Ready Mix Concrete Calculator

Ready Mix Concrete Calculator: How to Calculate Concrete for Any Slab, Footing, or Project

Whether you’re pouring a patio, driveway, foundation, sidewalk, or a set of columns, getting your concrete quantity right is one of the most important things you can do before breaking ground. Order too little and the truck leaves before the pour is finished. Order too much and you’re paying for concrete that ends up hardening in the drum.

A ready mix concrete calculator takes the guesswork out of the equation. But to use any concrete calculator confidently — and to verify the numbers you get back — you need to understand the math behind it. This guide covers everything: the formula, worked examples for every common project type, cost estimates, bagged vs. ready mix comparisons, and the questions contractors and homeowners ask most.

What Is Ready Mix Concrete?

Ready mix concrete is concrete that is batched and mixed at a central plant, then delivered to the job site in a rotating drum truck in a plastic (workable) state. Unlike site-mixed or bagged concrete, ready mix is engineered to a precise specification — a defined water-to-cement ratio, aggregate blend, admixtures, and target compressive strength — before it ever leaves the plant.

Ready mix is the standard choice for any project requiring more than about 0.5 cubic yards of concrete. It delivers consistency, strength, and speed that site mixing simply cannot match at scale.

The three major types of ready mix delivery are:

• Transit mix: Dry ingredients loaded into the truck, water added in transit. The drum mixes en route to the site.
• Shrink mix: Partially mixed at the plant, mixing completed during transit.
• Central mix (wet mix): Fully mixed at the plant, delivered ready to pour.

For most residential and commercial projects in the US, transit mix is the most common delivery method.

The Ready Mix Concrete Formula

The core formula for calculating concrete volume is the same whether you’re using a concrete calculator, doing the math by hand, or calling your local CEMEX, Quikrete, or redi mix supplier:

Volume (cubic feet) = Length × Width × Depth

Volume (cubic yards) = (Length × Width × Depth) ÷ 27

All measurements must be in feet. Since depth is almost always given in inches, convert it first by dividing by 12.

Step-by-Step Calculation

Step 1: Measure length and width in feet.

Step 2: Convert depth from inches to feet (divide by 12).

Step 3: Multiply length × width × depth to get cubic feet.

Step 4: Divide by 27 to convert to cubic yards.

Step 5: Add 5–10% for waste, spillage, and uneven subgrade.

Example: A concrete slab 20 feet × 20 feet × 4 inches thick.

• Depth in feet: 4 ÷ 12 = 0.333 ft
• Volume: 20 × 20 × 0.333 = 133.3 cubic feet
• Cubic yards: 133.3 ÷ 27 = 4.94 cubic yards
• Add 10% waste: 4.94 × 1.10 = 5.44 cubic yards — order 5.5 yards

You Can Also Check: Concrete Block Calculator

How to Calculate Ready Mix Concrete by Project Type

Different projects call for different shapes. Here’s how to calculate concrete yards for every common application.

Concrete Slab Calculator

A slab is the most common ready mix application — driveways, patios, garage floors, basement floors, sidewalks, and shed pads.

Formula: (Length × Width × Depth in feet) ÷ 27

These figures do not include the 5–10% waste factor. Always add overage before ordering.

Concrete Footing Calculator

Footings support the load of walls, columns, and structures. They are typically poured before walls are built.

Formula: (Width in ft × Depth in ft × Length in ft) ÷ 27

Example: A continuous footing 24 inches wide × 12 inches deep × 60 feet long.

• Width: 2 ft, Depth: 1 ft, Length: 60 ft
• Volume: 2 × 1 × 60 = 120 cubic feet ÷ 27 = 4.44 cubic yards

Concrete Column Calculator

Columns are cylindrical. Use the circle area formula.

Formula: π × radius² × height, then divide by 27

Example: 10 concrete columns, each 12 inches diameter × 4 feet tall.

• Radius = 0.5 ft
• Volume per column = 3.14159 × (0.5)² × 4 = 3.14 cubic feet
• 10 columns = 31.4 cubic feet ÷ 27 = 1.16 cubic yards total

Concrete Sidewalk Calculator

A standard sidewalk is typically 4 feet wide and 4 inches thick. Length is variable.

