Imperial volume
length ft × width ft × thickness in ÷ 12 = ft³
ft³ ÷ 27 = yd³
The waste factor is applied after base volume so ordering includes typical overage.
Material-only planning estimate
Concrete Slab Calculator: Estimate Concrete, Gravel, Forms, Rebar, and Cost
Plan a patio, walkway, shed pad, or small workshop slab with transparent estimates for concrete volume, ready-mix or bag quantities, gravel base, form boards, reinforcement, and rough material cost.
What this concrete slab calculator estimates
The calculator deep-links into the existing concrete calculator and uses the same tested formulas as the app. For slab inputs, it estimates:
- Concrete volume in cubic yards or cubic meters.
- Ready-mix recommendation for larger pours.
- 40 lb, 60 lb, and 80 lb bag counts for imperial estimates.
- Compacted gravel base volume and approximate weight.
- Form board counts from slab perimeter.
- Rebar stick counts and wire mesh planning area.
- Rough material cost range for concrete, gravel, forms, and rebar.
Concrete slab formula transparency
These are planning formulas for material estimating. They are not a structural design or local-code substitute.
Metric volume
length m × width m × thickness cm ÷ 100 = m³
Metric ready-mix guidance uses a cubic-meter threshold directly.
Planning add-ons
Gravel base uses the selected base depth, form boards use slab perimeter, and reinforcement estimates use default spacing plus planning waste.
Example concrete slab estimates
Rounded examples use M250 concrete, 4 in slab thickness, 5% waste, a 4 in gravel base, 24 in reinforcement spacing, and current app assumptions.
| Slab | Concrete | 80 lb bags | Gravel base | Forms | Rough materials |
|---|---|---|---|---|---|
| 10 × 10 × 4 in | 1.30 yd³ | 59 | 1.23 yd³ | 5 boards | $259–$417 |
| 12 × 12 × 4 in patio | 1.87 yd³ | 85 | 1.78 yd³ | 6 boards | $364–$584 |
| 20 × 20 × 4 in garage/workshop | 5.19 yd³ | 236 | 4.94 yd³ | 10 boards | $996–$1,592 |
Buying and planning checklist
Use the estimate as a starting point before calling suppliers or ordering material.
- Concrete: ready-mix cubic yards or bags, depending on pour size and access.
- Compacted gravel base at the selected depth.
- Form lumber, stakes, screws, and layout string.
- Reinforcement such as rebar or wire mesh when appropriate.
- Finishing and curing supplies not included in the current cost range.
- Delivery, labor, permits, excavation, compaction, vapor barrier, ties, and chairs are not included unless priced separately.
Concrete slab calculator FAQ
How much concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab that is 4 in thick needs about 1.23 yd³ before waste and about 1.30 yd³ with a 5% waste factor.
How many 80 lb bags are in a cubic yard?
Using the current app assumption of 0.022 yd³ per 80 lb bag, one cubic yard takes about 46 bags. Check the yield printed on the bag you buy.
How thick should a concrete slab be?
Many patios, walkways, and shed pads are planned around 4 in thickness, but vehicle loads, soil conditions, frost, and local codes can require different designs.
Do I need gravel under a slab?
A compacted gravel base is commonly used for drainage, support, and leveling. The calculator includes a default base estimate, but local site conditions matter.
Does this include labor or delivery?
No. The cost range is a rough material-only estimate. It excludes labor, delivery, permits, tools, taxes, excavation, compaction, vapor barrier, and finishing supplies.
Is rebar required?
Rebar needs depend on slab use, loads, soil, climate, and code. The calculator provides planning quantities only and does not replace structural engineering guidance.
Important caveats
These are rough planning estimates for materials. They are not purchasing guarantees, engineering designs, permit advice, or structural recommendations. Confirm local code, site preparation, subbase, reinforcement, slab thickness, drainage, control joints, curing method, and supplier yields before starting structural or load-bearing work.