mortar and sometimes grout, so the concrete volume is much less. For block walls, calculate the number of blocks instead (each 8×8×16 block covers 1.125 sq ft) and estimate mortar separately.
What if my wall has openings for windows or doors?
Subtract the volume of the openings from the total wall volume. For a window 3 ft × 4 ft in a 6-inch-thick wall, subtract 3 × 4 × 0.5 = 6 cubic feet (0.22 cu yd). Do this before adding the waste factor.