Free Concrete Calculator – Cubic Yards, Bags & Cost for Any Pour

Related calculators: If you’re framing walls above your concrete, use the wall framing calculator for stud counts and material costs. Need to spec insulation for the space? Try the insulation R-value calculator.

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How Many Bags of Concrete Do I Need?

The number of concrete bags you need depends on the project size and the bag weight you choose. A standard 80 lb bag of concrete mix yields approximately 0.60 cubic feet of mixed concrete. A 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields roughly 0.30 cubic feet. To calculate bags needed, divide your total cubic feet by the yield per bag.

For example, a 10×10 foot concrete slab at 4 inches thick requires 33.3 cubic feet (or 1.23 cubic yards). That equals approximately 56 bags of 80 lb concrete mix. Always buy 10% extra to account for spillage, waste, and uneven subgrades.

Concrete Calculator for a Slab: Common Sizes

Here are pre-calculated concrete volumes for the most common slab sizes at standard 4-inch thickness. A 10×10 slab at 4 inches needs 1.23 cubic yards (56 bags of 80 lb mix). A 12×12 slab at 4 inches needs 1.78 cubic yards (80 bags). A 20×20 slab at 4 inches needs 4.94 cubic yards (223 bags). A 24×24 slab at 4 inches needs 7.11 cubic yards (320 bags). Increasing thickness to 6 inches boosts the 10×10 requirement to 1.85 cubic yards (84 bags) and a 20×20 to 7.41 cubic yards (334 bags).

Above roughly 1.5 cubic yards, ready-mix is usually cheaper once the short-load fee is included; below that, run both numbers. At the prices this calculator ships with — $6.75 per 80 lb bag against $220 per cubic yard plus a $40-per-yard short-load charge under a 4 yard plant minimum — bags and ready-mix break even between 1.25 and 1.5 cubic yards, so a 1-yard pour is still cheaper in bags. The calculator runs both totals and tells you which is cheaper at your own prices. Ready-mix typically costs $180 to $250 per cubic yard delivered, while bagged concrete runs $5 to $7 per 80 lb bag — making pre-mix the economical choice at scale.

How Much Concrete for a Footing?

Concrete footings support the structural load of walls, columns, and posts. A typical residential continuous footing is 12 inches wide and 8 inches deep (16×8 under a two-storey wall). The IRC minimum thickness is 6 inches, which is a floor, not a typical. To calculate footing volume, multiply width x depth x length in feet, then convert to cubic yards by dividing by 27.

For a 40-linear-foot footing that is 12 inches wide and 8 inches deep: 1 ft x 0.67 ft x 40 ft = 26.67 cubic feet = 0.99 cubic yards. Footing concrete typically requires a mix with higher compressive strength — use 3,000 PSI or higher for load-bearing applications. Add 10 to 15% for form overpour and waste.

Concrete for Fence Posts and Deck Posts

Setting fence posts or deck posts in concrete requires calculating the volume of a cylindrical hole. Use the formula: pi x radius squared x depth. A standard 10-inch diameter hole that is 3 feet deep holds approximately 1.64 cubic feet of concrete — about 4 bags of 60 lb mix (three 60 lb bags yield only 1.35 cubic feet, 18% short).

For a typical 6-foot privacy fence with posts every 8 feet spanning 100 linear feet, you need 14 posts (posts = spans + 1). At 4 bags per post, that is 56 bags of 60 lb concrete mix. Fast-setting concrete (like Quikrete Fast-Setting) is ideal for posts — no mixing required; simply pour dry mix into the hole and add water.

Cubic Yards vs. Cubic Feet: Quick Conversion

Concrete is ordered by the cubic yard from ready-mix suppliers, but most DIY measurements are in feet and inches. Here is the conversion: 1 cubic yard equals 27 cubic feet. To convert cubic feet to cubic yards, divide by 27. To convert cubic yards to cubic feet, multiply by 27.

