The Concrete Slab Calculator estimates the volume and weight of concrete needed to cover a given area. Convert length, width, and thickness to cubic yards, 60-pound and 80-pound bag counts, weight, and ready-mix cost in seconds. Purchasing slightly more concrete than the estimated result reduces the probability of having insufficient concrete on the jobsite.
Cubic yards · bags · weight · cost — 100% free, no signup
4" — sidewalk / patio
Pouring multiple identical slabs? Set the count here.
Industry default is 10% to cover spillage and forms.
US average is ~$160/yd³ delivered. Override for your region.
100% free. No signup required. Results in seconds.
How the calculation flows
Each pour passes through four stops on the way to the supplier. The math is fixed; only the inputs change.
Length × width in feet, thickness in inches.
L × W × (T ÷ 12). Inches converted to feet.
ft³ ÷ 27, then × (1 + waste%).
60 bags of 60 lb · 11,025 lb of mud.
Fundamentals
A yard of concrete is 1 cubic yard, equal to 27 cubic feet (0.7646 m³) of cured material. Concrete is sold by volume, not weight, and three fundamentals carry most of the math behind every concrete calculator: the cubic yard, bag yields, and density.
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.
01 · Volume
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.
02 · Bags
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.
03 · Weight
Normal-weight concrete = 150 lb/ft³ (NRMCA standard). That's 4,050 lb per cubic yard — about two tons of mud per partial truckload.
Material primer
Concrete volume measures the cubic space a pour fills, expressed in cubic yards (yd³) for ready-mix orders or cubic feet (ft³) for bagged jobs. The math is identical worldwide; only the unit changes.
MATERIAL
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
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
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).
The formula
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.
Interactive diagram
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.
Live preview
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.
20 × 20 × 4″ · 10% waste included
How to use it
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.
Reference table
Pre-computed cubic yards, 60 lb bag counts, weight, and estimated ready-mix cost for the most common residential slabs — including 10x10, 12x12, 20x20, 30x24, and 40x30 dimensions. All values include the standard 10% waste factor.
| Slab size (ft) | Thickness | Cubic feet | Cubic yards | 60 lb bags | Weight | Ready-mix |
|---|---|---|---|---|---|---|
| 5 × 5 | 4″ | 9.2 | 0.34 | 21 | 1,375 lb | $114 |
| 10 × 10 | 4″ | 36.7 | 1.36 | 82 | 5,500 lb | $277 |
| 12 × 12 | 4″ | 52.8 | 1.96 | 118 | 7,920 lb | $373 |
| 15 × 15 | 4″ | 82.5 | 3.06 | 184 | 12,375 lb | $489 |
| 20 × 10 | 4″ | 73.3 | 2.72 | 163 | 11,000 lb | $495 |
| 20 × 20 | 4″ | 146.7 | 5.43 | 326 | 22,000 lb | $869 |
| 24 × 24 | 4″ | 211.2 | 7.82 | 470 | 31,680 lb | $1,252 |
| 30 × 24 | 5″ | 330.0 | 12.22 | 734 | 49,500 lb | $1,956 |
| 40 × 30 | 6″ | 660.0 | 24.44 | 1467 | 99,000 lb | $3,911 |
| 50 × 40 | 6″ | 1100.0 | 40.74 | 2445 | 165,000 lb | $6,519 |
Comparison
Same material, different delivery. Concrete mixer trucks deliver ready-mix as a viscous fluid in 9–11 cubic yard loads; pre-mixed 60-pound bags and 80-pound bags cover small DIY work. The crossover point sits right around 1 cubic yard — below that, bagged usually wins; above, ready-mix takes over.
Sizing
Industry rules of thumb for residential pours, with target compressive strength (psi) for each use case. Always verify slab thickness and rebar spacing with your local building department before placing the order.
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
Answers to the most common questions about calculating concrete volume, thickness, bag counts, weight, and cost.
More calculators
Slabs are just the start. Run these calculators for square footings, walls, holes, round footings, columns, circular slabs, tubes, curb and gutter barriers, and stairs — plus pre-mix bag counts, total cost, and concrete weight.
Continuous and spread footing volume calculator — yards, bags, and weight from length, width, and depth.
Open calculatorRound concrete column / pillar calculator — volume from diameter and height for any number of columns.
Open calculatorSolid concrete wall calculator — yards and bags from length, height, and thickness.
Circular concrete slab calculator from diameter and thickness.
Volume for cast-in-place concrete stairs by tread, rise, and width.
Concrete needed to fill cylindrical sonotubes.
Concrete for poured curbs and gutters.
Concrete needed per fence post or deck post hole.
Concrete for residential and heavy-duty driveways with thickness guidance.
Concrete for patios — bag and ready-mix options with cost estimate.
Sidewalk concrete by length, width, and thickness.
Concrete for residential foundations — slab + footings.
Concrete for garage slab including thickness guidance for vehicle weight.
Concrete pad for sheds and outbuildings.
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 calculatorConvert length, width, and depth to cubic yards of concrete with industry-standard 10% waste factor included.
Open calculatorEstimate ready-mix delivered cost or bagged DIY cost — includes typical short-load fee for orders under 3 yards.
Open calculatorConcrete weight in pounds and tons from cubic yards or cubic feet.
Cement, sand, and aggregate quantities for common DIY mix ratios (1:2:3, 1:2:4).
Rebar quantity, total linear feet, and grid spacing for slabs.
Gravel base needed beneath a concrete slab or footing.
Cure time to reach foot, vehicle, and full design strength.
Run the slab calculator, save the numbers, then call your supplier. The math is exact — your only job is to measure.
Privacy first
Inputs never leave your browser.
100% free
No signup, no upsell, no ads.
Supplier-ready
Numbers match how concrete is sold.
Built for both
DIY homeowners and pro contractors.
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