How Much Concrete Do I Need for a Slab?
Concrete slab volume depends on three things: length, width, and thickness. Here is how to calculate it, choose the right thickness for your project, and decide between bags and a ready-mix truck.
The formula for concrete slab volume is: length (ft) × width (ft) × thickness (inches) ÷ 12 ÷ 27 = cubic yards. The “divide by 12” converts inches to feet and the “divide by 27” converts cubic feet to cubic yards, which is how concrete is sold. For example, a 10 × 10 foot patio at 4 inches thick requires 10 × 10 × 4 / 12 / 27 = 1.23 cubic yards. Always add 10% overage to account for uneven subgrade, form imperfections, and spillage.
| Project Type | Recommended Thickness | Notes |
|---|---|---|
| Patio | 4 inches | Foot traffic only; 3,000 PSI mix standard |
| Sidewalk | 4 inches | Same as patio; check local code for width requirements |
| Driveway | 5–6 inches | Vehicle loads; 4,000 PSI recommended; some codes require 6″ |
| Garage Floor | 4–6 inches | 4″ for passenger cars; 6″ if storing heavy equipment |
| Shed Pad | 4 inches | Light load; ensure level subgrade before pouring |
Step-by-Step Calculation
Let us work through a real example. Say you are pouring a 12 × 20 foot driveway apron at 5 inches thick. Step 1: Multiply length by width: 12 × 20 = 240 square feet. Step 2: Convert thickness to feet: 5 / 12 = 0.417 feet. Step 3: Multiply area by thickness: 240 × 0.417 = 100 cubic feet. Step 4: Convert to cubic yards: 100 / 27 = 3.70 cubic yards. Step 5: Add 10% overage: 3.70 × 1.10 = 4.07 cubic yards. You would order 4 to 4.25 cubic yards from a ready-mix supplier, or calculate the bag equivalent if doing it by hand.
Bags vs Ready-Mix Truck
Concrete bags (40, 50, 60, or 80 pounds) are practical for small projects under about 0.5 cubic yards. That is roughly 23 bags of 80-lb mix. Between 0.5 and 1.0 cubic yards, the decision depends on your stamina and timeline — mixing 45 bags of 80-lb concrete is exhausting but doable for a fit person over a full day. Above 1.0 cubic yard, a ready-mix truck delivery is strongly recommended. Not only does mixing 50+ bags take longer than most people expect, but concrete needs to be poured continuously — a cold joint (where fresh concrete meets partially-set concrete) creates a structural weak point.
How Many Bags of Concrete per Cubic Yard?
The number depends on bag size: 90 bags of 40-lb, 72 bags of 50-lb, 60 bags of 60-lb, or 45 bags of 80-lb per cubic yard. An 80-lb bag yields approximately 0.6 cubic feet of mixed concrete. Our concrete calculator shows all four bag sizes at once and gives a buying recommendation (bags vs truck) based on your project volume.
Why 10% Overage Matters
Running short mid-pour is the worst outcome in concrete work. Unlike mulch or gravel, you cannot come back tomorrow and add more — the joint between old and new concrete will be weak and visible. Forms are never perfectly level, subgrade is never perfectly flat, and some concrete always sticks to the mixer, wheelbarrow, and tools. Ten percent overage is the standard safety margin. For first-time pourers or uneven ground, consider 15%. A small amount of leftover concrete is far better than a cold joint in the middle of your slab.
Ready-Mix Ordering Tips
Ready-mix concrete is sold by the cubic yard with a minimum order of typically 1 cubic yard. Orders under the full-truck minimum (usually 8 to 10 yards) incur a “short-load fee” of $50 to $150. A full truckload typically holds 8 to 10 cubic yards at $120 to $160 per yard. When ordering, specify the PSI strength (3,000 for patios, 4,000 for driveways), whether you need fiber reinforcement, and your pour schedule. Have your forms, subgrade, rebar or mesh, and crew ready before the truck arrives — ready-mix companies charge by the minute for time beyond the standard unload window (usually 5 to 7 minutes per yard).
Common Mistakes to Avoid
Pouring concrete too thin is the most common DIY mistake. A 3-inch patio slab will crack under normal use because concrete is strong in compression but weak in tension — it needs minimum thickness to resist bending. Skipping rebar or wire mesh is the second most common error; unreinforced slabs crack much more readily. Third, pouring on unprepared subgrade (soft dirt, organic material, or soil that was not compacted) leads to settling and cracking within the first year. Prepare a 4-inch compacted gravel base beneath any slab for proper drainage and support.