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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.