Concrete Slab Calculator
Estimate concrete volume in cubic meters, cubic feet and premix bags for slabs, patios and beton plates.
Enter Your Slab Dimensions
Table of Contents
Comprehensive Guide to Concrete Slabs
A concrete slab is a horizontal structural element poured in one layer: garage floors, terraces, sidewalks, shed bases and foundation plates (beton plates). Unlike footings or columns, slabs are defined by length, width and thickness over a flat area.
Ordering the right volume saves money and prevents cold joints from running out mid-pour. This calculator targets flat rectangular slabs and outputs volume in cubic meters (standard in Europe and engineering), plus cubic feet, cubic yards and an estimated premix bag count for DIY projects.
Common slab applications
- Monolithic foundation plates (dalle pleine / beton plate) for small buildings
- Outdoor terraces and patios exposed to weather
- Garage and workshop floors carrying vehicle loads
- Paths, shed pads and hot-tub bases
Volume formula
For a rectangular slab:
Volume = length × width × thickness
Example: 4 m × 3 m × 0.10 m = 1.2 m³
L-shaped slab: split into two rectangles, calculate each volume, then add.
Unit conversions
| From | To |
|---|---|
| 1 m³ | 35.3 cu ft |
| 1 cu yd | 27 cu ft / 0.765 m³ |
| 10 cm | 0.10 m / 3.94 in |
| 4 in | 0.333 ft / 10.2 cm |
Bags vs ready-mix
Bagged premix suits small slabs (typically under 1-2 m³). Larger pours are ordered from a ready-mix plant by the cubic meter or yard. Compare both options: bag labor and mixer rental vs delivery minimum charges.
| Bag type | Approx. yield |
|---|---|
| 40 lb (18 kg) | ~0.3 cu ft |
| 60 lb (27 kg) | ~0.45 cu ft |
| 80 lb (36 kg) | ~0.6 cu ft (default) |
Waste and safety margin
Always add 5-10% to the calculated volume for spillage, over-excavation and slight thickness variation. For your first pour, lean toward 10%.
Order volume = calculated volume × 1.10
Example: 1.2 m³ × 1.10 = 1.32 m³ to order
Frequently asked questions
- How thick should my patio slab be?
- Most residential patios use 10-12 cm (4-5 in). Driveways and garage floors need more. Check local building codes.
- Can I pour in cold weather?
- Concrete needs protection below ~5 C (41 F). Use cold-weather mixes, insulated blankets and avoid freezing during the first 24-48 hours.
- When can I walk on the slab?
- Light foot traffic is often possible after 24-48 hours. Wait about 7 days before placing furniture and 28 days for full design strength and vehicle loads.
- Do I need rebar?
- Wire mesh is common for patios. Rebar or thicker mesh is recommended for driveways, garage floors and structural plates.
Pro Tip
For complex shapes (L-shaped patios, multiple sections), calculate each rectangle separately and add the volumes. For non-slab work, use our general concrete calculator.
How to Calculate Slab Concrete
Accurate inputs depend on how you measure on site. Use the outside of forms for length and width if forms are already set.
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1Measure footprint: Length and width at the longest points. For irregular shapes, divide into rectangles.
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2Set thickness: From plans or the thickness guide below (often 10-15 cm for patios).
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3Match units: Meters + centimeters, or feet + inches. Do not mix systems in one field without converting.
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4Enter bag yield if you know your product (check the bag label; default 0.6 cu ft).
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5Calculate, then add 5-10% waste before ordering ready-mix or bags.
Quick check in imperial units
For a 4-inch slab, convert thickness to feet: 4 ÷ 12 = 0.333 ft. Then volume in cu ft = length (ft) × width (ft) × 0.333.
