Concrete Calculator

How to calculate how much concrete you need

Concrete is sold by the cubic yard for ready-mix delivery and by weight per bag for smaller DIY projects. Getting the volume right before you start saves money, avoids cold joints from stopping mid-pour, and prevents the frustration of running out of material before the form is full.

The calculation itself is simple geometry: length × width × thickness for rectangular pours, or π × radius² × depth for cylindrical shapes like post holes and columns. The challenge for most homeowners is remembering to convert inches to feet, accounting for waste, and choosing between ready-mix and bagged concrete based on the volume of the project.

This calculator handles all of that - enter your dimensions, select the shape, choose a bag size, and get an instant result in cubic yards, cubic feet, and cubic meters, along with a bag count and optional cost comparison.

Cubic yards vs. cubic feet vs. cubic meters

Understanding concrete volume units is essential before you order. Here is how they relate:

• Cubic yard (yd³): The standard unit for ordering ready-mix concrete in the United States. One cubic yard = 27 cubic feet. Most ready-mix trucks carry 8–10 cubic yards per load.
• Cubic foot (cu ft): Useful for calculating bag quantities. An 80-lb bag of concrete mix covers approximately 0.60 cubic feet. A 60-lb bag covers 0.45 cu ft and a 40-lb bag covers 0.30 cu ft.
• Cubic meter (m³): The standard unit in metric countries. One cubic meter = 35.31 cubic feet = 1.308 cubic yards.

A common mistake is confusing cubic feet with cubic yards. A 10 × 10 ft patio at 4 inches thick is 33.3 cubic feet - which is only 1.23 cubic yards. Ordering in cubic feet instead of cubic yards would result in ordering far too little material.

Remember: always convert thickness from inches to feet before calculating. Four inches thick = 4 ÷ 12 = 0.333 feet.

Bagged concrete vs. ready-mix: which should you use?

The decision between bagged and ready-mix concrete comes down to project volume and accessibility. Here is how to choose:

Use bagged concrete when:

• Your project requires less than 1 cubic yard (27 cubic feet)
• You are setting fence posts, mailbox posts, or deck footings
• The site is difficult for a concrete truck to access
• You want to work at your own pace and mix one bag at a time
• The project can be done over multiple days

Use ready-mix concrete when:

• Your project is larger than 1 cubic yard
• You are pouring a driveway, patio, garage floor, or foundation
• You need a consistent, engineered mix for structural applications
• Time matters - ready-mix is placed faster than hand-mixing bags

Cost comparison

Bagged 80-lb concrete typically costs $7–$10 per bag at retail, yielding about 0.60 cu ft. That works out to $315–$450 per cubic yard for bagged concrete. Ready-mix typically runs $130–$200 per cubic yard plus a delivery charge of $50–$150, with minimum orders of 1 yard.

For volumes under 1 cubic yard, bagged concrete is usually cheaper once you factor in the ready-mix minimum order and delivery fee. For larger pours, ready-mix becomes significantly more economical.

What thickness do you need?

Slab thickness is one of the most important decisions in a concrete project. Using the right thickness for the application saves money and ensures durability.

• 4 inches: Standard for residential patios, sidewalks, and garden paths with no vehicular traffic. The most common thickness for DIY pours.
• 4–6 inches: Garage floors and areas with occasional light vehicle traffic. Use 5–6 inches if heavy trucks or RVs will park on the surface.
• 6 inches: Driveways with regular vehicle traffic, agricultural slabs, and areas subject to freeze-thaw cycling.
• 8+ inches: Commercial truck traffic, heavy equipment pads, and structural applications. These typically require engineering review.

Structural slabs, foundations, and load-bearing footings must meet local building code requirements for thickness, reinforcement, and concrete strength. Consult a licensed structural engineer for any structural concrete work.

Calculating concrete for post holes

Post holes use a cylindrical volume formula: π × r² × depth, where r is the radius of the hole in feet and depth is the hole depth in feet. This calculator handles this automatically when you select the Column / Post Hole shape.

For a standard 10-inch diameter post hole that is 36 inches (3 feet) deep:

• Radius = 5 inches = 0.417 feet
• Volume = π × 0.417² × 3 = 1.637 cubic feet
• At 80 lb bags (0.60 cu ft each): 1.637 ÷ 0.60 = 2.73 → 3 bags per hole

The depth of post holes should extend below the frost line in your area. In cold climates, frost depth can reach 48 inches or more. Shallow post holes in freeze-thaw environments will heave over time, causing fence or deck movement. Contact your local building department for the required frost depth in your area.

For multiple post holes, multiply the per-hole volume by the number of holes and add 10–15% waste. Buy at least one extra bag per hole as insurance.

Concrete placement best practices

The right volume is only half the equation. Good placement technique and proper curing make the difference between concrete that lasts decades and concrete that cracks within a year.

Before the pour

• Compact the base: Concrete placed on soft or disturbed soil will settle unevenly. Compact the subgrade and add a 4-inch gravel base for slabs.
• Set forms precisely: Level forms produce level slabs. Stake and brace wooden forms so they do not move during the pour.
• Moisten the subgrade: Lightly wet the base before placing concrete to prevent the ground from drawing water out of the mix too quickly.

During the pour

• Consolidate concrete by rodding or tapping forms to eliminate air pockets.
• Use a screed board to level the surface. Work from one end of the slab toward the other.
• Float the surface to close voids and bring paste to the top.
• Do not overwork the surface or add water to the mix - both weaken the final product.

Curing

Concrete gains most of its design strength over 28 days, not overnight. Keep the surface moist for at least 7 days by covering with wet burlap, plastic sheeting, or a liquid curing compound. Do not allow concrete to dry out in the first week.

Avoid placing concrete in temperatures below 40°F (4°C) or above 90°F (32°C) without special precautions. Cold weather slows curing significantly and frost can permanently damage fresh concrete.

Common concrete calculation mistakes

• Thickness in wrong units: Entering 4 for thickness when the calculator expects feet (not inches) will give a result 12× too large. Always check which unit the form expects.
• Forgetting waste: Irregular ground, form variation, and spillage regularly add 5–10% to the theoretical volume. Never plan for 0% waste on a concrete project.
• Confusing cubic feet and cubic yards: A bag count calculation requires cubic feet; a ready-mix order uses cubic yards. This calculator shows both.
• Under-ordering ready-mix: Ready-mix is priced by the yard and most suppliers charge for the full minimum load. Add an extra 10% and order the nearest full 0.5 yard increment to avoid a short load.
• Not accounting for post displacement: For post hole concrete, the volume of the post itself displaces some concrete. For large posts in small holes, this can be significant.