The Concrete Floor Calculator calculates how much concrete you need for a floor slab by multiplying its length, width and thickness and adding waste.
It shows the volume in cubic yards or cubic meters, the number of 40, 60 or 80 lb bags if you mix it yourself, and an optional ready-mix cost when you enter a price per cubic yard.
Concrete Floor Calculator
Concrete volume and bags for a floor slab, and an optional ready-mix cost.
Formula
V = L × W × T × (1 + waste %)
Bags = V ÷ bag yield; ready-mix cost = V (yd³) × price
Where L = slab length, W = width, T = thickness.
How the Calculation Works
Step 1: Convert the thickness to feet (4 in = 0.333 ft) or meters.
Step 2: Multiply length × width × thickness and add waste.
Step 3: Divide cubic feet by 27 for cubic yards, or by the bag yield for bags.
Variables Explained
| Input | Meaning |
|---|---|
| Length, width | Slab size |
| Thickness | Slab depth |
| Bag size | 40, 60 or 80 lb premixed bags |
| Ready-mix price | Optional price per cubic yard |
Example Calculation
Given: 15 × 12 ft slab, 4 in thick, 5% waste, 80 lb bags
Volume: 15 × 12 × 0.333 = 60 ft³ × 1.05 = 63 ft³
Result: 63 ÷ 27 ≈ 2.33 yd³, or 105 bags of 80 lb mix
Concrete Volume Reference
| Slab thickness | Area covered by 1 yd³ |
|---|---|
| 3 in | 108 ft² |
| 4 in | 81 ft² |
| 5 in | 64.8 ft² |
| 6 in | 54 ft² |
Values before waste. Bag yields used: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³ (check your bag label).
Material Estimation Notes
Gravel base, vapor barrier, reinforcement (mesh or rebar), forms and control joints are separate items. The right slab thickness depends on its use and your local code.
Waste Factor Considerations
Uneven subgrade is the main reason slabs use more concrete than calculated — a small dip across a large area adds up. Allow waste to suit your base preparation.
Frequently Asked Questions
How much concrete do I need for a floor?
Length × width × thickness (in feet) ÷ 27 = cubic yards, plus waste.
How many bags of concrete for a 10 × 10 slab?
At 4 in thick: 33.3 ft³, or about 56 bags of 80 lb mix before waste.
How much area does a yard of concrete cover?
81 ft² at 4 in thick.
Should I use bags or ready-mix?
Bags suit small slabs; ready-mix is usually more practical for larger pours.
How thick should a concrete floor be?
It depends on use and code — follow your plans.