Construction Calculators
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Concrete Slab Calculator
Punch in your slab dimensions and we will give you the yardage to call into the batch plant, a Quikrete 5000 or Sakrete High Strength 80lb bag count if you are mixing on-site, rebar layout per ACI 332, and a ballpark material cost before the short-load fee.
Concrete Slab Calculator
Calculate concrete volume, bags, cost, and rebar for any slab size
Concrete Slab Thickness Guide
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.
| Thickness | Application | Notes |
|---|---|---|
| 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). |
How to Calculate Concrete for a Slab
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.
Step 1: Measure the Slab
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.
Step 2: Calculate Volume in Cubic Feet
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.
Step 3: Convert to Cubic Yards
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.
Step 4: Add Waste Factor
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.
Worked Examples
Example 1: 10 x 10 Patio Slab (4 inches thick)
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.
Example 2: 20 x 24 Driveway Slab (6 inches thick)
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.
Common Slab Sizes Reference
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).
| Slab Size | Thickness | Cubic Yards | 80lb Bags | Typical Use |
|---|---|---|---|---|
| 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 |
Concrete Slab Calculator FAQ
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.
How many cubic yards of concrete do I need?
Length (ft) x Width (ft) x Thickness (ft) / 27, then multiply by 1.10 for waste. Thickness has to be in feet, so a 4-in slab is 0.333 ft. A 20x20 pad at 4 in works out to 20 x 20 x 0.333 / 27 = 4.94 cu yd before waste, 5.43 after. Round up to the nearest quarter-yard when you order; the plant bills in quarter-yard increments anyway.
Do I need rebar in a concrete slab?
For anything you are driving on, yes. ACI 332 calls for reinforcement in residential slabs supporting vehicles -- #4 (1/2 in) bar at 12 in on center each way is the workhorse spec for driveways and garage floors. For a patio or shed pad at 4 in, 6x6 W2.9/W2.9 welded wire mesh (or fiber-reinforced mix) is plenty. Chair the steel up to mid-depth; rebar laid flat on the sub-base is decorative, not structural.
How long does concrete take to cure?
You can walk on it the next morning (24-48 hours). Pull forms at 48-72 hours. Light vehicle traffic at 7 days, when the slab is around 70% of design strength. Wait the full 28 days for full 4,000 PSI design strength before parking heavy equipment or backing a loaded trailer on it. Keep the surface damp or under plastic for the first week -- hot wind plus dry concrete plus no curing compound is how you end up with plastic shrinkage cracks at day three.
Should I use 4,000 PSI or 3,000 PSI mix?
For exterior flatwork in any climate that sees a freeze-thaw cycle, spec 4,000 PSI with 5-7% air entrainment. ACI 332 actually requires it for exterior residential slabs in freeze-thaw zones. 3,000 PSI is fine for interior basement slabs or a shop floor that never sees salt or freezing. The price gap is maybe $10-15 per yard; the repair gap if a 3,000 PSI driveway scales after one winter is the whole driveway.
What happens if the ground freezes deeper than my slab?
You crack. Frost heave lifts the slab unevenly, and a 4-in pad has no business resisting it. If your local frost line is below your slab base -- check your building department, but figure 30-48 in across most of the northern US -- you need either a frost-protected shallow foundation per IRC R403.3, a thickened perimeter footing down to the frost line, or 4 in of XPS insulation extending 24 in horizontally past the slab edge. Call this out in the proposal before you pour, not after the first January thaw.
Bidding more than one slab a week?
BuildVision AI reads your PDF plans, picks up slab outlines and thickened edges, and spits out yardage, rebar weight, and formwork linear feet straight onto a bid sheet. Stop scaling drawings with a digital wheel.
How the Concrete Slab Calculator Works
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.
Slab volume and material math
- Cubic yards = length ft x width ft x (thickness in / 12) / 27
- Shortcut: cubic yards = square feet x thickness in / 324
- 80-lb bags = cubic yards x 45 (60-lb bags = yards x 60)
- Order quantity = calculated yards x 1.05 to 1.10
- Vapor barrier = slab area x 1.10 for laps and turnups
- Mesh sheets or rebar grid = area / sheet coverage, or (span / spacing + 1) bars each way
Estimating Steps and Checks
Settle the real thickness before you measure anything
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.
Run the yardage from actual pour depth, not plan depth
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.
Add the ordering margin and round up
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.
Take off reinforcement and vapor barrier from the same dimensions
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.
Common Checks
- Coverage per yard by thickness, worth memorizing: one cubic yard covers about 81 sq ft at 4 inches, 65 sq ft at 5 inches, and 54 sq ft at 6 inches — divide slab square footage by the right number for a ten-second gut check on any quote.
- A half inch of subgrade slop is not a rounding error: on a 1,200 sq ft slab it is almost 2 extra yards. If the grading looks rough, shoot it and price the concrete off what is actually there.
- Bagged concrete stops making sense fast — at 45 80-lb bags per yard, even a 10 x 10 pad at 4 inches (1.23 yards) is roughly 56 bags to haul, mix, and place before the first batch starts setting. Anything over about a yard, call the ready-mix plant.
- Mesh that gets walked into the mud during the pour ends up at the bottom of the slab where it controls nothing. Spec chairs or bolsters to hold reinforcement in the upper third to middle of the slab, and count them in the takeoff.
Concrete Slab Calculator FAQs
How do I calculate cubic yards of concrete for a slab?
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.
How thick should a concrete slab be?
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.
How many 80-lb bags of concrete make a yard?
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.
How much extra concrete should I order?
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.
Do I need rebar or wire mesh in a concrete slab?
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.
Do I need a vapor barrier under a concrete slab?
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.