About

Designing a durable outdoor patio requires more than just pouring concrete; it involves creating a stable foundation to prevent cracking and shifting over time. This tool is designed for homeowners and contractors planning outdoor living spaces, where irregular shapes and proper ground preparation are common.

Accuracy is critical here because underestimating materials leads to cold joints (stopping a pour to buy more mix), which structurally weakens the slab. Conversely, over-ordering results in expensive waste and disposal fees. This calculator addresses the total system: the concrete slab itself and the essential sub-base layers (gravel and sand) that provide drainage and stability.

Formulas

The calculator uses geometric volume formulas adjusted for material density and waste factors.

Rectangular Volume:
V = L × W × T

Circular Volume:
V = π × r² × T

Total Weight:
W_total = V × D × (1 + waste)

Where D is the density of the material (Concrete, Gravel, or Sand) and waste is typically 5-10%.

Reference Data

Material	Typical Density	Use Case	Recommended Thickness
Standard Concrete	2,400 kg/m³ (150 lb/ft³)	Main Slab	10 cm (4 inches)
Compacted Gravel	1,600 kg/m³ (100 lb/ft³)	Sub-base (Drainage)	10-15 cm (4-6 inches)
Masonry Sand	1,500 kg/m³ (94 lb/ft³)	Leveling Layer	2.5-5 cm (1-2 inches)
High-Strength Mix	2,500 kg/m³ (156 lb/ft³)	Heavy Load Patios	15 cm (6 inches)
Rebar/Mesh	N/A	Reinforcement	Mid-depth of slab
Crushed Stone (Type 1)	2,200 kg/m³ (137 lb/ft³)	Heavy Duty Base	15 cm (6 inches)
Decorative Aggregate	1,450 kg/m³ (90 lb/ft³)	Exposed Finish	Surface Layer
Polystyrene	15-30 kg/m³	Insulation (Optional)	5-10 cm

Frequently Asked Questions

A sub-base of compacted gravel ensures proper water drainage and prevents the ground from shifting during freeze-thaw cycles. Without it, the concrete is likely to crack as the soil beneath it moves.

For standard foot traffic and patio furniture, 4 inches (10 cm) is the industry standard. If you plan to install a hot tub or heavy outdoor kitchen, consider increasing the thickness to 6 inches (15 cm).

Standard concrete contains gravel aggregate for strength, making it suitable for slabs. Sand mix (topping mix) lacks large aggregate and is used for thin repairs or smoothing, not for structural patio pours.

It is best practice to order 5% to 10% extra material to account for spillage, uneven sub-grade depths, and minor calculation rounding errors.

Not directly, but irregular shapes with sharp re-entrant corners are more prone to cracking. It is advisable to place control joints at these corners to guide potential cracks.