Calculate how much concrete you need for a slab from its length, width, and thickness. Get concrete volume in cubic yards, cubic feet, and cubic metres, plus bag quantities, waste allowance, estimated weight, and material cost for patios, driveways, garage floors, sheds, sidewalks, and other concrete slabs.
Concrete Slab Calculator
Cubic yards · bags · weight · material cost
4" — sidewalk / patio
Pouring multiple identical slabs? Set the count here.
10% is an editable planning allowance for spillage and form variation.
Planning value only. Replace it with your supplier's complete delivered quote.
Multiply the slab's length by width by thickness to calculate its volume. Convert the result to cubic yards for ready-mix concrete, then add an allowance for uneven subgrade and spillage. Enter your measurements above to calculate cubic yards, cubic metres, concrete bags, weight, and estimated cost.
Each pour passes through four stops on the way to the supplier. The math is fixed; only the inputs change.
Dimensions
Length × width in feet, thickness in inches.
Cubic feet
L × W × (T ÷ 12). Inches converted to feet.
Cubic yards
ft³ ÷ 27, then × (1 + waste%).
Bags & weight
163 bags of 60 lb · about 11,000 lb of concrete.
Two equations move your dimensions all the way to a supplier order. Multiply for cubic feet, divide by 27 for cubic yards, then add the waste factor.
Cubic feet
ft³ = L × W × (T ÷ 12)
Length × width in feet, thickness in inches converted to feet.
Cubic yards (with waste)
yd³ = (ft³ ÷ 27) × (1 + waste%)
10% waste is the residential default. Drop to 5% for sonotubes; bump to 15% on rough sub-grade.
Worked example · 20 × 10 × 4″ patio
~$500 in concreteStep 1
20 × 10 × (4 ÷ 12)
Step 2
66.67 ÷ 27
Step 3
2.47 × 1.10
Order ~2.75 yd³. At ~$160/yd³ + $60 short-load (under 3 yd³), that's about $500 in concrete delivered.
Methodology note: net volume is converted with 27 ft³ per cubic yard, then waste is applied. Bag estimates use published yields, round up to whole bags, and assume normal-weight concrete at 150 lb/ft³. See the full methodology and sources for limitations.
One cubic yard equals 27 cubic feet (0.7646 m³). Ready-mix concrete is commonly ordered by volume, so this calculator converts your slab dimensions into cubic yards, cubic feet and cubic metres before applying waste.
VOLUME
1 yd³
DIMENSIONS
3 ft × 3 ft × 3 ft
One cubic yard equals a cube measuring 3 feet on every side, divisible into 27 equal cubic-foot pieces. Concrete mixer trucks typically carry 9–11 cubic yards per load.
The cubic yard
A cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Ready-mix suppliers quote and bill in yd³, and one truck typically carries 9–11 yards.
Bag yields
A 60 lb bag of pre-mixed concrete yields 0.45 ft³ of cured slab. So 1 yard ≈ 60 bags. 80 lb bags yield 0.6 ft³ → 45 bags per yard.
Density & weight
The weight estimate uses 150 lb/ft³ as a planning density. Actual unit weight varies with the specified mix and aggregate.
Need a more detailed bag-by-bag estimate? Use the Concrete Bag Calculator or compare material pricing in the Concrete Cost Calculator.
MATERIAL
What concrete is made of
Concrete is a material made of coarse aggregate — sand, gravel, crushed stone, and slag — bonded by cement. Portland cement is the most commonly used binder, and the same cement appears in mortar, plasters, and stucco mixes. Concrete differs from mortar by including the heavier coarse aggregate that gives the cured slab its compressive strength.
DELIVERY
How concrete reaches the site
Concrete arrives in two forms. Pre-mixed bags (40-pound, 50-pound, 60-pound bags, and 80-pound bags) cover small DIY pours. Concrete mixer trucks deliver concrete as a viscous fluid for slabs over ~1 cubic yard (0.76 m³). Some specialty mixes feature a rapid set time for repairs, and precast concrete walls and panels are factory-cured before installation.
CURING
How concrete cures
Curing is the slow concrete hardening process that builds strength after placement. Concrete reaches over 90% of its final strength in four weeks (28 days), and strengthening continues for up to three years. Keeping the surface damp increases the strength of the concrete during early curing — through spraying concrete slabs with curing compounds, or ponding (flooding the slab with a shallow water layer under plastic sheeting).
Drag the length, width, and thickness sliders to watch the slab change shape. Cubic yards, bag counts, and weight update live — the same math the calculator above runs.
Includes 10% waste factor. Drag the sliders to see the slab and numbers update in real time.
The slab visualization scales length, width, and thickness from your entered dimensions. Volume is calculated independently from the visual representation; this is an explanatory diagram, not a precision engineering drawing.
Same math, applied to seven typical residential pours. Hit a preset or slide between them to see how yards, bags, and cost scale across patio, driveway, garage, and shop slab dimensions.