Concrete Driveway Calculator

Residential driveways are typically 4–6 inches thick. Standard single-car driveways run 10–12 feet wide; two-car driveways are 18–24 feet wide.

Concrete Cubic Yards: Quick Reference Table

For jobs where you already know the square footage, use this table to find cubic yards at common thicknesses.

Square Feet to Cubic Yards by Slab Thickness

Shortcut formula: Cubic yards = (Square feet × thickness in inches) ÷ 324

Bags of Ready Mix vs. Ready Mix Concrete Truck: Which Should You Use?

One of the most practical decisions for any project is whether to buy bagged concrete (pre-mixed dry concrete) or order ready mix concrete delivered by truck.

How Much Concrete Does a 50 lb Bag of Ready Mix Make?

A standard 50 lb bag of ready mix concrete yields approximately 0.375 cubic feet (about 0.014 cubic yards) of mixed concrete.

How Much Does Each Bag Size Yield?

How Many Bags Per Cubic Yard?

Is It Cheaper to Mix Concrete or Buy It Ready Mix?

This is one of the most commonly asked questions — and the answer comes down to project size, labor, and your time.

Bottom line: For small projects under half a cubic yard — a fence post, a small pad, a repair — bags are more practical. For anything larger, ready mix concrete is more cost-effective once you factor in labor time and the risk of inconsistent mixes. At roughly 1 cubic yard and above, bagged concrete typically costs more in materials alone, plus the labor to mix it.

Recommended Concrete Thickness by Application

Choosing the right slab thickness is as important as calculating the right volume. Too thin and the slab cracks under load. Too thick and you’re spending money you don’t need to.

Concrete Thickness Guide

Do You Need Rebar for a 4 Inch Slab?

This is a critical question that competitors barely address, and it directly affects how much you spend and how long your slab lasts.

The short answer: Rebar is not always required for a 4-inch residential slab, but it is strongly recommended for anything beyond a simple shed pad or garden patio.

Here’s a practical breakdown:

Rebar vs. Wire Mesh vs. Fiber Reinforcement

When rebar is required for a 4-inch slab:

• Driveways that carry vehicles regularly
• Garage floors with constant car traffic
• Any slab on unstable, expansive, or poorly compacted soil
• Slabs in areas with freeze-thaw cycles
• Any slab that will bear significant point loads (columns, heavy equipment)

When a 4-inch slab may be poured without rebar:

• Light-use residential patio
• Shed floor pad on well-compacted, stable soil
• Short sidewalk in a stable climate

For driveways and garage floors, standard practice is to use #3 rebar on 18-inch centers, or #4 rebar on 24-inch centers, placed at mid-depth (about 1.5 to 2 inches from the bottom of the slab).

Concrete Strength Grades: Which PSI Do You Need?

Not all ready mix concrete is the same. Concrete is specified by compressive strength measured in PSI (pounds per square inch) in the US, or MPa in metric markets.

Concrete PSI by Application

When ordering ready mix concrete, you’ll specify the PSI alongside your cubic yardage. For most residential work — patios, driveways, garage floors, and sidewalks — 3,000 PSI is the standard starting point. Check your local building code, as some jurisdictions require a minimum of 3,500 PSI for driveways in freeze-thaw climates.

What Is the 90 Minute Rule for Concrete?

The 90-minute rule is one of the most important practical facts about ready mix concrete that every project manager and contractor should know.

According to ASTM C94 (the standard specification for ready mixed concrete), concrete must be discharged from the drum within 90 minutes of water being added to the mix, or before the drum has completed 300 revolutions — whichever comes first.

This is not a soft guideline. It exists because concrete begins to lose workability as soon as the cement hydration reaction starts. After 90 minutes, the concrete may still be pourable in warm conditions, but:

• Slump (flowability) decreases significantly
• Strength development may be compromised if water is added to restore workability
• The pour will be uneven and difficult to finish

Key Practical Takeaways for the 90 Minute Rule

• Plan your pour before the truck arrives. Forms should be ready, rebar should be set, and the crew should be positioned.
• In hot weather, the clock runs faster. High ambient temperatures accelerate hydration. On days over 85°F, you may have less than 90 minutes of workable time.
• Cold weather extends working time — but cold slows curing too, requiring insulation or heating.
• Adding water on site is not the answer. If the concrete starts to stiffen, adding water reduces compressive strength by increasing the water-to-cement ratio. Never add water without explicit instructions from the batch plant.
• Large pours need a pump or multiple trucks. If your project is larger than one truckload (~10 cubic yards), coordinate truck arrivals so each load is placed within the 90-minute window.

