Concrete sub-clusterSlab spine

Concrete slab calculator — yards, bags, and the slab-specific cushion.

Slab math is length × width × thickness, divided by 27. A 22-foot by 22-foot detached-garage slab at 4 inches thick comes out to 5.98 cubic yards — call it 6.5 yards on a ready-mix truck once you add the cushion. Type your dimensions, and the calculator does the math plus tells you what to actually order at the dispatcher.

Most slab pours fall into one of four buckets: a foundation slab-on-grade for new construction, a detached-garage floor, a driveway extension, or a patio. The pour is fast, the math is unforgiving, and the weather window is narrower than you'd think. The notebook below the calc tells you what to call the dispatcher with — and whether the slab is small enough that ready-mix isn't worth the short-load fee.

Slab pour, residential interior

Exact · 5.98 yd³

Bags 80 lb · 270

Bags 60 lb · 359

Material order cushion

The math5.98 yd³— (22 ft × 22 ft × 4″ / 12) / 27 = 5.98 yd³

What I’d actually order6.5 yd³

Why the cushionSmall grade variation, edge forms that bow a quarter-inch, and wheelbarrow loss can eat the difference fast. On a slab this size you're finishing while the truck is still chuting, and there's no time to do math when you realize the south corner is short.

When NOT to over-orderAbove 3 yd³ ready-mix beats bagged on cost and on your back. But the truck has a 90-minute discharge window after batching (ASTM C94) — over-cushion the order and you'll be chasing the finisher with a calculator instead of a screed. Quarter-yard up is the cushion; half-yard up is the regret.

Quick tipsWhat I'd want you to know in 60 seconds

Concrete will crack. Your job is to choose where — cut joints every 8 to 12 feet on a 4-inch slab.
Below 1 yard, buy bags. Above 1 yard, call the truck. The fee for a small ready-mix order eats the math in between.
Don't pour below 40°F overnight or above 95°F at discharge. Both ends ruin the cure window.
Use a 10-mil vapor barrier under any slab on dirt. The 6-mil sheets at the hardware store don't cut it anymore.
Walk on it in 24 to 48 hours. Drive on it at 7 days. Don't backfill against fresh forms before day 14.

Worked exampleDetached 2-car garage, 24×24

A 24×24 detached-garage slab — ready-mix and the weather window.

24 ft × 24 ft × 4″ slab. Volume math: 24 × 24 × (4/12) / 27 = 7.11 cubic yards exact. With a 5% waste cushion that rounds up to 7.50 yd³ on a quarter-yard dispatcher. I'd call Ozinga and ask for 7.50 to leave a working cushion, then check the forecast for the pour day — anything under 40°F overnight and the cure protection gets expensive (ACI 306R-16 protection period kicks in), anything over 95°F at discharge and slump loss eats the finishing window (ACI 305.1-14).

Bagged is the wrong call here. 7.11 yd³ is roughly 320 bags of 80 lb mix — a logistical nightmare even with a crew of three and a mixer. The breakpoint between bagged and ready-mix sits around 1 yd³ for solo work and 2 yd³ for a small crew. A garage slab is ready-mix territory every time. Where bagged makes sense on this page is a 4×4 stoop or a small mechanical-room pad — anything under a yard, where the short-load fee from the supplier outweighs the carry.

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Quick answers about SiteworkMath's calculators and material take-offs — concrete yardage, topsoil + mulch volume, tile box-count, deck-surface materials. Free, no signup. Not structural-engineering or code advice — for joist / beam / footing / permit decisions, talk to a structural engineer, licensed contractor, or your local building department.

Hi, I'm the SiteworkMath assistant. I answer questions about the calculators and guides on this site — concrete yardage, topsoil, mulch, tile, and decking material math. I'm not a structural engineer or a licensed contractor; I'm a calculator built around the math that James Wu — Chicago-area flipper — uses on his own jobs. For structural decisions (joist sizing, beam spans, footing depth in your soil + climate, anything that needs a permit) talk to a licensed structural engineer or your local building department.

How I'd actually use this on a flip

The slab pours that earn this calculator's keep are the ones you build new — a foundation slab-on-grade on a teardown rebuild, a detached garage floor on a rear-yard build, a driveway extension when the buyer wants a third bay. The math is the same across all of them; the operational decisions are the cushion and the dispatcher call.

