How Much Concrete Do I Need?
To find how much concrete you need, multiply the length by the width by the depth of the pour in feet, then divide by 27 to convert cubic feet to cubic yards. A 10-foot by 10-foot patio poured 4 inches thick needs roughly 1.24 cubic yards before adding the standard 10 percent waste factor.
The formula is the same for slabs, footings, and columns. The inputs change, but the arithmetic does not. Every section below shows real numbers so you can work the calculation for your specific project.
Key facts
- Divide length × width × thickness (all in feet) by 27 to get cubic yards of concrete needed.
- A 4-inch slab requires 0.333 feet of depth in the formula; a 6-inch slab requires 0.5 feet.
- Order 10% more than your calculated volume to account for spillage, uneven subgrade, and consolidation.
- Ready-mix trucks typically deliver a minimum of 1 cubic yard and a maximum load of 10 cubic yards per truck.
- Bagged 80-pound concrete mixes yield approximately 0.6 cubic feet per bag, so 45 bags equal roughly 1 cubic yard.
- A residential driveway slab poured at 4 inches thick over a 400 square foot area requires approximately 5 cubic yards.
What Is the Formula for Calculating Concrete Volume?
The standard formula converts three linear measurements into cubic yards, which is the unit ready-mix suppliers price and deliver by.
How does the cubic yard formula work?
Cubic yards = (length in feet × width in feet × depth in feet) ÷ 27. The number 27 converts cubic feet to cubic yards because one cubic yard contains 27 cubic feet. Depth must be entered in feet, not inches. A 4-inch slab is 0.333 feet. A 6-inch slab is 0.5 feet. A 3.5-inch slab is 0.292 feet.
Worked example for a 20-foot by 30-foot driveway at 4 inches thick:
- 20 × 30 × 0.333 = 199.8 cubic feet
- 199.8 ÷ 27 = 7.4 cubic yards calculated
- 7.4 × 1.10 = 8.1 cubic yards ordered (10 percent waste added)
Always round up to the nearest half cubic yard when ordering. Suppliers do not sell fractions below 0.5 cubic yards, and running short mid-pour ruins the slab.
How Do I Calculate Concrete for Different Project Types?
Slabs, footings, columns, and steps each use the same cubic yard formula but measure different dimensions. Three project shapes cover almost every residential job.
How much concrete does a slab need?
A concrete slab uses length × width × depth ÷ 27. Residential patios and sidewalks are typically 4 inches thick. Driveways that carry passenger vehicles are poured at 4 to 5 inches. Driveways that carry trucks or RVs are poured at 5 to 6 inches. A 200-square-foot patio at 4 inches requires 2.5 cubic yards before waste. The same area at 6 inches requires 3.7 cubic yards.
How much concrete does a footing need?
A rectangular footing uses the same length × width × depth formula. A continuous footing for a 40-linear-foot wall, 12 inches wide and 12 inches deep, requires 1.48 cubic yards. A stepped footing or one with variable depth must be broken into segments and summed. Footing dimensions below frost depth vary by region. In Louisiana, frost depth is minimal, but typical residential footings run 12 to 24 inches deep depending on the structure and engineer specification.
How much concrete does a round column or post base need?
A cylindrical form uses: volume = 3.14159 × radius² × depth ÷ 27. A 12-inch-diameter form (radius 0.5 feet) poured 3 feet deep holds 0.087 cubic yards per column. Ten columns of that size require 0.87 cubic yards total. Tube form sizes are sold in 8-inch, 10-inch, 12-inch, and 16-inch diameters. Measure the radius from the center, not across the outside of the form.
Should I Use Ready-Mix or Bagged Concrete?
The break point is roughly 1 cubic yard. Below that volume, bagged concrete is practical. Above 1 cubic yard, ready-mix is faster, stronger, and more consistent.
When does bagged concrete make sense?
Bagged mix is practical for projects under 0.75 cubic yards, which is about 34 standard 80-pound bags. Each 80-pound bag yields 0.6 cubic feet mixed. At 0.6 cubic feet per bag, a 1-cubic-yard pour requires 45 bags, which is 3,600 pounds of dry material to mix and move by hand. For anything larger than a small post-base job or a utility pad under 50 square feet, the labour cost of bagged concrete exceeds the delivery premium for ready-mix.
When should I order ready-mix concrete?
Order ready-mix for any pour above 1 cubic yard, any pour requiring a specific PSI rating, or any pour that must be placed in under 90 minutes. Ready-mix arrives at a consistent 3000 PSI, 3500 PSI, 4000 PSI, or higher depending on the spec you order. Residential driveways typically use 3500 PSI. Exposed aggregate and stamped flatwork typically use 4000 PSI for better surface hardness. Minimum delivery loads are usually 1 cubic yard, and some suppliers charge a short-load fee under 3 cubic yards.
What Waste Factor Should I Add to My Calculation?