Our concrete calculator handles all unit conversions automatically. Enter your dimensions in feet and inches and it will output both cubic feet and cubic yards, along with the number of 40, 60, or 80 lb bags needed.

Short Load Fees: What to Know Before Ordering Ready-Mix

Ready-mix concrete trucks carry a full load of 8 to 10 cubic yards. If your project requires less than the minimum load (typically 3 to 5 cubic yards depending on the supplier), you may be charged a short-load fee of $50 to $200 or more. Always ask your supplier about their minimum order and short-load charges before scheduling delivery.

For projects under 1 cubic yard, bagged concrete from a home improvement store is almost always the better choice. For projects between 1 and 3 cubic yards, compare the total cost of bags against the ready-mix short-load fee to decide which is more economical.

Gravel Base, Rebar and Wire Mesh Quantities

A slab needs a compacted gravel base of at least 4 inches, and that base has its own volume. Use the same arithmetic as the slab: length times width times base depth in feet, divided by 27. A 20 x 20 slab on a 4 inch base needs 400 x 0.333 = 133.3 cubic feet, or 4.94 cubic yards of gravel. Gravel is sold by the ton in most of the country; crushed stone runs roughly 1.4 tons per cubic yard, so that slab needs about 6.9 tons. Order 5 to 10 percent extra for compaction.

For reinforcement, #3 rebar on 18 inch centres in both directions needs roughly (width divided by 1.5, plus one) bars running one way and (length divided by 1.5, plus one) the other. On a 20 x 20 slab that is 14 bars each way at 20 feet, about 560 linear feet, plus 10 percent for laps. Welded wire mesh is sold in 5 x 150 foot rolls covering 750 square feet before overlap; a 400 square foot slab takes one roll with room for the 6 inch side laps. Chair the steel up to the middle third of the slab depth, not on the ground.

What Does a Concrete Slab Cost Per Square Foot?

Most people search by the square foot, not the cubic yard. Divide your total by the slab area. At 180 to 250 dollars per cubic yard delivered, a 4 inch slab uses 0.0123 cubic yards per square foot, so the concrete alone is roughly 2.20 to 3.10 dollars per square foot. A 6 inch slab is half again more. That figure is material only: it excludes the gravel base, reinforcement, forms, pump or wheelbarrow labour, finishing and sealing, which together usually cost more than the concrete. Use the calculator above for your volume, then divide by your square footage.

Concrete Mix Ratios by Project Type

Different concrete projects require different mix strengths. Standard residential slabs use a 3,000 PSI mix (1 part cement, 2 parts sand, 3 parts gravel). Footings and structural elements typically need 3,500 to 4,000 PSI. Decorative flatwork and countertops may use a 5,000 PSI mix for superior finish and durability.

Most bagged concrete mixes from brands like Quikrete, Sakrete, and ProMix are pre-blended to 4,000 PSI when properly mixed with the correct water-to-cement ratio. Use the minimum water needed for workability — too much water weakens the final strength significantly.

Frequently Asked Questions About Concrete Calculations

How thick should a concrete driveway be?

Residential driveways should be at least 4 inches thick for passenger vehicles. If you park heavy trucks or RVs, use 5 to 6 inches for better load capacity and crack resistance.

How thick should a garage floor be?

A standard garage floor slab is 4 inches thick. For workshops with heavy equipment, 5 to 6 inches is recommended. Always pour on a compacted gravel base of at least 4 inches to prevent settling and cracking.

How long does it take concrete to cure?

Concrete reaches 70% of its final strength in 7 days and full strength (typically 4,000 PSI) at 28 days. Keep newly poured concrete moist and avoid heavy traffic for at least 24 to 48 hours. Avoid freezing temperatures during the first 24 hours of cure.

Do I need rebar in a concrete slab?

For structural slabs, footings, and driveways, yes. Use #3 rebar (3/8 inch) on 18-inch centers for most residential applications. Fiber mesh can substitute for light-duty flatwork like sidewalks and patios but is not a replacement for rebar in load-bearing applications.