Slab Thickness Guide
Thickness depends on load and soil. These are typical residential starting points; always verify with local building codes.
| Use | Thickness |
|---|---|
| Garden path / light duty | 7-10 cm (3-4 in) |
| Patio / terrace | 10-12 cm (4-5 in) |
| Driveway (cars) | 12-15 cm (5-6 in) |
| Garage floor | 12-15 cm + reinforcement |
Poor soil (clay, peat, uncompacted fill) may require a thicker slab, geotextile, gravel sub-base or engineered design. Frost depth and drainage also affect minimum thickness in cold climates.
Slope and drainage
Exterior slabs should slope away from buildings, typically 1-2% (about 1-2 cm per meter). Slope does not change volume much but must be planned in the formwork before pour day.
Reinforcement and Sub-base
Concrete slabs need support from what is underneath and often steel or mesh inside to control cracking. These layers are not included in the volume above but are essential for durability.
Gravel sub-base
A compacted crushed stone layer (often 10-15 cm / 4-6 in) spreads load and improves drainage. Order gravel separately with our gravel calculator; typical rule: same footprint as the slab, depth per engineer or local practice.
Reinforcement options
- Welded wire mesh: Common for patios and light slabs; place in the upper third of the thickness.
- Rebar grid: Used for driveways, garage floors and structural plates; follow spacing on drawings.
- Fiber additive: Mixed into the concrete; reduces shrinkage cracking but does not replace structural rebar.
Control joints
Joints guide cracks in straight lines. Plan joint spacing roughly 24-30 times the slab thickness (e.g. every 3-4 m for a 12 cm slab). Tooled joints during finishing or early saw cuts both work.
Pouring and Curing Tips
A successful pour depends on preparation, weather and finishing discipline. Concrete begins to set within an hour or two in warm conditions - have enough help and tools ready.
Before the pour
- Compact gravel sub-base and check level
- Install forms, vapor barrier if required
- Place rebar or mesh on chairs
- Plan control joint locations
- Oil forms for easy release
- Confirm truck access or mixer position
After the pour
- Screed, bull-float and finish before bleed water pools
- Cure 7+ days; mist or use curing compound
- Avoid heavy loads until full strength (~28 days)
- Saw cut joints within 6-24 hours if needed
- Protect from rain and freezing while green
Weather considerations
- Hot weather: Pour early morning, use retarders, keep surface moist.
- Cold weather: Insulated blankets, heated water/mix, no freeze for 48 h.
- Rain: Cover with plastic; do not work surface water into the mix.
Concrete mix types for slabs
Specify strength (e.g. C25/30 or 3000-4000 psi) and exposure class with your supplier. Exterior slabs need air entrainment in freeze-thaw climates. Garage floors may need higher strength and fiber or mesh.
Common Mistakes to Avoid
- Wrong thickness unit: Entering 10 inches instead of 10 cm multiplies volume by ~25. Double-check thickness unit.
- No waste factor: Running short mid-pour forces a cold joint. Order 5-10% extra.
- Skipping sub-base: Pouring on soft soil leads to settlement and cracks.
- Overworking the surface: Too much troweling while bleed water is present weakens the top layer.
- Driving too early: Vehicles on a green slab cause spalling and deep cracks.
Practical Examples
Example 1 4 m x 3 m terrace, 10 cm
Volume = 4 x 3 x 0.10 = 1.2 m³
~42 cu ft / ~1.6 cu yd
With 10% waste: 1.32 m³
Example 2 12 ft x 10 ft patio, 4 in
Volume = 12 x 10 x (4/12) = 40 cu ft
~1.13 m³ / ~67 bags at 0.6 cu ft
Example 3 6 m x 5 m garage, 15 cm
Volume = 6 x 5 x 0.15 = 4.5 m³
Ready-mix order ~5 m³ with waste
Example 4 2 m x 2 m shed pad, 8 cm
Volume = 2 x 2 x 0.08 = 0.32 m³
~11 cu ft / ~19 bags at 0.6 cu ft
Also see: building materials calculator for gravel and multi-material estimates, general concrete calculator for other shapes.