Slab Calculator - Concrete Yards, Bags, Joints & Waste
Slab volume is length times width times thickness. Add waste for uneven subgrade, then round ready-mix to cubic yards or bagged concrete to whole bags.
Percent added for cuts, waste, settlement, or field loss.
Finished mixed volume from one bag. Use the product label or data sheet, not only bag weight.
Bag count is shown separately; cost uses ready-mix volume.
Change any value and the results, formula, and diagram update immediately. Use the same unit system throughout one estimate.
Slab Volume Quick Reference
Ready-mix yards and 80 lb bag counts for common slabs at standard thicknesses. Use these for the dispatcher conversation; the calculator runs the exact numbers for your dimensions.
| Slab size | 4 in thick | 5 in thick | 6 in thick | 80 lb bags @ 4 in |
|---|---|---|---|---|
| 8 x 8 ft | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ | 36 |
| 10 x 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ | 56 |
| 12 x 16 ft (driveway pad) | 2.37 yd³ | 2.96 yd³ | 3.56 yd³ | 107 |
| 20 x 20 ft (single garage) | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ | 222 |
| 20 x 30 ft (RV / shop) | 7.41 yd³ | 9.26 yd³ | 11.11 yd³ | 333 |
| 24 x 24 ft (double garage) | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ | 320 |
Add 10% waste before ordering. Over about 30 bags, switch to ready-mix - the bag math stops working when mixing time outruns the cure of the first batch.
How Slab Volume Actually Works
Slab volume is length x width x thickness, all in the same unit. The classic trap is leaving thickness in inches when length and width are in feet. A 12 x 16 ft slab at 4 inches is 12 x 16 x 0.333 = 64 cubic feet, or 2.37 cubic yards. Drop the inches into the formula as 4 and you get 64 yards - the same call dispatchers take once a month.
The metric version: a 4 x 5 m slab at 100 mm is 4 x 5 x 0.1 = 2 m³. Same logic, different units. The calculator keeps unit conversions visible so you can spot when something looks 10x off.
Picking The Right Slab Thickness
3 in (75 mm)
Interior decorative pads, mowing strips, light pathways. Minimum for any slab that holds weight, even a person.
4 in (100 mm)
The residential default. Patios, basement floors, walkways, interior floors, light garage floors. With welded wire fabric or fibre.
5-6 in (125-150 mm)
Driveways, garage floors with normal vehicle traffic, sheds with equipment. Add #4 rebar grid at 16 in OC if loads are heavy.
6-8 in (150-200 mm)
Commercial light, heavy-truck driveways, agricultural floors. Always with rebar. Joint pattern matters.
8-12 in (200-300 mm)
Industrial slab-on-grade, equipment foundations, point-load areas. Engineered. Double mat of rebar.
Monolithic with edge beam
Slab thickness in the middle but 12-18 in thick at the perimeter. Common for slab-on-grade homes in southern US, slab huts, and shed bases. Calculator-wise: compute field and beam volumes separately and add.
Base Prep - What Goes Under The Concrete
A slab sitting on bare dirt cracks in the first cold season. Build the base from the bottom up:
- Subgrade: compacted native soil. Remove organics, top 6-8 in.
- Sub-base: 4-6 in of compacted crushed stone or gravel (3/4 minus, road base). Compacted in lifts.
- Vapor barrier: 6-mil (0.15 mm) poly under any heated, interior, or moisture-sensitive slab. Overlap 12 in at seams.
- Insulation: 2 in of XPS rigid foam under heated slabs or in cold-climate basements.
- Reinforcement: welded wire fabric (6x6 W2.9), fibre, or #4 rebar grid placed at mid-slab thickness.
Skipping the gravel base on expansive or clay soils is the single biggest reason residential slabs crack.
Reinforcement and Control Joints
Slabs crack. Reinforcement controls where the cracks happen and limits their width.
- Welded wire fabric (WWF / WWM): 6x6 W1.4 or W2.9. Cheap, easy to install, fine for residential. Must be lifted to mid-slab during pour.
- Synthetic fibre (macro or micro): dosed into the mix at the plant. Adds tensile strength, controls plastic shrinkage cracks. Standard in many residential pours.
- Rebar grid: #4 (1/2 in / 12 mm) at 16 in (400 mm) OC, both ways. For driveways, garages, structural slabs.
- Post-tensioning: tensioned steel cables after pour. Commercial and high-end residential. Not DIY.
Control joints (tooled or saw-cut) every 8-12 ft in each direction, at no more than 24-36 times the slab thickness. Joint depth should be 1/4 of slab thickness (1 in deep for a 4 in slab). Cut within 12 hours of pouring, before the slab develops random cracks.
Curing - Day 1 to Day 28
Concrete cures by hydration, not drying. Keep it damp. Standard curing schedule:
- First 24-48 hours: walkable but soft. Cover with plastic or curing blankets. Keep off heavy traffic.
- Days 2-7: spray with water 2-3 times per day, or cover with wet burlap. Slab is at 70% of design strength by day 7.
- Day 7: parking light vehicles OK. Forms can be stripped.
- Day 28: 90%+ of design strength. Full structural use.
Curing compound (a sprayed-on film) replaces water-based curing for large pours. Hot weather demands more curing attention - early-life cracks form in the first 6 hours when surface dries faster than the body.
