Concrete Slab Calculator
Plan a concrete slab from start to finish — concrete volume (cubic yards, bags, truck loads), rebar or wire mesh, gravel base, vapor barrier, and a full cost breakdown with a downloadable PDF estimate.
Material quantities are geometric estimates and costs use 2024-2025 U.S. averages. Prices vary by region and contractor — always order a little extra and get 3+ quotes. For planning purposes only.
| Thickness | Cubic Yards | 80 lb Bags | Truck Loads | Concrete Cost |
|---|
For your 400 sq ft slab (waste included). Concrete cost = ready-mix only, at your $/cu yd.
How to use the concrete slab calculator
Four quick steps turn your slab dimensions into concrete volume, reinforcement, base material, and a full installed-cost estimate — no math required on your end.
Size your slab
Enter length × width (or total sq ft), pick a thickness, and set a quantity for identical pours. Or tap a preset like Patio 12×12 or 2-Car 20×20.
Add reinforcement & base
Choose wire mesh or a rebar grid, then toggle a gravel subbase, 6-mil vapor barrier, or old-slab removal to match your build.
Set waste, mix & rates
Dial in a 5–10% waste allowance, pick a bag size, and enter your local ready-mix and labor prices for accurate costing.
Read results & save a PDF
Get cubic yards, bag counts, truck loads, material quantities, and a full cost breakdown — then download a two-page PDF estimate.
How a slab estimate is calculated
It all starts from one idea: volume = area × thickness, worked in matching units. From there the calculator layers on reinforcement, base, and cost. Here is the exact math behind each figure.
Worked example — a 20×20×4″ 2-car pad
Concrete: 400 × (4/12) = 133.3 cu ft = 4.94 cu yd
With 10% waste: 146.7 cu ft = 5.43 cu yd (order ~5.5)
Wire mesh: 400 × 1.10 ÷ 50 = 9 sheets
Gravel base: 400 × (4/12) ÷ 27 = 4.94 cu yd
At ~$160/cu yd + mesh + gravel + $3/sq ft finishing → roughly $2,500–$3,500 all-in (~$6–$9/sq ft).
Slab estimating charts
Handy lookups for the questions people ask before a pour — common slab volumes, installed cost per square foot, thickness, and reinforcement. Volume figures exclude waste, so add 5–10% to your final order.
| Slab | Area | Cu yd @ 4″ | Cu yd @ 6″ | Supply |
|---|---|---|---|---|
| Shed pad 8×10 | 80 ft² | 0.99 yd³ | 1.48 yd³ | Bags or ready-mix |
| Patio 12×12 | 144 ft² | 1.78 yd³ | 2.67 yd³ | Bags or ready-mix |
| 2-car pad 20×20 | 400 ft² | 4.94 yd³ | 7.41 yd³ | Ready-mix |
| Garage 24×24 | 576 ft² | 7.11 yd³ | 10.67 yd³ | Ready-mix |
| Shop 40×60 | 2,400 ft² | 29.63 yd³ | 44.44 yd³ | Ready-mix |
Above ~2–3 cubic yards, bagging becomes impractical and ready-mix is usually cheaper with far less labor. At 5″ thick, multiply the 4″ figure by 1.25.
| Scope | Typical cost |
|---|---|
| Materials only (you pour) | $3–$5 / ft² |
| Basic slab, professionally installed | $6–$10 / ft² |
| Reinforced w/ gravel base & finishing | $8–$14 / ft² |
| Stamped, colored, or decorative | $12–$20+ / ft² |
Figures reflect 2024–2025 U.S. averages and vary widely by region, access, and site prep. Get 3+ local quotes before committing.
| Application | Typical thickness |
|---|---|
| Patios, walkways, shed pads | 4 in |
| Standard car driveway | 4–5 in |
| Garage floor, heavy / RV pad | 5–6 in |
| Heavy equipment / structural | 6–8 in |
| Footings & structural slabs | Per code / engineer |
Going from 4″ to 6″ adds 50% more concrete. Always follow local building codes and any engineer’s spec.
| Item | Typical cost | Best for |
|---|---|---|
| Wire mesh | ~$0.35 / ft² | Patios, sheds, light slabs |
| Rebar grid | ~$0.75 / linear ft | Driveways, garages, structural |
| Gravel base (4″ compacted) | ~$1.50 / ft² | Drainage & uniform support |
| Vapor barrier (6-mil poly) | ~$0.10 / ft² | Interior & moisture-sensitive |
| Old slab removal | ~$2.50 / ft² | Replacing an existing slab |
Rebar is typically set 16–18″ on center in both directions for residential slabs. Reinforcement, base depth, and spacing should follow local code.
Everything the calculator works out
One set of slab details gives you every number needed to plan the pour and price the job — from concrete volume to a line-item cost breakdown.
Key figures every slab depends on
Built for any slab project
From a weekend shed pad to a full garage floor, the same calculator scales to the job — and prices it out.