On a foundation slab-on-grade for a new build, the math returns the volume and the practical-order amount, but the calculator doesn't know your subgrade. If the engineer spec'd a stiffer mix (4500-5000 psi for a slab-on-grade with point loads), the volume doesn't change but the price does. If the soil report flagged expansive clay, the slab gets thicker (5-6") or gets reinforcement upgrades the calculator can't see. The number above is volume; structural decisions live with the engineer and the inspector.

On a detached-garage floor, 4" is the standard pour over a compacted aggregate base. I default to 5% cushion on a level subgrade and bump to 10% if the form bows or the prep is rough. Driveway extensions get the same defaults but I add a saw-cut control joint at the seam where the new pour meets the existing slab — concrete shrinks toward joints, and a controlled crack beats a random one every time.

Where this number breaks down

A few traps that put the calculator on the wrong side of the order:

Vapor barrier non-conformance.ACI 302.1R-15 specifies a 10-mil vapor retarder per ASTM E1745 under any slab over earth. Sites still ship the old 6-mil polyethylene roll because it's on the shelf — it doesn't meet E1745, and you find out 18 months later when the finished flooring delaminates from slab moisture migration. Spec the 10-mil at order time.
Pour temperature window.ACI 305.1-14 caps general-construction concrete at 95°F at discharge. ACI 306R-16 triggers cold-weather protection at 40°F (air falls or is expected to fall below). On a Chicago shoulder season, check the dispatch forecast for both ends of the day — a 7am pour at 38°F means the truck arrives outside the protection envelope, and you're negotiating an admixture or a rescheduled pour at 6:30am.
Edge-form bow.A long form fights hydrostatic pressure when the truck starts chuting. On a 24-foot run with a 4" pour the head pressure is small, but if you're forming a 6" thickened-edge slab, the bottom of the form sees ~75 lb/sq ft of lateral force. Stake the form every 4 feet on the long side, every 2 feet at the corners.
Control-joint spacing.PCA spec is 24 to 36 times the slab thickness in inches — for a 4" slab, joints every 8-12 feet in both directions. Skip this and the slab cracks where it wants to. Joint depth at least one-quarter the slab thickness; cut within 6-18 hours of the pour.
Finishing window vs discharge window. ASTM C94 gives you 90 minutes from batching to discharge. A 24-foot garage slab is one truck most of the time, but if the placer and the finisher are the same person, the bleed water timing on a hot day cuts the finishing window short. Two-person crew is the operational floor on anything over 4 yd³.
The 28-day cure clock.Concrete reaches roughly 70% strength at 7 days and 100% at 28 days (ACI 318). Vehicle traffic at day 7 for a residential driveway, full heavy load at day 28. Backfilling against a freshly stripped footing or stem wall before day 14 risks lateral movement and settlement. The calculator output doesn't schedule anything — your project plan does.

Methodology

Every number on this page traces to one of four layers — site arithmetic for the volume math, manufacturer data sheets for bag yields, ACI / ASTM / NRMCA / PCA standards for pour and joint guidance, and IRC 2024 R506 for residential slab-on-grade code minimums. The per-cluster sourcing tier in methodology spells out which sources back which kinds of claims.

Show the formulas

Slab volume: (L ft × W ft × T″ / 12) / 27 = cubic yards.
Practical order: exact × (1 + waste%/100), rounded UP to the next 0.25 yd³ for Chicago-suburb residential dispatch. Strict half-yard dispatchers add one further step.
Bag count: ceiling of the EXACT yardage at 45 bags / yd³ for 80 lb (Quikrete #1101 yields 0.60 ft³/bag) or 60 bags / yd³ for 60 lb (yields 0.45 ft³/bag). Bagged is practical below ~1 yd³; ready-mix is the call above.
Control-joint grid: 24-36× slab thickness in inches (PCA). 4″ slab → 8-12 ft. Joint depth ≥ ¼ slab thickness, cut within 6-18 hours.

Frequently asked

How thick should a residential concrete slab be?

A standard interior slab or detached-garage floor is 4 inches over a compacted base. Driveway slabs designed for passenger vehicles are typically 4 inches; heavy-truck driveways or workshop floors with rolling loads bump to 6 inches. IRC 2024 R506 sets the minimum residential slab-on-grade at 3.5 inches, but 4 inches is the practical floor — below 4 you lose tolerance for the next problem (a subgrade dip, a form bow, a finishing window that closes early).

