How many bags of concrete do I need for a slab?

Work out the volume first (length × width × thickness, all in feet), divide by 27 to get cubic yards, then divide by the yield printed on the bag. An 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet. For anything over roughly 1 cubic yard, ready-mix delivery is usually cheaper and far less work than mixing a hundred bags by hand.

How thick should a concrete slab be?

Four inches is standard for a patio, shed floor or garden path on well-compacted ground. Use 4–6 inches for a driveway carrying cars and 6 inches or more for heavy vehicles or where the subgrade is weak. Thickness matters more than mix strength for load capacity, but a slab that is too thin will crack regardless of the mix.

How do I convert cubic feet to cubic yards of concrete?

Divide cubic feet by 27, because a cubic yard is 3 × 3 × 3 feet. A 10 ft × 10 ft slab at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards. Always order 5–10% more than the calculated figure, since running short mid-pour means a cold joint that never bonds properly.

Should I add extra concrete for waste and spillage?

Yes, order 5–10% above your calculated volume. Concrete is lost to uneven subgrade, form deflection, spillage and over-excavation, and you cannot pause a pour to fetch more. For a 1.2 cubic yard slab, ordering 1.3 cubic yards is normal practice. Leftover concrete is far cheaper than a cold joint or a short pour.

How much does concrete cost per cubic yard?

Ready-mix concrete typically runs roughly $125–$175 per cubic yard nationally before delivery and short-load fees, and most suppliers apply a minimum charge or a fee for loads under about 3 cubic yards. Bagged concrete costs more per cubic yard of equivalent volume but makes sense for small jobs. Regional pricing varies widely, so confirm current rates with two local suppliers before budgeting.

Concrete Calculator - Cubic Yards, Bags and Cost

Complete Guide to Concrete Calculations and Pouring

Understanding Concrete Measurement: Cubic Yards Explained

Concrete is sold by the cubic yard in the United States. One cubic yard equals 27 cubic feet (3 feet × 3 feet × 3 feet). To calculate concrete needs, determine the volume of your project in cubic feet (length × width × thickness), then divide by 27 to convert to cubic yards. For example, a 10' × 10' patio at 4" thick: 10 × 10 × 0.33 (4 inches = 0.33 feet) = 33 cubic feet ÷ 27 = 1.22 cubic yards.

Always order 5-10% more concrete than calculated. Variations in ground level, slight measurement errors, spillage, and form imperfections mean you'll need extra. Running short mid-pour creates serious problems—cold joints where old and new concrete meet are weak points prone to cracking. Ordering excess is cheap insurance. Most suppliers have minimum delivery charges (often 1 cubic yard), making small projects expensive per yard. For very small jobs, consider bagged concrete mix.

Bagged Concrete vs Ready-Mix: When to Use Each

Bagged concrete (sold in 60-80 pound bags) makes sense for projects under 1 cubic yard. Each 80-pound bag yields approximately 0.6 cubic feet, meaning you need 45 bags per cubic yard. Bags are convenient for small projects like setting fence posts, small repairs, or when access prevents delivery trucks. However, mixing multiple bags is labor-intensive and inconsistent—each batch varies slightly. Bags cost $4-6 each, making a cubic yard of bagged concrete $180-270 plus your labor.

Ready-mix concrete delivered by truck is economical for projects over 1 cubic yard. Price is $125-200 per cubic yard depending on location, mix specifications, and delivery distance. Ready-mix arrives perfectly mixed, consistent throughout your pour, and saves enormous labor. You'll need enough helpers to move concrete quickly—trucks typically allow 7-10 minutes per yard before charging "wait time" fees. For 2+ cubic yards, ready-mix is almost always the better choice.

Concrete Mix Designs and PSI Ratings

Common Concrete Mix Applications:

2,500 PSI: Residential foundation footings, basement walls. Basic structural application with minimal exposure to elements.
3,000 PSI: Most common for residential driveways, sidewalks, patios. Good balance of strength and workability for typical homeowner projects.
3,500 PSI: Driveways with heavy vehicle traffic, garage floors. Enhanced durability for high-stress applications.
4,000 PSI: Commercial applications, warehouse floors, roads. Higher strength for commercial loads and extreme conditions.
5,000+ PSI: Specialized applications like bridges, parking structures, industrial facilities. Requires specific expertise and conditions.

PSI (pounds per square inch) measures compressive strength—how much weight concrete can support before failing. Higher PSI costs more and typically has lower workability (harder to spread and finish). For most residential projects, 3,000-3,500 PSI is appropriate. Specify air-entrained concrete for freeze-thaw climates—microscopic air bubbles allow expansion space, preventing cracking from freeze-thaw cycles.

Proper Site Preparation for Concrete Pours

Excavate to proper depth, accounting for concrete thickness plus base material. Most flatwork requires 4-6 inches of compacted gravel base for drainage and stability. Compact base material thoroughly with a plate compactor—uncompacted base settles over time, causing concrete to crack. Build forms from 2× lumber, staking securely every 3-4 feet. Forms must be level and at the correct height—concrete follows the forms.

For slabs larger than 10' × 10', install expansion joints every 8-10 feet to control cracking. Concrete expands and contracts with temperature changes; expansion joints provide space for movement. Use fiber mesh or rebar reinforcement for strength. Welded wire mesh (6×6–10/10) works for most residential slabs. Place mesh in the middle third of slab thickness for maximum effectiveness. Rebar (#3 or #4 bars in 18" grid) provides superior strength for driveways or heavy-use areas.

