How Much Concrete Do I Need? Slab Math, Bag Counts, and When to Call the Truck
Yards and bag counts for real projects — patios, shed slabs, and deck piers — with the ordering math nobody tells you.
By Mohamed Zakrya · Updated · 5 min read
Concrete is the one material where running short isn't an inconvenience — it's permanent. A pour that stops mid-slab sets with a cold joint you'll look at for twenty years. So the estimate matters more here than anywhere else in the yard, and the good news is the math is three lines long.
The quick answer
Common projects at their standard thickness, with the recommended 10% order margin already included:
| Project | Size | Order (yd³) | 80 lb bags | Verdict |
|---|---|---|---|---|
| Sidewalk section | 4 × 8 ft, 4 in | 0.43 | 20 | Bags |
| Shed slab | 8 × 8 ft, 4 in | 0.87 | 40 | Borderline |
| Patio | 10 × 10 ft, 4 in | 1.36 | 62 | Truck |
| Patio | 12 × 12 ft, 4 in | 1.96 | 88 | Truck |
| Parking pad | 10 × 20 ft, 5 in | 3.40 | 153 | Truck, no question |
Your slab isn't on the table? The calculator runs the same math with your dimensions, your thickness, and your bag size:
Free calculator
Concrete Calculator
Slab volume in cubic yards and bags — with the order margin that prevents a short pour.
Three lines of math
Volume is length × width × thickness (in feet), and concrete is sold by the cubic yard — 27 cubic feet. The only judgment call is the margin: add 10% for real-world subgrade dips, spillage, and form bulge, because the alternative to a small surplus is the cold joint above. That's the whole formula. Everything else is deciding how the concrete arrives.
Three projects, fully worked
A 12 × 12 patio at 4 inches. Volume: 144 × (4 ÷ 12) = 48 cubic feet. With the 10% margin: 52.8 — call it 1.96 cubic yards. A ready-mix plant rounds that to 2 yards. In bags it's 88 eighty-pounders: over 7,000 lb of dry mix through a wheelbarrow. This is a truck job wearing a DIY costume.
Deck piers — the cylinder version. Round piers use πr²h instead of the slab formula. A 12-inch sonotube set 36 inches deep holds 3.14 × 0.5² × 3 = 2.36 cubic feet — four 80 lb bags per pier. A four-pier deck is 16 bags: a real but very doable Saturday, and exactly the job bagged concrete exists for.
The 4-versus-5-inch decision, priced. A 10 × 16 pad at 4 inches orders at 2.17 yd³ (98 bags); the same pad at 5 inches is 2.72 yd³. The extra inch costs exactly 25% more material — worth every penny if a vehicle will ever park on it, pure waste if it's a patio table and a grill. Match the thickness to the load, not to "stronger is better."
How ready-mix ordering actually works
The part nobody explains until you're on the phone: plants sell in quarter-yard increments and most charge a short-load fee — often $75–$150 — for orders under about 3–4 yards, because the truck costs the same to roll either way. Give them the rounded-up yard number, the pour date, and honest site access (can a 30-ton truck reach the forms, or are you wheelbarrowing 80 feet?). Book help before you book the truck: once the drum arrives, you typically have under an hour of workable time, and concrete does not wait for a second pair of hands to show up.
The mistakes that ruin pours
Skipping the base. A slab is only as good as the 4 inches of compacted gravel under it. Pouring on soft or organic soil is how slabs crack in year one — calculate the base layer first:
Free calculator
Gravel Calculator
Cubic yards and tons for any area and depth — loose fill or compacted base.
Pouring naked. Wire mesh or rebar isn't optional on anything you'll drive on, and it's cheap insurance on patios. It doesn't prevent cracks — it holds the cracked pieces together so you never see them open.
Inconsistent bag mixing. Bag concrete is forgiving until you freelance the water. Too wet is weak; mix every batch to the same peanut-butter consistency or batches cure at different strengths and shades.
Ignoring the weather. Below about 40°F, curing stalls and freezing ruins it; on hot, windy days the surface dries before the slab cures and shrinkage cracks appear by evening. Concrete wants a mild, overcast day — and so do you.
What it costs
In most US metros, ready-mix runs roughly $140–$200 per cubic yard delivered, plus the short-load fee on small orders. Bagged 80-pounders typically run $5–$8 each. On the 1.36-yard patio that's surprisingly close in dollars — about $400 in bags versus $300-and-change delivered — which is exactly why the decision isn't really about money. It's about whether you want to lift, mix, and place two and a half tons by hand against a setting clock. Past a yard, the truck is buying back your spine, not your wallet.
Three quick questions
Can I pour concrete in cold weather? Below 40°F, curing slows dramatically, and fresh concrete that freezes in the first day or two is permanently weakened. If a cold snap is coming: use a cold-weather mix or warm water, pour early in the day, and cover the slab with insulating blankets overnight. Below freezing with no protection plan, reschedule — the slab can't be un-ruined.
Do I really need rebar or mesh in a small slab? For walkways and pads nothing heavier than feet will touch, fiber-mix bags or plain concrete on a proper gravel base are generally fine. For patios, wire mesh is cheap and worth it; for anything vehicles touch, rebar on a grid is non-negotiable. Steel doesn't stop concrete from cracking — it stops cracks from becoming gaps.
How long until I can use it? Stay off it for 24 hours, walk on it after 24–48, keep vehicles off for a full 7 days, and consider it fully cured at 28. Curing is hydration, not drying — misting the slab or covering it with plastic for the first week makes it stronger, not weaker.