Cost Estimates Around The World
2026 ready-mix prices for 3000 PSI / M20 standard slab mix, plus installed cost including base prep and finishing.
| Region | Ready-mix (3000 PSI / M20) | Installed slab (base + concrete + finish) | Per 80 lb bag (small pours) |
|---|---|---|---|
| United States (USD) | $140-180 / yd³ | $6-12 / sq ft installed | $5-7 |
| Canada (CAD) | C$200-260 / m³ | C$8-15 / sq ft installed | C$7-10 |
| United Kingdom (GBP) | £110-140 / m³ | £70-130 / m² installed | £4-6 per 25 kg bag |
| Eurozone (EUR) | €120-160 / m³ | €80-150 / m² installed | €5-8 per 25 kg bag |
| Australia (AUD) | A$220-300 / m³ | A$130-220 / m² installed | A$8-12 per 20 kg bag |
| India (INR) | ₹5,500-7,500 / m³ (M20 RMC) | ₹180-350 / sq ft installed | ₹380-500 per 50 kg cement bag |
| Mexico (MXN) | MX$2,800-3,800 / m³ | MX$1,400-2,800 / m² installed | MX$170-230 / 50 kg cement bag |
| Philippines (PHP) | PHP 5,200-7,000 / m³ | PHP 2,200-4,500 / m² installed | PHP 280-360 / 40 kg cement bag |
Installed cost includes the slab plus base preparation, rebar or mesh, formwork, finishing, and labour - not the structure on top of it. Decorative finishes (stamped, coloured, polished) add 30-100% to the installed cost.
Common Mistakes
- Entering slab thickness in feet when the field expects inches (or vice versa).
- Skipping the gravel base on expansive or clay soils.
- No vapor barrier under living-space or heated slabs.
- Forgetting control joints - the slab cracks where it wants, not where you want.
- Cutting joints too late (after 24 hours); cracks have already happened.
- Ordering exact yards with no waste allowance, then paying a $200 short-load fee.
- Skipping reinforcement on a driveway slab - cracks within two freeze-thaw cycles.
- Walking heavy equipment on the slab before day 7 - leaves wheel ruts that never go away.
Slab Calculator FAQ
How much concrete do I need for a 20 x 24 ft slab at 4 inches?
20 x 24 x 0.333 = 160 cu ft, or 5.93 cubic yards. Add 10% waste and order 6.5 yards. With ready-mix at $160 / yd, concrete alone is $1,040. Bagged would be about 270 of the 80 lb bags - definitely a ready-mix job.
What thickness should my slab be?
Patio or walkway: 4 inches (100 mm). Driveway: 5-6 inches with rebar. Garage floor: 5-6 inches. Heavy truck or shop floor: 6-8 inches. Engineered structural slab: per drawings, often 8-12 inches.
Do I need rebar in a 4 inch patio slab?
Not strictly required for a light-use 4 in patio - welded wire fabric or fibre mesh is enough. Driveways, garages, and any slab carrying vehicles should use #4 rebar at 16-18 in OC. Code in your jurisdiction usually sets the minimum.
How far apart should control joints be?
Every 8-12 ft in both directions. The rule of thumb is no further apart than 24-36 times the slab thickness in inches. A 4 in slab wants joints every 8-12 ft; a 6 in slab can stretch to 12-18 ft.
How long before I can park a car on a new slab?
Light passenger vehicle after 7 days. Full design strength at 28 days. Heavy vehicles, RVs, or trucks should wait the full 28 days to avoid wheel ruts in still-curing concrete.
How much should the slab slope?
For drainage, 1/8 in per foot (1% slope) is the minimum on exterior slabs. Driveways are usually 1.5-2% to shed water away from the house. Interior slabs can be flat but garage floors should slope 1-2% toward the door.
What is the minimum slab thickness for a structural floor?
Slab on grade with normal residential loads: 4 inches. Engineered floors (basements, suspended): 5 inches minimum per ACI 318, often 6-8 in. Local code, soil reports, and engineering drive this number.
How much waste should I add for a slab?
10% is the residential default and covers form bulge, washout, low spots, and the truck minimum buffer. For perfectly level interior slabs with rigid forms, 5% can be enough. Rough excavation or over-dug footings: 15%.
Can I pour a slab over an existing slab?
Yes, if the original is sound and clean. Bond with a slurry coat or chemical bonding agent. The new layer needs at least 2 inches of thickness to avoid feather-edge failures. Pouring a thin topping over a cracked slab transfers the cracks up to the new surface within 1-2 seasons.
What is the difference between this and the concrete calculator?
This Slab Calculator is focused on flat slab pours - length x width x thickness, with slab-specific guidance on joints, base, and curing. The Concrete Calculator handles multiple shapes (columns, tubes, curbs, stairs) in one place. Use Slab for clean rectangular pours; use Concrete for everything else.
Related Construction Calculators
For non-slab concrete shapes (columns, tubes, curbs, stairs), use the Concrete Calculator. For cement bag math from raw ingredients, see the Cement Calculator. For reinforcement bars, use the Rebar Calculator. For footings under the slab edge, see the Footing Calculator. More tools live on the Construction Calculators hub.
Sources
- ACI 318 - Building Code Requirements for Structural Concrete
- ACI 302 - Guide for Concrete Floor and Slab Construction
- Portland Cement Association concrete resources
- National Ready Mixed Concrete Association
- Bureau of Indian Standards - IS 456 Plain & Reinforced Concrete code of practice
- NIST Handbook 44 unit conversion tables
This calculator is for planning and ordering conversations. Local code, project drawings, engineered design, and manufacturer instructions control the final work.