Concrete Slab Calculator
Quick Presets
Concrete Slab Volume Reference
Cubic yards needed by slab size and depth.
| Slab Size | 4" Deep | 6" Deep | 80-lb Bags (4") |
|---|---|---|---|
| 4' x 20' | 0.99 | 1.48 | 45 |
| 10' x 10' | 1.24 | 1.85 | 56 |
| 10' x 20' | 2.47 | 3.70 | 111 |
| 12' x 12' | 1.78 | 2.67 | 80 |
| 12' x 24' | 3.56 | 5.33 | 160 |
| 20' x 24' | 5.93 | 8.89 | 267 |
| 10' x 40' | 4.94 | 7.41 | 222 |
How We Calculate This
This concrete slab calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
Calculating concrete for a slab is simple: multiply length, width, and depth to get volume, then convert to cubic yards — the standard ordering unit for ready-mix concrete.
The basic rule:
- Measure length and width in feet, depth in inches. Formula: L(ft) × W(ft) × D(in)/12 = cubic feet, then divide by 27 for cubic yards
- Standard slab depth is 4 inches for patios and sidewalks, 6 inches for garages and driveways with heavy traffic
- Always order 5-10% extra concrete to account for spillage, uneven subgrade, and form movement
Ready-mix concrete is sold by the cubic yard. Most trucks carry 8-10 cubic yards. For small jobs under 1 yard, bagged concrete (60 lb or 80 lb bags) may be more practical. This calculator shows both options.
When Would You Use This Calculator?
This concrete slab calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
A 10x10 slab at 4 inches thick requires 1.24 cubic yards of concrete (about 33 cubic feet). That's roughly 56 bags of 80-lb concrete mix or 1.3 yards of ready-mix. Always order 5-10% extra for waste and spillage.
How many bags of concrete are in a cubic yard?
One cubic yard of concrete equals approximately 45 bags of 80-lb concrete mix or 60 bags of 60-lb mix. For jobs over 1 cubic yard, ordering ready-mix delivery is usually more cost-effective and easier than mixing bags by hand.
How thick should a concrete slab be?
Sidewalks and patios: 4 inches. Garages and driveways: 4-6 inches. Heavy equipment pads: 6-8 inches. The slab should rest on 4-6 inches of compacted gravel base for proper drainage and to prevent cracking.
How much does a yard of concrete cost?
Ready-mix concrete costs $100-$150 per cubic yard delivered, depending on your region and the mix design. Short-load fees ($15-$50 per yard under the minimum) apply for orders under 3-5 yards. An 80-lb bag costs about $5-$7, which works out to roughly $225-$315 per cubic yard.
Do I need rebar or wire mesh in my slab?
For most residential slabs, welded wire mesh (6x6 W1.4/W1.4) provides adequate reinforcement. Use #4 rebar on 24-inch centers for garage floors, driveways, or any slab over 5 inches thick. Fiber-reinforced concrete is another option for crack control in patios and walkways.
How long does concrete take to cure?
Concrete reaches about 70% of its full strength in 7 days and 99% in 28 days. You can walk on it after 24-48 hours and drive on it after 7 days. Keep it moist for the first 7 days by misting or covering with plastic for best results.