The Concrete Pouring Process Step-by-Step

Start in the furthest corner from the truck, working backward so you don't walk through wet concrete. Pour concrete slightly higher than forms—you'll screed (level) it down. Use a screed board (straight 2×4 or specialized tool) across the top of forms in a sawing motion to level concrete. Have enough people ready—you can't pause mid-pour. Work quickly but carefully; concrete begins setting within 30-90 minutes depending on temperature and mix.

After screeding, use a bull float to smooth the surface and bring cream (fine cement paste) to the top. Wait for bleed water to evaporate before additional finishing—working wet concrete causes surface weakness. Once bleed water disappears, use a hand float for smoother finish or broom for non-slip texture (standard for sidewalks, driveways). Edge the perimeter with an edging tool to create rounded, chip-resistant edges. Cut control joints with a grooving tool or saw-cut within 12 hours—these guide cracking to occur in straight lines at joints rather than randomly.

Concrete Curing: The Most Critical Phase

Curing is the process of maintaining adequate moisture and temperature for concrete to reach full strength. Concrete doesn't dry—it cures through a chemical reaction (hydration) between cement and water. This reaction requires moisture and takes weeks to complete. Concrete reaches approximately 70% of full strength in 7 days, 90% in 28 days. Proper curing can double concrete's final strength; improper curing causes weak, brittle concrete prone to cracking and dusting.

Keep concrete continuously moist for at least 7 days (longer in hot or dry conditions). Methods include covering with plastic sheeting (trap moisture), wet burlap, frequent water spraying, or applying curing compound (liquid membrane that prevents moisture loss). Don't let concrete dry out, especially in the first 48-72 hours. Avoid walking on new concrete for 24 hours, driving for 7 days. Full vehicle weight capacity requires 28 days of curing.

Temperature Considerations for Concrete Work

Ideal concrete pouring temperature is 50-80°F. Cold weather (below 40°F) dramatically slows curing—concrete may take days to set and weeks to achieve strength. Cold-weather pours require heated enclosures, insulated blankets, or accelerating admixtures. Concrete that freezes before curing suffers permanent strength loss and surface scaling. In cold climates, avoid concrete work from November through March unless you can provide adequate protection.

Hot weather (above 90°F) causes rapid evaporation, leading to plastic shrinkage cracks, reduced strength, and difficult finishing. Pour early morning or late evening when temperatures are cooler. Use retarding admixtures to slow setting time, spray surfaces with water mist during finishing, and begin curing immediately. Shade wet concrete if possible. Hot, dry, windy conditions are worst-case scenario—evaporation occurs faster than concrete can bleed water to the surface, causing severe cracking.

Understanding Concrete Thickness Requirements

Thickness requirements depend on application. Sidewalks: 4 inches minimum on properly compacted base. Patios: 4 inches for pedestrian traffic only; 5-6 inches if furniture or grills will be present. Driveways: 5-6 inches minimum; 6 inches strongly recommended for heavier vehicles. Garage floors: 6 inches, reinforced with rebar or fiber mesh. Thicker isn't always better—proper base preparation and reinforcement matter more than extra thickness.

Each additional inch adds approximately 0.25 cubic yards per 100 square feet. Going from 4" to 6" on a 20' × 20' driveway adds 1 cubic yard ($125-200 in material), but provides significantly more durability. Skimping on thickness to save money creates problems—thin concrete cracks easily under loads. Loading areas, RV pads, or areas with heavy equipment require 8+ inches with rebar reinforcement. Consult local building codes for minimum requirements.

Common Concrete Problems and Prevention

Cracking is the most common concrete problem. Some cracking is inevitable—control joints and expansion joints minimize it. Causes include inadequate base preparation (settling), insufficient reinforcement, improper curing (rapid moisture loss causes shrinkage), too much water in mix (weakens concrete), and lack of expansion joints. Hairline cracks are cosmetic; structural cracks (wide, displacement, continuous) indicate serious issues.

Scaling (surface flaking) results from freeze-thaw damage, poor finishing technique (overworking wet concrete), or deicing chemicals. Use air-entrained concrete in freeze-thaw climates and avoid deicing salts the first winter. Dusting (powdery surface) indicates weak concrete from overwatering, poor curing, or finishing too early. Efflorescence (white deposits) occurs when water migrates through concrete, depositing salts. It's cosmetic and can be cleaned with specialized cleaners.

DIY vs Professional Concrete Work: Making the Decision

Small projects (under 2 cubic yards) like small patios, walkways, or shed foundations are DIY-friendly for capable homeowners. You'll need basic tools (wheelbarrow, shovels, screed board, floats, edgers), helpers, and physical stamina. Watch instructional videos, work methodically, and don't attempt your first pour on a large or critical project. Simple rectangular pours are easier than complex shapes or slopes.

Large projects (3+ cubic yards), driveways, structural applications, or decorative work (stamped, colored concrete) warrant professional installation. Professionals have experience, proper equipment, and can work quickly before concrete sets. Poor concrete work is expensive to remove and redo. Professional installation costs $6-15 per square foot depending on complexity. While DIY saves labor costs, mistakes can cost more than hiring professionals initially. Structural applications (foundations, retaining walls) should always involve professionals or engineers.