Concrete Slab Calculator

How it's calculated

The calculator first finds the volume of the slab in cubic feet: length (ft) × width (ft) × thickness (in ÷ 12). That's converted to cubic yards by dividing by 27, since a cubic yard is 3 ft × 3 ft × 3 ft. A waste percentage is added on top to cover over-excavation and form loss, and the adjusted cubic yardage is multiplied by concrete's density (tons per cubic yard) to get the weight in tons. Bag counts divide the adjusted cubic feet by each bag's yield; ready-mix truckloads divide the adjusted cubic yards by the truck's capacity.

Worked example

A 10 ft × 10 ft patio slab at 4 inches thick: volume = 10 × 10 × (4 ÷ 12) = 33.33 cubic feet. In cubic yards, that's 33.33 ÷ 27 = 1.23 yd³. Adding a 10% waste factor: 1.23 × 1.10 = 1.36 yd³. At 2.02 tons per cubic yard, that's 1.36 × 2.02 = 2.74 tons — or 2.65–2.76 tons across the full 1.95–2.03 density range. In 80 lb bags, the adjusted 36.67 cu ft (33.33 × 1.10) would need 62 bags — well past the roughly half-cubic-yard point where ready-mix delivery starts making more sense than mixing bag after bag by hand.

How deep should it be?

Slab thickness is driven almost entirely by what's going to be on top of it, not by preference — but the actual number depends on the load the slab carries, your soil's bearing capacity, and what your local code requires for that specific structure, not a generic rule of thumb this calculator can supply. Foot traffic and vehicle loads are different problems: a driveway has to resist repeated point loads from tires in a way a patio or walkway never does, which is why the two aren't interchangeable even though both are "just a slab." For guidance specific to what you're pouring, see the driveway, patio, and sidewalk calculators — each covers the considerations for that particular use in more detail than a general slab page can. Whatever the thickness, a slab poured directly onto undisturbed or ungraded soil is asking for trouble — a compacted gravel or crushed-stone base underneath gives the slab a stable, well-drained foundation and is standard practice for any slab-on-grade pour.

Common mistakes

• –Pouring directly onto undisturbed or ungraded soil instead of a compacted gravel or crushed-stone base — the slab is far more likely to crack or heave without one.
• –Using a patio or walkway thickness under a driveway or anywhere vehicles will park. Vehicle loads need more mass and support than foot traffic — check your local code or a contractor's spec, or see the driveway calculator's guidance, rather than assuming one thickness works for both.
• –Not checking your ready-mix supplier's minimum order. Most plants charge a "short load" fee below about 1 cubic yard, which can make a small bagged job cheaper even where ready-mix would normally win.
• –Mixing bags for a job that's really a ready-mix job. Past roughly half a cubic yard, ready-mix delivery is generally cheaper and dramatically less labor than mixing dozens of bags by hand.
• –Forgetting that a waste factor here covers over-excavation and form loss, not moisture — unlike the landscape materials on this site, a wetter mix doesn't mean you need more of it.

Where the density figure comes from

Unlike the landscape materials on this site, concrete is a manufactured product with a tightly controlled mix, so its density varies far less than gravel, sand, or mulch. CEMEX, a major ready-mix producer, puts a solid unbroken slab at about two tons (4,000 lbs) per cubic yard, and says one cubic yard takes roughly 45 eighty-pound bags or 90 forty-pound bags. ACI 318 and ASTM C138 define normal-weight concrete at 145–150 lb/ft³ — ACI 318 uses 150 lb/ft³ as its default structural value — which works out to 3,915–4,050 lbs (1.96–2.03 tons) per cubic yard. The 1.95–2.03 ton/yd³ range here reflects that normal-weight span, not moisture variation. Lightweight and heavyweight concrete mixes fall outside this range and aren't covered by this calculator. The 40 lb and 80 lb bag counts above are CEMEX's own; the 60 lb bag's 0.45 ft³ yield is Quikrete's own dealer training figure instead, confirmed against SiteOne's published spec. We use 2.02 tons/yd³ as the display figure, drawn from:

• CEMEX — Ready-Mix Concrete Calculator
• ACI 318 & ASTM C138 — normal-weight concrete density, 145–150 lb/ft³
• Quikrete — Dealer Training: Mixing & Placing Concrete (PDF)

Read more in our density methodology.

FAQ

How much concrete do I need for a slab?

For a 12 ft × 12 ft slab poured 4 inches thick, that's 12 × 12 × (4 ÷ 12) = 48 cubic feet, or about 1.78 cubic yards before waste — use the calculator above to add your waste factor and get a bag count or ready-mix order. For a driveway, patio, or sidewalk specifically, the calculators for those pours walk through examples closer to that use case.

Should I use bags or ready-mix concrete?

Bags make sense for small jobs — a few stepping stones, a post footing, a small patch. Past roughly half a cubic yard, ready-mix delivery is generally cheaper per yard and dramatically less labor than mixing dozens of 40 or 80 lb bags by hand. Check your local ready-mix plant's minimum order too, since a small job might fall below it and incur a short-load fee.

Why does concrete's weight vary so little compared to gravel or sand?

Concrete is a manufactured product with a tightly controlled mix ratio, so its density is far more consistent than a quarried or organic material's. The range on this site comes from ACI 318 and ASTM C138's normal-weight concrete specification (145–150 lb/ft³), not from moisture swings the way gravel, sand, or mulch vary. Lightweight and heavyweight specialty mixes fall outside this range.

How thick should a concrete slab be?

It depends on what's going to be on top of it. Foot traffic and vehicle loads put very different demands on a slab, and the actual number is set by that load, your soil's bearing capacity, and your local code — not a generic rule of thumb. See the driveway, patio, or sidewalk calculators for guidance specific to what you're pouring.

Do I need a gravel base under a concrete slab?

Yes, for almost any slab. A compacted gravel or crushed-stone base gives the pour a stable, well-drained foundation and helps prevent cracking and heaving, especially in freeze-thaw climates. Skipping it to save a delivery is one of the most common reasons a DIY slab fails early.

Is bagged concrete cheaper than ready-mix for a small job?

For a true small job — a post footing or a couple of stepping stones — yes, bags are usually cheaper once you factor in a ready-mix plant's delivery fee and order minimum. But the per-yard cost of bagged concrete is higher than ready-mix, so that advantage disappears fast as the job grows past a few dozen bags.

How many ready-mix truckloads will I need?

Divide your adjusted cubic yardage by the truck's capacity — standard ready-mix trucks carry about 10 cubic yards per full load. The calculator above uses 10 cubic yards as a default and rounds up, since a partial load still requires dispatching a truck, and most plants charge extra for small or split loads.