Ready Mix Concrete Cost: How Much Does It Cost Per Yard?

Concrete prices vary significantly by region, mix design, and market conditions. These are current general benchmarks.

Concrete Price Per Cubic Yard

Additional charges to account for:

• Short load fee: Suppliers charge a fee for orders under their minimum (usually 5–7 yards). Typically $50–$150 per truck.
• Saturday/after-hours delivery: Add $50–$200 depending on supplier.
• Pump truck rental: $500–$1,500 depending on boom size and region.
• Fuel surcharges: Common in high-fuel-cost periods.

How Much Would a 20×20 Slab Cost?

A 20×20 concrete slab is one of the most commonly poured sizes — it’s a standard two-car parking pad, a garage addition, or an outdoor patio foundation.

Material Calculation

• Slab area: 400 square feet
• Thickness: 4 inches (standard residential)
• Cubic yards: (400 × 4) ÷ 324 = 4.94 cubic yards
• With 10% waste: 5.44 cubic yards — order 5.5 yards

Total Cost Breakdown for a 20×20 Slab

Professional installation for a 20×20 concrete slab typically runs $5.00–$8.75 per square foot all-in, or $2,000–$3,500 for the full job including labor, materials, and finishing.

How to Order Ready Mix Concrete: What Information You Need

When you call a ready mix supplier — whether it’s CEMEX, a local redi mix plant, or a regional provider — you’ll need to provide the following:

• Volume in cubic yards (use the calculator to determine this)
• PSI strength (3,000 PSI for most residential work)
• Slump (usually 4–5 inches for slabs, stiffer for footings)
• Aggregate size (3/4-inch is standard for most slabs)
• Any admixtures (air entrainment for freeze-thaw climates, accelerator or retarder based on temperature)
• Delivery date and time
• Site access (truck size limitations, overhead clearances)

Your supplier may also ask about the intended use so they can recommend the right mix design for your application.

Tips for Getting an Accurate Concrete Estimate

Even with the best concrete calculator, errors creep in. Here’s how to avoid the most common mistakes.

Always Measure Twice

A 1-foot error in length on a 20-foot slab represents a 5% error. On a 4-inch thick slab, that’s an additional 0.25 cubic yards. Small measurement errors compound quickly on large pours.

Use the Actual Poured Depth, Not the Nominal

If your forms are set for 4 inches but the subgrade is uneven, the effective depth may average 4.5 or 5 inches. For large slabs on uneven ground, use 4.5 inches as your planning depth even if your target is 4 inches.

Add 10% for Slabs, 5% for Walls and Columns

Slabs have more surface area exposed to spillage, edge waste, and subgrade variation. Walls and columns are more precisely formed and lose less material.

Account for Steps and Cutouts

If your slab has an interior cutout (a drain, a post base, or a landscaping island), calculate and subtract that volume. If it has steps or raised sections, calculate each section separately and add them together.

Confirm the Truck Minimum

Most ready mix suppliers have a minimum order of 1 cubic yard. Some charge a short load fee for orders under 5 or 7 yards. For very small pours, confirm whether bagged concrete is more economical.

Concrete Unit Weight and Volume Conversions

Understanding the weight of concrete helps when planning delivery access, calculating structural loads, and estimating subgrade requirements.

Concrete Weight Reference

Concrete Unit Conversions

Concrete Curing: What Happens After the Pour

Calculating the right amount of concrete is only half the story. What happens after the pour determines whether your slab reaches its design strength.

Concrete does not “dry” — it cures through a chemical reaction called hydration. Strength develops over time:

• 24 hours: Approximately 16% of 28-day design strength
• 3 days: Approximately 40% of design strength
• 7 days: Approximately 65% of design strength
• 28 days: 100% of design strength (design specification point)
• 1 year: Up to 120% of 28-day strength (concrete continues to strengthen slowly)

Concrete Curing Best Practices

• Keep the slab moist for at least 7 days after pouring. Cover with wet burlap, curing blankets, or a curing compound.
• Do not allow the slab to freeze within the first 24–48 hours.
• In hot weather, mist the surface to prevent rapid moisture loss and surface cracking.
• Do not apply loads (parking vehicles, for example) within the first 7 days. Full design loads should wait 28 days.

