Why the Math Matters
Running out of concrete mid-pour is one of the worst things that can happen on a slab job. The new batch may not match the set time or consistency of the first load, and cold joints create weak points in the finished slab. Ordering too much wastes money, since ready-mix suppliers charge by the cubic yard and unused concrete cannot be returned. Getting the number right before you order takes five minutes and prevents both problems.
The Concrete Volume Formula
Concrete volume is calculated in cubic yards. All measurements must be in feet before you divide by 27 (since one cubic yard equals 27 cubic feet).
The key conversion most people miss: thickness is almost always given in inches, so divide it by 12 to get feet. A 4-inch slab is 4 ÷ 12 = 0.333 feet thick.
Common Slab Thicknesses
- 4 inches (0.333 ft): standard residential slab, sidewalk, patio, driveway
- 5 inches (0.417 ft): light vehicle traffic, heavier residential use
- 6 inches (0.5 ft): heavy vehicle traffic, commercial applications
- 8 inches (0.667 ft): structural foundations, footings
Add a Waste Factor
Always order more than your calculated volume. Concrete is lost to uneven subgrade, form spillage, and the small amount left in the truck or mixer. The standard practice is to add 10% for ready-mix orders and 5% to 10% for bagged concrete.
Round the final number up to the nearest quarter yard for ready-mix orders, or up to the nearest whole bag for bagged concrete.
Worked Example: 10 × 12 Foot Patio Slab
A patio slab that is 10 feet wide, 12 feet long, and 4 inches thick.
Convert thickness to feet:
Calculate volume:
Convert to cubic yards:
Add 10% waste factor:
Order 1.75 cubic yards (rounded up to the nearest quarter yard). Most ready-mix suppliers have a minimum order of around 1 cubic yard, so this job qualifies for a truck delivery. For fast results on any slab size, use our concrete calculator.
Converting to Bags
For smaller jobs, bagged concrete is often more practical than a ready-mix truck. An 80 lb bag of pre-mixed concrete yields approximately 0.60 cubic feet of finished concrete.
For the 10 × 12 patio (1.63 cubic yards with waste factor):
73 bags is a lot to mix by hand. For any slab over about 0.5 cubic yards, renting a portable mixer or ordering ready-mix is strongly recommended. The bags are cheaper per yard but the labour cost quickly outweighs the savings.
Tips for Getting It Right
- Measure twice. An error of 6 inches in length or width on a large slab can mean a difference of half a cubic yard or more. Double-check your form dimensions before ordering.
- Account for irregular shapes. For L-shaped or irregular slabs, break the area into rectangles, calculate each section separately, and add the volumes together before applying the waste factor.
- Order in the morning. Ready-mix trucks are dispatched early. If you order for an afternoon pour, the plant may not have availability, especially on short notice in busy seasons.
- Have your forms built before you call. Suppliers want final dimensions at the time of order. Having everything ready prevents last-minute changes that can shift your order amount.
- Specify your mix. Standard residential slabs use a 3,000 psi mix. Driveways and higher-load applications often call for 3,500 to 4,000 psi. Ask your supplier if you are unsure which strength you need.
Frequently Asked Questions
What is the minimum ready-mix concrete order?
Most ready-mix suppliers have a minimum order of 1 cubic yard. Some will deliver smaller amounts (down to 0.5 yards) but charge a short-load fee that can be significant. For jobs under 1 cubic yard, bagged concrete is often more cost-effective despite the extra labour. Always call your local supplier to confirm their minimum and fee structure before deciding.
How thick should a concrete driveway be?
For residential driveways with passenger vehicles only, 4 inches is the standard minimum. If you park heavy trucks, RVs, or have a garbage truck rolling over it regularly, 5 to 6 inches provides better long-term durability. Thicker slabs also benefit from rebar or wire mesh reinforcement to control cracking.
Do I need rebar in a residential slab?
It depends on the application. Patios and sidewalks can often use wire mesh or no reinforcement at all with proper subgrade preparation. Driveways typically benefit from wire mesh or #3 rebar on a 24-inch grid. Structural slabs (garage floors, foundations, footings) should use rebar sized and spaced per local building code. When in doubt, check with your local permit office.
How long does concrete take to cure?
Concrete reaches initial set in about 24 to 48 hours - enough to walk on carefully. It reaches roughly 70% of its design strength in 7 days and 99% at 28 days. You can drive on it after about a week for passenger vehicles. Keep it moist during the first 7 days (covered with plastic sheeting or dampened burlap) to prevent the surface from drying too quickly, which causes cracking and surface weakness.
What causes concrete to crack?
The most common causes are: shrinkage during curing (controlled with control joints cut every 8 to 10 feet), subgrade settlement (prevent with a compacted gravel base), tree roots growing underneath, freeze-thaw cycles in cold climates, and loading beyond the slab's design capacity. Control joints give the concrete a designated place to crack so the crack follows a straight line instead of running randomly across the surface.