Concrete Floor Calculator
This concrete floor calculator works out the concrete for a garage, basement, shed or shop floor slab. Enter the length and width, or just the floor area, plus the thickness, the number of identical rooms or bays and a waste allowance, and it returns cubic yards, floor area, weight and bag counts. Open the optional cost panel to price the concrete, delivery and finishing labor.
How to use this calculator
- Enter the length and width, or type the floor area in square feet, square yards or square meters. Need the area first? Use the square footage calculator.
- Set the thickness (it starts at 4 inches).
- Pouring several identical rooms or bays? Enter the number and the volume multiplies.
- Keep the 10 percent waste allowance, then read cubic yards for a ready-mix order or the bag count for hand mixing.
Cubic yards = floor area (sq ft) × thickness (ft) ÷ 27
Example: a 20 × 20 ft garage floor
400 sq ft × 0.333 ft = 133.3 ft³, ÷ 27 = 4.94 yd³. Pour it at 6 inches and it's 7.4 yd³, half again as much concrete for the same footprint.
How thick should a concrete floor be?
| Thickness | yd³ per 100 sq ft | Concrete per sq ft | Where the number comes from |
|---|---|---|---|
| 3-1/2 in | 1.08 | Not priced | IRC R506.1 minimum for a floor |
| 4 in | 1.23 | $1.98 | ACI 330R-08 practical minimum; the usual home floor |
| 5 in | 1.54 | $2.47 | NRCS farm floors, axles under 15,000 lb |
| 6 in | 1.85 | $2.96 | NRCS farm floors, axles of 15,000 to 20,000 lb |
The code minimum is 3-1/2 inches: IRC R506.1, as reproduced in Seattle's residential code, says floors "shall be a minimum 3-1/2 inches (89 mm) thick." Most floors go to 4, and ACI 330R-08 explains why: "there is no benefit in building slabs less than 4 in. (100 mm) thick." That guide is for parking lots, so treat it as reasoning rather than a garage rule.
For floors under vehicles and equipment, USDA's NRCS floor design guide sets a 5 inch minimum for single axle loads under 15,000 lb, 6 inches for 15,000 to 20,000 lb, 8 inches for 20,000 to 30,000 lb and 10 inches for 30,000 to 40,000 lb. It's written for farm buildings, so use it as a guide for a shop floor, not a code. More in the floor thickness guide.
What the cost estimate includes
- Concrete and delivery: at the $160 per cubic yard default, that's $1.98 per sq ft at 4 inches, $2.47 at 5 and $2.96 at 6.
- Finishing labor: pre-filled at $2 to $8 per sq ft, $4.5 in the middle, for placing, screeding, floating and a broom or trowel finish.
- Not included: a minimum job charge. The estimate multiplies area by a rate, so on a small floor add any minimum your contractor charges.
Prices are US averages as of 2025; edit them for your area. For finishes like polish, stain or epoxy, see the concrete floor cost guide. For a full slab with edges and reinforcement, use the slab cost calculator.
Bags or ready-mix?
One cubic yard is 45 bags of 80 lb mix, since each bag makes about 0.60 ft³. At the $8 per bag default, that's $360 a yard in bags against $160 delivered. A 10 × 12 ft room at 4 inches is 1.48 yd³, or 67 bags to mix by hand.
The truck minimum here defaults to 10 cubic yards, with a $30 per yard surcharge below it. The calculator prices a part load pro rata, while real suppliers set their own minimums and fees, so ask yours. See ready-mix delivery and short loads and the bag calculator.
Vapor retarder: which code edition applies
| IRC edition | Requirement |
|---|---|
| 2018 | 6 mil polyethylene or approved retarder, joints lapped 6 in (R506.2.3) |
| 2021 | 10 mil, meeting ASTM E1745 Class A (R506.2.3) |
| 2024 | 6 mil construction-grade polyethylene allowed again (NAHB summary) |
Garages are exempt. The code doesn't require a retarder under garages, utility buildings and other unheated accessory structures, unheated storage rooms under 70 sq ft, carports, or driveways, walks and patios unlikely to be enclosed and heated later.
ACI goes further than the code. ACI 302.1R-15 says conventional polyethylene "does not fully conform to the requirements of ASTM E1745" and shouldn't be used as below-slab moisture protection, and that the best protection comes when the retarder is "placed in direct contact with the slab." It matters most under vinyl, wood, carpet or other moisture-sensitive flooring. See basement concrete floors and under-slab insulation.
Reinforcement
Steel doesn't change the concrete volume, and it won't stop cracks. ACI 302.1R-15 puts it plainly: "Reinforcement will not prevent cracking. If the reinforcement is properly sized and located, cracks should remain tightly closed." Joints decide where cracks form.
Placement matters. The IRC requires reinforcement to be "supported to remain in place from the center to upper one-third of the slab," and ACI says welded wire "should not be placed on the ground and pulled up after placement of the concrete." Use chairs. The rebar calculator lays out the grid, and the reinforcement guide covers joints.
Topping or leveling an existing floor
Leveling
A thin self-leveling underlayment is sold by the bag and coverage, not by the yard. Use the self-leveling concrete calculator and the floor leveling guide.
Bonded topping
A topping of 2 inches or more over a sound slab is a volume job: enter the area and the topping thickness here. Concrete floor resurfacing covers bonding.
Radiant heat
Pouring over tubing? The heated concrete floor guide covers cover depth over the tubes.
Frequently asked questions
How much concrete do I need for a floor?
Multiply the floor area in square feet by the thickness in feet, then divide by 27 for cubic yards. A 400 sq ft floor at 4 inches is 133.3 cubic feet, or about 4.94 cubic yards before waste.
How many yards of concrete for a 20x20 floor?
A 20 × 20 ft floor is 400 sq ft. At 4 inches thick it needs about 4.94 cubic yards, so order 5 plus waste. At 6 inches it needs about 7.4 cubic yards.
How thick should a concrete floor be?
The IRC minimum for a slab-on-ground floor is 3-1/2 inches, but most floors are poured at 4, and ACI 330R-08 says there is no benefit in going thinner. For floors under vehicles or equipment, USDA NRCS farm-building guidance uses 5 inches for axle loads under 15,000 lb, 6 inches for 15,000 to 20,000 lb, and 8 to 10 inches for heavier axles.
How many bags of concrete for a floor?
Divide the volume in cubic feet by the bag yield: about 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. A cubic yard takes 45 bags of 80 lb mix, so a 10 × 12 ft floor at 4 inches (40 cubic feet) needs 67 bags before waste.
How much does it cost to pour a concrete floor?
Open the cost panel on the calculator. It prices the concrete and delivery from the thickness and adds finishing labor per square foot, using US averages you can edit. It doesn't include a minimum job charge, so on a small floor add any minimum your contractor charges.
Do I need a vapor barrier under a concrete floor?
Under a heated interior floor, yes. The 2018 IRC called for 6 mil polyethylene, the 2021 edition for 10 mil meeting ASTM E1745 Class A, and the 2024 edition allows 6 mil again, so check which edition your area uses. Garages, carports, unheated accessory buildings and outdoor flatwork are exempt. ACI 302.1R-15 recommends against conventional polyethylene and says the retarder works best in direct contact with the slab.