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Free concrete slab calculator — cubic yards, bags, and costs for any slab size. Rebar, ready-mix trucks, and materials for patios and foundations.
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Enter measured project values. Results update only when you choose Calculate.
Method
A 30 x 40 shop slab at a nominal 4 inches figures 14.8 cubic yards — but if the subgrade runs a half inch low across the pour, the real depth is 4.5 inches and the slab actually takes 16.7 yards. Order exact and you are watching a finisher stand around while a second truck fights traffic with two yards on board, paying a short-load fee, and cold-jointing a floor that was supposed to be monolithic. Slab math is simple; the margin decisions around it are where pours go sideways.
This calculator is for converting plan dimensions into a batch-plant order: garage and shop floors, driveways, patios, equipment pads, and the flatwork portion of monolithic pours. Use it to build the takeoff, sanity-check a sub's yardage before you sign the ticket, and size the reinforcement and vapor barrier buy in the same pass — because those quantities come off the same length and width you already measured.
4 inches is the standard for interior floors, patios, and walkways; driveways and garage slabs carrying vehicle loads want 5 to 6 inches, and anything seeing truck or equipment traffic should be 6 inches with a thickened edge. The thickness decision moves the yardage more than any measuring error — going from 4 to 6 inches is a 50 percent increase in concrete.
Length x width x thickness in feet, divided by 27. A 24 x 24 garage at 4 inches is 24 x 24 x 0.333 / 27 = 7.1 yards. Walk the forms and shoot the subgrade first: the plan says 4 inches, but the concrete fills whatever is actually there, including every low spot and the thickened edges and haunches the drawing shows in section.
Carry 5 percent extra on a well-graded, well-formed slab and 10 percent when the subgrade is rough or the pour includes footings and turndowns. Round up to the half-yard increment the plant batches. Running short mid-pour means a short-load fee, a waiting crew, and a cold joint; a half yard left in the truck is the cheapest insurance on the job.
For mesh, divide slab area by sheet coverage and add a square of overlap at every lap; for rebar, a grid at 12 to 18 inches on center each way is (span / spacing) + 1 bars per direction, typically #3 or #4. Under interior slabs, figure vapor barrier at slab area plus 10 percent for the 6-inch lap requirement, and count chairs — reinforcement lying on the poly is doing nothing.
Multiply length in feet x width in feet x thickness in inches divided by 12, then divide by 27. A 20 x 20 patio at 4 inches is 20 x 20 x 0.333 / 27 = 4.9 yards. The one-line shortcut: square feet x thickness in inches / 324 gives the same answer.
4 inches for residential floors, patios, sidewalks, and shed pads on decent soil. Go 5 to 6 inches for driveways and garage slabs, and 6 inches where trucks or equipment will sit. Thickened edges and interior footings on the structural drawings add volume beyond the flat-slab math, so take them off separately.
About 45. An 80-lb bag yields roughly 0.60 cubic feet, and 27 / 0.6 = 45; 60-lb bags run about 60 per yard and 40-lb bags about 90. That is why bagged concrete only pencils out for small pads and repairs — a modest 12 x 12 slab at 4 inches is already about 80 bags.
5 to 10 percent over the calculated volume. Subgrade variance, spillage, pump-line prime, and form deflection all eat concrete, and the downside is asymmetric: a leftover half yard costs little, while coming up short means a short-load fee for a second truck and a cold joint in the middle of the slab. Rough subgrade or integral footings push you to the 10 percent end.
For most 4-inch residential flatwork, 6x6 welded wire mesh or fiber handles shrinkage-crack control; slabs at 5 inches and up carrying vehicle loads typically get #3 or #4 rebar at 12 to 18 inches on center each way. Either way it only works if it is chaired up into the slab, not lying on grade. Check the local code and soils report — expansive soil can dictate heavier reinforcement regardless of thickness.
Under any interior or conditioned slab, yes — code generally requires a polyethylene vapor retarder (commonly 10 to 15 mil in practice, 6 mil minimum) under floors that will see flooring or living space. Figure slab area plus about 10 percent for the 6-inch overlaps at seams and turnups at edges. Exterior flatwork like driveways and patios normally pours directly on the compacted base without one.
Spec it by load. 4 in for foot traffic, 5 in for daily car traffic, 6 in for one-ton+ pickups or RV pads, 8 in if you are parking equipment. IRC R506 sets 3.5 in as the floor for slab-on-grade in habitable space, and ACI 332 governs the residential numbers most inspectors actually check. Skinny slabs crack; oversized slabs eat your margin.
The math is third-grade arithmetic, but the gotcha is units. Tape out length and width in feet, pull thickness off the plans in inches, then bring everything to feet before you multiply. I have watched a crew over-order four yards because someone left thickness in inches.
Pull length and width off the forms after they are squared and braced, not off the plan. A 4-inch slab is 4 / 12 = 0.333 ft. Take depth measurements at four points across the pour; if your sub-base sags an inch in the middle, your yardage is wrong.
Multiply through. For an L-shape or a thickened edge, break the pour into rectangles and add them up. Thickened edges around the perimeter add yards quickly.