Pouring a patio, shed pad, or small slab and wants the exact bag count, mesh, and gravel for one supply run.
- Start from a preset like Patio 12×12
- Compare bags vs ready-mix by volume
- Add gravel base and vapor barrier
Prices flatwork daily and needs a fast, defensible per-sq-ft figure with reinforcement and labor built in.
- Enter local $/cu yd and $/sq ft rates
- Switch mesh to a rebar grid instantly
- Hand the PDF estimate to the client
Scoping a larger build and setting a materials budget across concrete, base, and reinforcement.
- Model thickness options side by side
- Check truck loads for pour-day logistics
- Batch identical pads by quantity
7 tips for a slab that lasts
Small habits keep your order right the first time and your slab crack-free — avoiding the two classic mistakes: running short mid-pour, or skipping prep that shows up as cracks a year later.
Concrete slab calculator FAQ
The cost, volume, and material questions people ask most before pouring a slab.
Most concrete slabs run $6 to $14 per square foot installed in 2024–2025, depending on thickness, reinforcement, and finish.
• Materials only (DIY pour): ~$3–$5 / ft²
• Basic slab, professionally installed: ~$6–$10 / ft²
• Reinforced with gravel base & finishing: ~$8–$14 / ft²
• Stamped, colored, or decorative: ~$12–$20+ / ft²
As a guide, a 20×20 ft (400 ft²) 2-car pad at 4″ with mesh, gravel, and finishing typically lands around $2,500 to $3,500. Always get 3+ local quotes.
Volume is area × thickness in matching units. Multiply length by width for square feet, multiply by thickness in inches divided by 12 for cubic feet, then divide by 27 for cubic yards.
Example: A 20 ft × 20 ft slab at 4 inches = 400 × (4/12) = 133.3 cu ft = 4.94 cu yd. Add 5–10% for waste, so order about 5.5 cubic yards of ready-mix.
• 4 inches: Standard for patios, walkways, sidewalks, and shed pads.
• 4 to 5 inches: Recommended for standard passenger-vehicle driveways.
• 5 to 6 inches: Ideal for garage floors, heavy trucks, or RV parking pads.
• 6 to 8 inches: Heavy equipment or structural slabs.
• Footings & structural slabs: Per local code and soil (often 8 to 12+ inches).
Thicker slabs need proportionally more concrete — going from 4 to 6 inches increases volume by 50%.
Reinforcement is recommended for almost every slab to control cracking as concrete shrinks and settles.
• Wire mesh (~$0.35/ft²): Good for patios, sheds, and other light, non-structural slabs.
• Rebar grid (~$0.75/linear ft, typically 16 to 18″ on center): Better for driveways, garage floors, and any slab carrying vehicles or structural loads.
Reinforcement doesn’t prevent hairline cracks entirely, but it holds them tight and keeps the slab acting as one piece.
A compacted gravel base — usually 4 inches (~$1.50/ft²) — provides drainage and uniform support so the slab doesn’t settle unevenly. It’s strongly recommended for almost all slabs on grade.
A 6-mil poly vapor barrier (~$0.10/ft²) sits between the gravel and the concrete to block ground moisture. Use it under any interior slab, garage floor, or slab that will get flooring, insulation, or moisture-sensitive contents.
A 20 ft × 20 ft (400 ft²) slab at 4 inches thick needs about 4.94 cubic yards of concrete (5.4 with 10% waste).
At typical 2024–2025 rates — ready-mix around $160/cu yd, wire mesh, a 4″ gravel base, and $3/ft² finishing — the all-in cost usually runs about $2,500 to $3,500, or roughly $6 to $9 per square foot. Removing an old slab first adds about $2.50/ft².
Divide total cubic feet by the bag yield, then round up:
• 80 lb bag: ~0.60 cu ft (45 bags per cu yd)
• 60 lb bag: ~0.45 cu ft (60 bags per cu yd)
• 40 lb bag: ~0.30 cu ft (90 bags per cu yd)
A 10×10 ft slab at 4 inches (33.3 cu ft) needs about 56 80-lb bags. Bags are practical under roughly 2 cubic yards; above that, ready-mix is cheaper and far less labor.
Bagged concrete is manageable and cost-effective for small slabs under 1 to 2 cubic yards — shed pads, small patios, or step footings.
Ready-mix delivery is faster and far easier for anything over about 2 cubic yards, and it gives one continuous pour with no cold joints. Suppliers often charge a short-load fee for deliveries under 4 to 6 cubic yards, so weigh delivery fees against the labor of mixing dozens of bags.
General Estimating Notice: This calculator provides material quantities and cost estimates based on user input, standard material densities, and 2024–2025 U.S. average pricing. Actual job-site requirements and costs vary with subgrade preparation, formwork, aggregate, finishing, regional rates, and contractor pricing. Always order a little extra, verify structural specs and footing dimensions against local building codes, and get multiple quotes before ordering.