Driveway
20 × 20 × 4″ · 10% waste included
Pre-computed volume, cubic yards, 60 lb bag counts, weight, and estimated ready-mix cost for common residential slabs. Calculated volume is shown before waste; the planning quantity includes an editable 10% allowance.
| Slab size (ft) | Thickness | Calculated volume (ft³) | With 10% waste (yd³) | 60 lb bags | Weight | Ready-mix |
|---|---|---|---|---|---|---|
| 5 × 5 | 4″ | 8.3 | 0.34 | 21 | 1,375 lb | $114 |
| 10 × 10 | 4″ | 33.3 | 1.36 | 82 | 5,500 lb | $277 |
| 12 × 12 | 4″ | 48.0 | 1.96 | 118 | 7,920 lb | $373 |
| 15 × 15 | 4″ | 75.0 | 3.06 | 184 | 12,375 lb | $489 |
| 20 × 10 | 4″ | 66.7 | 2.72 | 163 | 11,000 lb | $495 |
| 20 × 20 | 4″ | 133.3 | 5.43 | 326 | 22,000 lb | $869 |
| 24 × 24 | 4″ | 192.0 | 7.82 | 470 | 31,680 lb | $1,252 |
| 30 × 24 | 5″ | 300.0 | 12.22 | 734 | 49,500 lb | $1,956 |
| 40 × 30 | 6″ | 600.0 | 24.44 | 1467 | 99,000 lb | $3,911 |
| 50 × 40 | 6″ | 1000.0 | 40.74 | 2445 | 165,000 lb | $6,519 |
Run the Concrete Slab Calculator above in three steps: measure, pick a thickness, and read the order quantity in cubic yards or pre-mix bags.
Length, width, and thickness. For irregular shapes, break it into rectangles and sum the volumes.
4″ for sidewalks and patios, 5–6″ for driveways and shop floors. The calculator has presets for each.
Cubic yards (with waste) is the number to give your supplier. Bag counts are listed for DIY pours under ~1 yd³.
Same material, different delivery. Mixer trucks deliver ready-mix in larger loads; pre-mixed bags suit small DIY work. As project volume increases, ready-mix often becomes more practical because bag mixing requires substantial labor. Compare your local bag price, ready-mix rate, delivery fee, and short-load charges.
Ready-mix delivered
Often practical at scaleBagged concrete
Often practical for small poursTypical residential starting points — not engineering specifications. Appropriate thickness depends on loading, soil and subgrade conditions, reinforcement, climate, local codes and project design. For structural or vehicle-bearing slabs, verify the design with a qualified professional.
Compressive strength
3,500 psi
Use
Standard residential driveways for passenger vehicles.
Reinforcement
#3 rebar at 18" centers
Walkway / sidewalk
4"
3,000 psi
Patio
4"
3,000 psi
Driveway
4–5"
3,500 psi
RV / heavy
6"
4,000 psi
Garage floor
4"
3,500 psi
Shed foundation
4"
3,000 psi
Shop / barn slab
6"
4,000 psi
Light commercial
6"+
4,000+ psi
The calculation is intentionally simple and auditable. Use the assumptions below to decide whether they fit your project before placing an order.
Constants follow standard concrete-industry conventions and manufacturer yield data. Pricing is a user-editable estimate, not a quote. Thickness guidance is a residential rule of thumb; local codes, soil, loads, reinforcement, and project specifications control the final design.
Bag quantities use the published yield for the selected bag size and are rounded up to whole bags.
Cost covers the estimated concrete material and applicable ready-mix short-load fee; labor, forms, reinforcement, preparation, and finishing are excluded.
Read formulas, sources, and limitationsAnswers to the most common questions about calculating concrete volume, thickness, bag counts, weight, and cost.
Plan slab quantities, ordering and material costs with these focused supporting tools.
By shape
Concrete Footing Calculator
Continuous and spread footing volume calculator — yards, bags, and weight from length, width, and depth.
Open calculatorRound Column Calculator
Round concrete column / pillar calculator — volume from diameter and height for any number of columns.
Open calculatorRound Slab Calculator
Circular concrete slab calculator from diameter and thickness.
Open calculatorBy project
Driveway Concrete Calculator
Concrete for residential and heavy-duty driveways with thickness guidance.
Open calculatorPatio Concrete Calculator
Concrete for patios — bag and ready-mix options with cost estimate.
Open calculatorSidewalk Concrete Calculator
Sidewalk concrete by length, width, and thickness.
Open calculatorGarage Floor Concrete Calculator
Concrete for garage slab including thickness guidance for vehicle weight.
Open calculatorShed Slab Calculator
Concrete pad for sheds and outbuildings.
Open calculatorBy output
Concrete Bag Calculator
How many 40, 50, 60, or 80 lb concrete bags do you need? Side-by-side bag counts and total weight for any volume.
Open calculatorCubic Yard Calculator
Convert length, width, and depth to cubic yards of concrete with an editable waste allowance.
Open calculatorConcrete Slab Cost Calculator
Compare ready-mix and bagged material costs with editable local prices and short-load assumptions.
Open calculatorRelated
Concrete Slab Thickness Calculator
Planning thickness ranges and concrete quantities for common residential slab projects.
Open calculatorRebar Calculator for Concrete Slab
Rebar quantity, total linear feet, and grid spacing for slabs.
Open calculatorGravel Base Calculator for Concrete Slab
Gravel base needed beneath a concrete slab or footing.
Open calculatorPlan your pour with confidence
Run the slab calculator, save the numbers, then call your supplier. The math is transparent — your key job is to measure carefully and confirm local requirements.
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Inputs never leave your browser.
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Supplier-ready
Numbers match how concrete is sold.
Built for both
DIY homeowners and pro contractors.
Methodology & disclaimer
Volume math uses standard formulas (1 yd³ = 27 ft³, normal-weight concrete = 150 lb/ft³). Bag yields are pulled from manufacturer datasheets. Estimates only — confirm dimensions, code requirements, and pricing with your contractor or local supplier.
Read full methodology