Concrete slab estimator
Concrete Slab Calculator for Square Feet, Bags, and Cost
Estimate patios, driveway pads, garage slabs, shed bases, and walkways from square feet or dimensions. Get yards, bags, waste, material cost, and quote scope checks before ordering.
Answer first
100 sq ft at 4 in = about 1.36 yd3
With 10% waste, that is about 62 80-lb bags before you compare ready-mix delivery.
Enter Slab Size
Save this estimate or download a PDF report
The PDF report uses the current calculated result. Save the estimate first when you want to reuse the inputs later.
Results
Live estimateSlab pour scope plan
Turn slab yards into a quote-ready scope check
This slab result is marked as Site scope check needed with a slab scope score of 60/100. Use it to move from concrete volume into the jobsite and quote items that usually change the real slab price.
Slab area
120 sq ft / 11.1 m2
Thickness
4 in / 10.2 cm
Buying pressure
Bags versus ready-mix check
Missing slab scope before approval
- Subgrade compaction, gravel base depth, drainage, and vapor barrier.
- Inside-form dimensions, form bracing, thickened edges, and turndowns.
- Control joint layout, saw-cut timing, finish type, curing, and cleanup.
- Crew pace, truck access, chute reach, pump, buggy, or wheelbarrow distance.
- Slope away from structures, drainage path, and exposed-edge finish.
Slab quote questions to ask
- Does the quote include excavation, gravel base, compaction, forms, and form removal?
- Are reinforcement, vapor barrier, control joints, curing, cleanup, and haul-off separated?
- Does the concrete line match the calculator volume, waste factor, PSI, slump, and delivery window?
- Are access limits, wheelbarrow distance, pump or buggy needs, and waiting time priced?
- Are finish texture, drainage slope, steps, drains, and backyard access included?
Rebar and mesh quote check
Concrete slab reinforcement takeoff
Rebar calculators usually stop at sticks and weight. This slab worksheet connects the grid takeoff to the quote scope: bar size, spacing, cover, lap, chairs, tie wire, inspection timing, and whether wire mesh or fiber is being substituted.
Reinforcement signal
Quote scope check
Use the takeoff to compare whether rebar, mesh, chairs, labor, and inspection are included before approving the slab quote.
Grid bars
7 + 8
Bars in each direction
Order length
172.2 ft
156.5 ft before lap/waste
20 ft sticks
9
115 lb approx
Material check
$81
8 chair/support check
| Scenario | Spacing | Order length | Sticks | Material check | Quote question |
|---|---|---|---|---|---|
| Current grid assumption | 18 in | 172.2 ft | 9 | $81 | Does the written quote match this bar spacing and size? |
| Tighter 12 in spacing check | 12 in | 251.9 ft | 13 | $117 | Would tighter spacing change material, labor, or inspection scope? |
| Wider 24 in spacing check | 24 in | 126 ft | 7 | $63 | Is wider spacing actually allowed by the plan or local requirement? |
Reinforcement quote questions
Download takeoff rows
Use this as a planning worksheet. Replace every spacing, cover, lap, and price assumption with the drawing, supplier, or contractor quote before buying steel.
Concrete calculator square feet
Square feet to concrete yards
When you know the slab area but not the yardage, use square feet and thickness first. The shortcut is square feet x thickness in / 12 / 27, while one cubic yard covers about 324 / thickness inches square feet before waste.
Current area
120 sq ft
11.15 m2
Coverage per yd3
81 sq ft
before waste
This estimate
1.6296 yd3
74 bags
| Thickness | 1 yd3 covers | Yards with waste | 80 lb bags |
|---|---|---|---|
| 2 in | 162 sq ft | 0.8148 yd3 | 37 |
| 3 in | 108 sq ft | 1.2222 yd3 | 55 |
| 3.5 in | 92.57 sq ft | 1.4259 yd3 | 65 |
| 4 in | 81 sq ft | 1.6296 yd3 | 74 |
| 5 in | 64.8 sq ft | 2.037 yd3 | 92 |
| 6 in | 54 sq ft | 2.4444 yd3 | 110 |
Table uses the current waste factor and 80 lb bag yield of 0.6 ft3 unless your calculator result uses a different bag yield.
Formula
Slab area
Area = length x widthArea = 12 x 10 = 120.00 sq ftConcrete volume
Volume = length x width x thicknessYards from square feet = square feet x thickness in / 12 / 271 yd3 coverage = 324 / thickness in sq ftVolume = 12 x 10 x 0.333Waste and bags
Final volume = 40.0000 x (1 + 10%/100)Bags = ceil(final volume / 0.6)Example Slab Estimates
Slab Thickness Guide
| Project | Typical | Note |
|---|---|---|
| Patio or walkway | 4 in | Common for light foot traffic on a prepared base. |
| Shed base | 4 in | Often enough for small storage sheds with stable subgrade. |
| Garage slab | 4 in | Common residential starting point, subject to loads and code. |
| Driveway pad | 5 in+ | Heavier vehicles, poor soil, or edges may need more concrete. |
Before You Order
- Measure inside the forms, not the outside lumber length.
- Account for thickened edges, turndowns, steps, or drains separately.
- Confirm base depth, reinforcement, joints, and permits before pouring.
- Compare bag count with local ready-mix minimums for larger slabs.
Planning a concrete patio?
For patios, compare the slab dimensions, thickness, waste, 40/60/80 lb bag count, ready-mix material cost, backyard access, finish type, and installed quote scope before using the result as a buying number. For 12x12 and 20x20 examples, use the concrete patio calculator guide.
Planning a concrete sidewalk?
For sidewalks, split the run into straight sections, then compare cubic yards, 40/60/80 lb bags, ready-mix delivery, square-foot pricing, and linear-foot quote checks. For a 40 ft x 4 ft example, use the concrete sidewalk calculator.
Planning a concrete driveway?
For driveways, use a driveway-specific planning pass for thickness, apron tie-ins, replacement scope, drainage, base prep, access, and quote review. Use the concrete driveway calculator.
Planning a concrete garage slab?
For garage floors, compare the finished slab dimensions, thickness, waste, bag pressure, and ready-mix assumptions before using the result as a buying number. For a garage-specific tool, use the concrete garage slab calculator. For the full guide, use the concrete garage slab calculator guide for 20x20 and 24x24 examples, vapor barrier notes, apron checks, and quote questions.
Planning a concrete shed base?
For sheds, use the finished concrete pad size, not only the shed model name. A concrete shed base may need a manufacturer base size, border, ramp landing, gravel base, anchors, drainage, or backyard access plan before the calculator result becomes a buying number. For a shed-specific tool, use the concrete shed base calculator. For the full buying guide, use the shed base concrete mix calculator guide for 8x10, 10x12, and 12x16 examples, 40/60/80 lb bag counts, ready-mix cost checks, and shed foundation quote questions.
Frequently Asked Questions
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Disclaimer: This calculator is for concrete material planning only. Site conditions, base preparation, reinforcement, drainage, joints, form accuracy, and local building rules can change actual requirements. Consult a qualified contractor or engineer for structural work.