A 20×20 slab is one of the most common sizes homeowners pour — big enough for a two-car garage, a workshop, or a solid patio, small enough that most driveways can handle the concrete truck. Here’s the math contractors use, so you know exactly what you’re paying for before anyone shows up with a mixer.
The Quick Answer
For a standard 4-inch thick slab, a 20×20 area needs about 4.94 cubic yards of concrete. Most contractors round up to 5 cubic yards to cover spillage, uneven subgrade, and minor waste — and you should too.
How That Number Is Calculated
Concrete is ordered by the cubic yard, not the square foot, so the math has three steps:
Area: 20 ft × 20 ft = 400 square feet
Volume: 400 sq ft × (4 in ÷ 12) = 133.3 cubic feet
Convert to yards: 133.3 ÷ 27 = 4.94 cubic yards
If you’re pouring a thicker slab — say for a garage that’ll hold vehicle weight — the number changes fast:
Thickness Cubic Yards Needed Rounded Order
4 inches (standard patio/walkway) 4.94 5 yards
5 inches (light-duty driveway) 6.17 6.25 yards
6 inches (garage floor, heavy vehicles) 7.41 7.5 yards
This is exactly where most DIYers get burned — they price out a 4-inch slab, then find out mid-project their local code or their garage’s intended use requires 6 inches, and now they’re short by two and a half yards with a truck already on-site paying by the minute.
What This Actually Costs
Ready-mix concrete typically runs $150–$180 per cubic yard delivered, depending on your region and the mix strength (PSI rating) you need. For a 20×20 slab at 4 inches:
Concrete only: roughly $750–$900 for 5 yards
Labor and finishing: often doubles or triples the total, landing a fully finished slab somewhere between $3,600 and $7,200 depending on region, site prep, and whether you’re adding rebar or a vapor barrier
Get a bid that’s dramatically below this range for a 20×20, and it’s worth asking exactly what’s included — subgrade prep, rebar, control joints, and finishing are where corners get cut.
Don’t Guess — Run Your Exact Numbers
Every slab is a little different — your thickness requirements, your local concrete pricing, and whether you’re adding reinforcement all change the final number. Instead of eyeballing it, run your exact dimensions through the free QuickSlabCalc slab calculator to get a precise cubic yard and cost estimate before you call for a quote — so you walk into that conversation knowing exactly what a fair number looks like.
Frequently Asked Questions
Do I need rebar for a 20×20 slab?
For a patio or walkway, most codes don’t require it, though wire mesh is common. For anything holding vehicle weight — garages, driveways — rebar or thicker wire mesh is standard practice and often required by local code.
How many bags of concrete would this take if I mix it myself?
An 80 lb bag yields about 0.6 cubic feet. For a 4-inch, 20×20 slab (133.3 cubic feet), that’s roughly 222 bags — which is why virtually everyone orders ready-mix delivery at this size instead of hand-mixing.
How long does a 20×20 slab take to cure?
It’s typically walkable in 24–48 hours and can handle light traffic in about a week, but full structural cure — the point at which it reaches its rated strength — takes around 28 days.