Ready-mix is priced by the yard (27 cu ft is a 3x3x3 cube). A standard mixer truck holds about 10 cu yd, and most plants charge a short-load fee of roughly $40-90 per yard short of a full truck, so plan the trip count before you call.
Add 10% on a clean rectangular pour with stable subgrade. Bump it to 15% if your sub-base is uneven, you are working with curved forms, or it is your first time running the wheelbarrow line. You can always send the leftover yard back to make a couple of splash blocks, but you cannot stretch a load that came up short.
1. Thickness in feet: 4 / 12 = 0.333 ft
2. Volume: 10 x 10 x 0.333 = 33.3 cu ft
3. Yardage: 33.3 / 27 = 1.23 cu yd
4. With 10% waste: 1.23 x 1.10 = 1.36 cu yd
Call it 1.5 yd from the plant, or mix on-site with about 46 bags of Quikrete 5000 80lb (each yields 0.6 cu ft). At 1.36 yd you are under the 3-yd plant minimum most places enforce, so expect a short-load fee or plan to bag it.
1. Thickness in feet: 6 / 12 = 0.5 ft
2. Volume: 20 x 24 x 0.5 = 240 cu ft
3. Yardage: 240 / 27 = 8.89 cu yd
4. With 10% waste: 8.89 x 1.10 = 9.78 cu yd
Order a full 10-yard truck. You are 0.22 yd over the load, which is well inside the slump you will lose to spillage and rebound off the chute. Pad it half a yard if the sub-base is sandy or you have a thickened edge at the apron.
Yardages crews call in week after week, pre-waste. Add 10% before you pick up the phone with the batch plant -- 15% if the sub-base is loose or you are pouring a thickened edge. Bag counts assume 80lb bags at 0.6 cu ft yield (Quikrete 5000, Sakrete High Strength, and equivalents all sit there).
Production field guide
4" — Sidewalks, patios, shed floors — IRC R506 minimum for unheated slab-on-grade. Fiber mesh in the mix saves a layout step on small pads.
5" — Residential driveways, garage floors — Passenger cars and half-ton pickups. Drop in #3 rebar at 18 in on center or 6x6 W2.9 WWR.
6" — Heavy vehicle driveways, RV / boat pads — Three-quarter and one-ton pickups, RVs, dump trailers. #4 rebar at 12 in on center, both directions.
8" — Foundations, loading docks, monolithic pours — Per ACI 318 you are designing this slab, not eyeballing it. Two mats of #4 or #5 typical.
10-12" — Industrial, heavy equipment pads — Forklift aisles, machinery anchors, fire-truck turnouts. Engineer-stamped mix design (4,000-5,000 PSI).
10 x 10 ft — 4" — 1.23 — 56 — Shed pad, fits a 10x10 Tuff Shed; below plant minimum, plan to bag it
12 x 12 ft — 4" — 1.78 — 80 — Backyard patio or hot tub pad (350 gal tub fully loaded)
20 x 20 ft — 4" — 4.94 — 222 — Two-car parking pad in front of a detached garage
24 x 24 ft — 4" — 7.11 — 320 — Detached garage floor; bump to 5 in if vehicles park inside
10 x 20 ft — 5" — 3.09 — 139 — Single-car driveway strip from the apron to the garage
20 x 24 ft — 6" — 8.89 — 400 — F-250 or RV pad; one ready-mix truck covers it with room for the chute
How many bags of concrete do I need for a 10x10 slab? — A 10x10 ft pad at 4 in thick is 1.23 cu yd, which works out to roughly 56 bags of 60lb mix or 46 bags of 80lb Quikrete 5000 / Sakrete High Strength (0.6 cu ft yield per 80lb bag) once you include 10% waste. Be honest with yourself: 56 bags is two pallets of mixing, plus water, plus a mixer rental. Anything over a single yard, most crews just call the plant and eat the short-load fee.
How thick should a concrete slab be? — Spec it by load. 4 in for foot traffic (sidewalks, shed pads, patios) per IRC R506. 5 in for residential driveways and garage floors taking passenger cars or half-ton pickups. 6 in for three-quarter- and one-ton pickups, RVs, and boat trailers. 8 in once you cross into foundation or loading-dock territory, where ACI 318 takes over and you need an engineer on the design. 10-12 in for industrial floors and equipment pads.
How much does a concrete slab cost per square foot? — Plain broom-finish runs $6-12 per sq ft installed in most US markets in 2025-2026, with ready-mix delivered at roughly $160-200 per cu yd plus a $40-90 short-load fee under 8-10 yards. Materials by themselves (concrete, #4 rebar, 2x lumber for forms, vapor barrier) sit around $3-5 per sq ft. Stamped, integral-color, or exposed-aggregate finishes add $4-12 per sq ft depending on the pattern and how much sample-board approval the homeowner wants.
Separate plan workflow
This calculator solves one bounded formula from the inputs shown. BuildVision AI supports reviewed plan takeoff, complete-document CSV, and editable quote lines; the estimator owns pricing and final bid approval.