About

A garage floor experiences stresses that a standard room does not: the dynamic weight of moving vehicles, hot tires, oil drips, and often the freeze-thaw cycles near the door. Consequently, garage slabs are typically thicker and require a higher compressive strength mix (PSI) than residential interior floors.

This calculator helps plan the pour by accounting for the main slab and the "curb" or "stem wall" often found around the perimeter to protect drywall from water and debris. It assumes a standard slope (usually 1/8 inch per foot) towards the overhead door for drainage, though this affects volume minimally compared to the overall thickness choices.

Formulas

The calculator separates the main slab volume from the perimeter curb volume.

Slab Volume:
V_slab = L × W × T_slab

Curb/Stem Wall Volume:
V_curb = P × W_curb × H_curb

Total Needed:
V_total = (V_slab + V_curb) × 1.05

Where P is the length of the curb (usually 2 sides + rear wall).

Reference Data

Parameter	Light Duty (Car)	Heavy Duty (Truck/Workshop)	Notes
Slab Thickness	10 cm (4 inches)	15 cm (6 inches)	Min 4" recommended for any vehicle.
Concrete Strength	3000 - 3500 PSI	4000 - 4500 PSI	Higher PSI resists road salt damage better.
Rebar Size	#3 (10mm)	#4 (13mm)	Grid pattern typically 18" or 24".
Stem Wall Width	15 cm (6 inches)	20 cm (8 inches)	Supports framing above grade.
Slump	100 mm (4 inches)	100 mm (4 inches)	Lower water content = stronger concrete.
Curing Time	7 Days	28 Days	Do not drive on it before 7 days.
Vapor Barrier	6 mil Poly	10-15 mil Poly	Prevents moisture wicking up.
Expansion Joints	Every 3-4 meters	Every 3-4 meters	Prevents random cracking.

Frequently Asked Questions

For a standard garage, 3500 PSI is the sweet spot. If you own heavy trucks or use the garage as a workshop with heavy machinery, upgrade to 4000 PSI. In cold climates, air-entrained concrete is crucial to prevent freeze-thaw damage.

Yes. While wire mesh was common in the past, a grid of #3 or #4 rebar spaced 18-24 inches apart provides superior structural integrity and crack resistance for vehicle loads.

A standard slope of 1/8 inch per foot removes water but doesn't significantly change the concrete volume compared to a flat slab calculation. The volume lost at the thin end is roughly gained at the thick end, or the subgrade is sloped to match.

It is the raised concrete lip around the edge of the garage. It protects the wooden framing plates from water on the floor (from washing cars or melting snow). It is usually poured monolithically with the floor or just after.

Wait at least 7 days for light vehicles. Full cure takes 28 days. Driving on it too early can cause permanent ruts or cracking.