Add 10 percent to every calculated volume as a baseline waste factor. Subgrade surfaces are never perfectly flat, so the actual depth varies across the pour area. Concrete consolidates slightly when vibrated. Some material stays in the chute and is lost. On irregular terrain or projects with many corners, increase the waste factor to 15 percent. On a simple rectangular slab on prepared ground, 10 percent is adequate. Ordering short costs a second delivery fee and leaves a cold joint in the slab where the two pours meet, which is a structural weakness.
What Are the Most Common Calculation Mistakes?
Four errors account for nearly every concrete shortage or significant overorder on residential projects.
- Converting inches to feet incorrectly. Entering 4 inches as 4 instead of 0.333 overstates the volume by a factor of 12. The calculation produces a number 12 times too large.
- Forgetting the waste factor. A calculation without a 10 percent buffer runs short on roughly 3 in 10 residential pours where subgrade is uneven.
- Measuring the wrong dimension. Footings are measured to the inside of the form, not the outside. A 2-inch-thick form board on each side of a 16-inch footing makes the interior dimension 12 inches, not 16 inches.
- Mixing units mid-calculation. Length in feet, width in inches, and depth in feet produces a meaningless number. All three dimensions must be in feet before multiplying.
Is This a DIY Job or Does It Need a Contractor?
Pours under 1 cubic yard on a flat surface with no rebar, no forms to build, and no finish requirement beyond a rough broom are within reach of a capable DIYer. Post bases, small utility pads, and repair fills fall into that category. A 400-square-foot driveway at 4 inches thick is 5 cubic yards of concrete arriving in a single truck, which must be placed, screeded, floated, and jointed before the surface stiffens. In summer in Louisiana, working time is 45 to 60 minutes from discharge. That pace demands at least 3 to 4 experienced workers and the right tools. A slab with reinforcing steel, a specific PSI spec, or a decorative finish should be contracted. Getting the volume calculation wrong by 10 percent on a 5-cubic-yard pour is a 55-gallon overorder or a cold joint in the middle of the driveway.
Does Subgrade Condition Change How Much Concrete Is Needed?
In Greater New Orleans and surrounding parishes, soft alluvial clay and organic silt compress under a slab's weight, which increases effective depth and can consume 5 to 12 percent more concrete than the calculation predicts. In areas where the subgrade has settled or been disturbed, field experience here shows that actual poured volume consistently runs above the formula by that margin. Compacting and crowning the subgrade before forming is the correct fix, not simply ordering extra concrete. A subgrade that is not compacted to a uniform surface produces a slab with variable thickness, which creates inconsistent strength across the pour. On the north shore, sandy subgrade drains faster but erodes under the form if not contained, which can open voids under the slab edge. In both cases, the calculation is accurate only after the subgrade is final-graded and measured.
If you are working through the numbers for a driveway, patio, or footing in Greater New Orleans and want a material quantity confirmed against an actual site condition, a concrete contractor can walk the job, verify the subgrade, and give you a volume and a cost together. That review takes less time than a second concrete delivery.
Common questions
- How many bags of concrete do I need for a 10x10 slab?
- A 10x10-foot slab at 4 inches thick requires 1.24 cubic yards of concrete, which equals approximately 56 standard 80-pound bags. Each bag yields 0.6 cubic feet, and 56 bags produce 33.6 cubic feet, or 1.24 cubic yards. Add a 10 percent buffer and purchase 62 bags. At that volume, ready-mix delivery is worth pricing as an alternative.
- How do I convert square feet to cubic yards for concrete?
- Square feet alone cannot convert to cubic yards without a depth. Multiply the square footage by the depth in feet, then divide by 27. A 300-square-foot area at 4 inches thick is 300 × 0.333 ÷ 27 = 3.7 cubic yards. Depth in inches must be divided by 12 before entering the formula.
- What is the minimum concrete order from a ready-mix truck?
- Most ready-mix suppliers require a minimum order of 1 cubic yard per delivery. Orders under 3 cubic yards often carry a short-load surcharge of $50 to $150 depending on the supplier and region. Confirm the minimum load and any surcharge with your local plant before calculating whether bagged concrete is cheaper for small pours.
- How much does concrete cost per cubic yard in 2026?
- Ready-mix concrete in the Greater New Orleans area runs $140 to $180 per cubic yard for standard 3000 to 3500 PSI residential mix in 2026. Higher PSI mixes, fiber reinforcement, or accelerating admixtures add $10 to $30 per cubic yard. Delivery fees, short-load charges, and fuel surcharges are separate and vary by supplier and distance.
- How thick should a concrete driveway be?
- A residential driveway that carries passenger vehicles is poured at 4 inches minimum, and 5 inches is a stronger standard in areas with soft subgrade. Driveways that carry trucks, RVs, or heavy equipment are poured at 6 inches. Thickness below 4 inches on a driveway produces a slab that cracks under repeated load within 3 to 7 years in most soil conditions.
- How long does concrete take to cure before I can drive on it?
- Foot traffic is safe after 24 to 48 hours. Passenger vehicle traffic is safe after 7 days. Full structural cure to design strength takes 28 days. Driving on a slab before 7 days risks surface cracking and tire impressions, particularly in hot weather when the surface cures faster than the interior of the slab.
Last updated August 5, 2026
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