How much concrete do I need for a 24×24 garage slab?

A 24-foot by 24-foot slab at 4 inches thick is 7.11 cubic yards exact. With a 5% waste cushion that rounds up to 7.50 yd³ as the practical-order amount on a quarter-yard Chicago-suburb dispatcher; on a strict half-yard dispatcher, order 7.5 or 8.0. Bagged is impractical at this volume — about 320 bags of 80-pound mix — so ready-mix is the call. Use the calculator above for any slab dimensions; the math is length × width × thickness, divided by 27.

Do I need a vapor barrier under a concrete slab?

Yes for any habitable space, finished basement, attached garage, or pour you'll later cover with flooring. ACI 302.1R-15 specifies a vapor retarder of at least 10 mils per ASTM E1745; the older 6-mil polyethylene practice doesn't meet ASTM E1745 and shouldn't be used as below-slab moisture protection. For heavy traffic or angular subbase, 15 mils is the recommended thickness. The cost difference between 6-mil and 10-mil sheeting is small; the cost of replacing finished flooring after the slab sweats moisture is not.

What temperature can you pour a concrete slab in?

ACI 306R-16 (cold weather) defines cold conditions as air temperature falling, or expected to fall, below 40°F during the protection period — at and below that threshold you need insulating blankets, a heated enclosure, or accelerating admixtures. ACI 305.1-14 (hot weather) caps general-construction concrete temperature at 95°F at the point of discharge — above that, slump loss and premature set kill the finishing window. (The 85°F figure floating around the internet is the mass-concrete threshold for elements over 1 meter thick — not residential slabs.) On a Chicago shoulder season — March or October — both ends of the day can swing through risky territory; pour mid-morning to give the cure window all the daylight you can.

How long until I can walk on a new concrete slab?

Light foot traffic in 24 to 48 hours depending on temperature and mix. Vehicle traffic typically in 7 days for a residential driveway — concrete reaches roughly 70% of design strength at 7 days and full design strength at 28 days per ACI 318. Don't backfill against fresh forms before day 14; lateral pressure on uncured concrete is how cracks happen. The calculator returns volume; the cure schedule belongs on your project plan.

How far apart should control joints be in a slab?

PCA's rule of thumb is 24 to 36 times the slab thickness in inches — equivalent to 2 to 3 times the thickness expressed in feet. A 4-inch residential slab gets joints every 8 to 12 feet in both directions. Joint depth should be at least one-quarter of the slab thickness — for a 4-inch slab, that's a 1-inch saw cut. Cut joints within 6 to 18 hours of the pour (early-entry saws extend the front of that window). If you skip control joints, the slab will pick its own crack lines; planned cracks are aligned, unplanned cracks are random.

What I'd do next

Footing, post-hole, or stair math
If the slab is one piece of a bigger pour — footing, post hole, stair stoop — the parent calculator has all four shape modes.
Slab math, expanded
The rectangle math, the cushion, the bag-vs-truck breakpoint — long-form.
Bag count vs ready-mix economics
Once you have the yardage, decide bagged or ready-mix. The economics flip around 1 yd³.

Also in this cluster

By James Wu. Volume math is site arithmetic, formulas above. Bag yields trace to Quikrete Concrete Mix #1101 data sheet and Sakrete High-Strength Concrete Mix (ASTM C387). Ready-mix delivery minimums and short-load policy from NRMCA CIP 31 — Ordering Ready Mixed Concrete; discharge window from ASTM C94/C94M — Standard Specification for Ready-Mixed Concrete. Hot-weather practices from ACI 305.1-14 — Specification for Hot Weather Concreting (95°F discharge cap, general construction). Cold-weather practices from ACI 306R-16 — Guide to Cold Weather Concreting (40°F protection threshold). Slab-on-grade vapor retarder from ACI 302.1R-15 — Guide to Concrete Floor and Slab Construction and ASTM E1745 (10-mil minimum). Control-joint spacing from NRMCA CIP 29 — Vapor Retarders Under Slabs on Grade and PCA technical guidance (24-36× slab thickness in inches). Residential code minimums from IRC 2024 R506 (slab-on-grade requirements). Engine logic in lib/sitework/concrete.ts. Not structural-engineering advice — for your specific subgrade, frost line, and reinforcement spec, consult the local building inspector or a licensed structural engineer. Full methodology.