Flatwork Concrete Estimator

Concrete Slab Calculator

Estimate concrete for patios, driveway pads, garage slabs, shed bases, and walkways. Enter slab dimensions to get cubic yards, cubic feet, cubic meters, bag count, waste, and material cost.

Slab Volume
Uses length, width, and thickness for flat pours.
Thickness Presets
Quick presets for patio, driveway, garage, shed, and walkway.
Bags or Yards
Compare bag count with ready-mix cubic yard volume.

Enter Slab Size

Slab area and raw volume before waste

120.00 sq ft / 40.00 ft3 / 1.48 yd3

Results

Concrete Volume

1.6296 yd³

44 ft³ / 1.2459 m³

Bags Needed

74 bags

Rounded up to whole bags

Estimated Cost

$370.00

Based on bag price

Waste Factor

10%

Included in total volume

Formula

Slab area

Area = length x widthArea = 12 x 10 = 120.00 sq ft

Concrete volume

Volume = length x width x thicknessVolume = 12 x 10 x 0.333

Waste and bags

Final volume = 40.0000 x (1 + 10%/100)Bags = ceil(final volume / 0.6)

Example Slab Estimates

12 ft x 10 ft patio

Backyard patio slab at 4 inches thick with 10% waste.

Result: 1.6296 yd3 / 44 ft3 / 74 bags

20 ft x 12 ft driveway pad

Small driveway or parking pad at 5 inches thick.

Result: 4.0741 yd3 / 110 ft3 / 184 bags

10 ft x 8 ft shed base

Simple shed foundation slab at 4 inches thick.

Result: 1.0864 yd3 / 29.3333 ft3 / 49 bags

Slab Thickness Guide

ProjectTypicalNote
Patio or walkway4 inCommon for light foot traffic on a prepared base.
Shed base4 inOften enough for small storage sheds with stable subgrade.
Garage slab4 inCommon residential starting point, subject to loads and code.
Driveway pad5 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.

Frequently Asked Questions

How do I calculate concrete for a slab?
Multiply slab length by slab width by slab thickness after converting thickness to feet or meters. Then add a waste factor and round bag counts up to whole bags.
How thick should a concrete slab be?
Many patios, walkways, shed bases, and garage slabs use about 4 inches of concrete. Driveway pads often use 5 inches or more depending on loads, base preparation, reinforcement, and local code.
How much waste should I add for a slab?
A 10% waste factor is a practical starting point for many flat slabs. Use more for uneven subgrade, irregular forms, thickened edges, first-time DIY pours, or projects where running short would be costly.
Should slab concrete be ordered in cubic yards?
In the United States, ready-mix concrete is usually ordered by cubic yard. Bagged concrete is usually labeled by cubic feet of yield. This calculator shows both units plus metric volume.
Does this calculator include gravel base or reinforcement?
No. This page estimates concrete material only. Gravel base, reinforcement, vapor barrier, forms, joints, finishing, delivery, and labor should be estimated separately.

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Concrete Volume CalculatorConcrete Bag CalculatorConcrete Steps CalculatorConcrete Footing CalculatorConcrete Cost CalculatorReady-Mix vs Bags Calculator

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.