Common Mistakes When Calculating Concrete Yardage

These errors are responsible for the majority of over- and under-orders on ready mix jobs.

Forgetting to convert inches to feet. If your depth is 4 inches, that’s 0.333 feet — not 4. Multiplying by 4 instead of 0.333 gives you 12 times too much concrete.

Skipping the waste factor. Even perfectly formed slabs lose 5–8% of concrete to spillage, overfill at the edges, and surface variations. Always add overage.

Not accounting for the subbase. If your sub-base isn’t compacted and level, you’ll use more concrete than calculated. Over-excavated areas, depressions, and soft spots all consume extra volume.

Forgetting to subtract openings. If your slab has a column base, drain basin, or tree well cut out of it, subtract those areas from your total before calculating cubic yards.

Rounding down instead of up. Always round up to the nearest quarter-yard when ordering. Concrete that doesn’t arrive is a much bigger problem than a small amount left over.

Frequently Asked Questions

How Do You Calculate Ready Mix Concrete?

Multiply the length × width × depth of your project in feet to get cubic feet, then divide by 27 to get cubic yards. For depth in inches, divide by 12 first to convert to feet, or use the shortcut: (length × width × depth in inches) ÷ 324. Add 5–10% for waste. That final number is what you order from your ready mix supplier.

Example for a 10×10 ft pad at 4 inches deep: (10 × 10 × 4) ÷ 324 = 1.23 cubic yards. Add 10% = *1.35 cubic yards — order 1.5 yards.

How Much Concrete Does a 50 Pound Bag of Ready Mix Make?

A 50 lb bag of ready mix concrete yields approximately 0.375 cubic feet of finished concrete. You would need 72 bags to make one cubic yard. At typical retail prices of $6–$8 per bag, that’s $432–$576 per cubic yard in material alone — significantly more expensive than ready mix delivery for projects of any meaningful size.

What Is the 90 Minute Rule for Concrete?

ASTM C94 requires that ready mix concrete be fully discharged within 90 minutes of water being added to the mix at the plant, or before the drum completes 300 revolutions — whichever occurs first. After 90 minutes, hydration has progressed to a point where workability is significantly reduced, and adding water to restore slump will compromise compressive strength. Always have your site, crew, and forms completely ready before the truck arrives.

How Much Would a 20×20 Slab Cost?

A 20×20 concrete slab requires approximately 5.5 cubic yards of ready mix concrete (4 inches thick, with 10% waste). Material cost alone runs approximately $800–$1,000 at current pricing. Professional installation including labor, subbase preparation, forming, rebar, pouring, and finishing typically comes to $2,000–$3,500 total. DIY pours with rented equipment and self-purchased materials can be done for $1,200–$1,600.

Is It Cheaper to Mix Concrete or Buy It Ready Mix?

For projects under 0.5 cubic yards, bagged concrete is often the more practical choice — no minimum order, no delivery fee, no scheduling. For anything over 0.5 yards, ready mix concrete is almost always more economical when you factor in the cost of bagged concrete (roughly $130–$200 per yard in bags vs. $120–$170 per yard in ready mix), the labor to mix it, and the quality consistency that only a batched mix provides. At one cubic yard and above, ready mix wins on every metric except accessibility.

Do You Need Rebar for a 4 Inch Slab?

Rebar is not legally required for every 4-inch residential slab, but it is strongly recommended for driveways, garage floors, and any slab subject to vehicle loads or freeze-thaw cycles. For light-use patios and shed pads on stable, well-compacted soil, wire mesh or fiber reinforcement can be a sufficient and more economical alternative. When in doubt, the cost of adding #3 rebar on 18-inch centers is modest compared to the cost of repairing or replacing a cracked slab.

Summary: Ready Mix Concrete Calculator Quick Reference

Use the ready mix concrete calculator at the top of this page to get your cubic yardage instantly. Enter your length, width, and depth — the calculator handles the conversion, the division by 27, and the waste factor automatically so you can order with confidence.