Instantly calculate how many 80 lb, 60 lb, or 40 lb bags of Quikrete 5000 or Sakrete High Strength you need for any balcony slab — cantilevered, rooftop deck, residential, and more.
Enter your balcony dimensions to calculate bags needed, cubic yards, weight & estimated cost.
Extends from building without external columns. Min. 6 in. thick, #4 rebar, 5,000 PSI (ACI 318).
Typical residential balcony: 8–16 ft
Cantilever projection typically 4–8 ft from wall
Cantilevered: 6–8 in · Simply supported: 5–6 in · Pool deck: 4–5 in
A concrete balcony slab is a structural reinforced concrete platform that projects from a building's exterior wall or rests on supporting beams. In the United States, balcony slabs are governed by ACI 318 (Building Code for Structural Concrete) and local IBC requirements, which mandate a minimum compressive strength of 4,000–5,000 PSI and proper reinforcement to handle live loads of at least 60 psf (pounds per square foot). The amount of concrete you need depends on your balcony's length, width, and slab thickness — typically 5 to 8 inches for residential cantilevered designs.
For a standard 10 ft × 6 ft cantilevered balcony at 6-inch thickness, you need approximately 1.11 cubic yards of concrete — roughly 50 × 80 lb bags of Quikrete 5000 including 10% waste. Always use minimum 5,000 PSI mix for cantilevered slabs per ACI 318-19 Section 26.4.
The most common residential type — extends from the building without external columns. Requires minimum 6–8 inch thickness, top-mat #4 rebar at 12 in. o/c, and 5,000 PSI concrete per ACI 318. Projection should not exceed 6–8 ft without engineering review.
Rests on beams or columns at both ends, allowing thinner slabs of 5–6 inches. Requires 4,000–5,000 PSI concrete and #4 rebar in both directions at 12 in. o/c. Common in apartment buildings and commercial construction across the USA.
Flat rooftop decks and pool surrounds typically use 4–5 inch slabs with fiber-reinforced or 4,000 PSI concrete. A waterproof membrane beneath is mandatory. Live load capacity must meet minimum 100 psf per IBC 2021 for public rooftop areas.
Calculating concrete for a balcony slab follows the same volume formula used for any rectangular slab, then converts cubic feet to cubic yards and divides by bag coverage. The key difference for balconies is using the correct minimum thickness per structural code and adding a 10–15% waste factor for form-work edges and reinforcement displacement.
Any cantilevered concrete balcony slab must be designed by a licensed structural engineer (PE) per IBC 2021 and ACI 318-19. This calculator provides material quantity estimates only — it does not substitute for engineered structural drawings, rebar specifications, or a building permit. Contact your local AHJ (Authority Having Jurisdiction) before pouring.
The table below shows typical concrete balcony slab thicknesses, required PSI strength, and approximate 80 lb bag counts for common balcony sizes used in residential and light commercial projects across the United States.
| Balcony Size (ft) | Slab Type | Thickness | Volume (cu yd) | 80 lb Bags (+10%) | Min. Strength |
|---|---|---|---|---|---|
| 8 × 4 ft | Cantilevered | 6 in. | 0.59 cu yd | 27 bags | 5,000 PSI |
| 10 × 6 ft | Cantilevered | 6 in. | 1.11 cu yd | 50 bags | 5,000 PSI |
| 12 × 6 ft | Cantilevered | 6 in. | 1.33 cu yd | 60 bags | 5,000 PSI |
| 12 × 8 ft | Simply Supported | 5 in. | 1.48 cu yd | 67 bags | 4,000 PSI |
| 16 × 8 ft | Simply Supported | 5 in. | 1.98 cu yd | 89 bags | 4,000 PSI |
| 20 × 10 ft | Rooftop Deck | 4 in. | 2.47 cu yd | 111 bags | 4,000 PSI |
| 20 × 12 ft | Rooftop / Pool Deck | 5 in. | 3.70 cu yd | 167 bags | Ready-Mix Rec. |
Understanding your balcony slab type is critical before calculating concrete quantities, because each type has different thickness requirements, reinforcement needs, and PSI minimums. Choosing the wrong mix or undercalculating thickness is one of the leading causes of balcony structural failures reported to OSHA.
Projects outward from building wall with no external support. Minimum 6 in. thick with top-mat reinforcement. Requires licensed PE design per ACI 318-19. Most common in residential builds.
Supported at both ends by beams or walls. Allows 5–6 in. thickness. Bi-directional #4 rebar at 12 in. on center. Common in apartment complexes and multi-family housing.
Flat slab over waterproofed roof structure. Typically 4–5 in. with fiber-reinforced or 4,000 PSI concrete. Mandatory waterproofing membrane beneath. Slope minimum 1/8 in./ft for drainage.
At-grade or slightly elevated outdoor terrace. Typically 4 in. thick on compacted gravel base. Use 4,000 PSI concrete with fiber reinforcement for crack resistance in freeze-thaw climates.
The cost of a concrete balcony slab in the USA depends on size, thickness, mix type, local labor rates, and whether you pour DIY with bagged concrete or hire a contractor for a ready-mix delivery. For balconies larger than 2 cubic yards, ready-mix concrete ordered by the yard is more economical than bagged product.
Using 80 lb Quikrete 5000 at ~$9.50/bag (Home Depot / Lowe's 2026), a 10×6 ft cantilevered slab (50 bags) costs approximately $475 in materials only. Add rebar (~$120), form lumber (~$80), and rental mixer (~$60) for a total DIY cost of ~$735.
For slabs exceeding 1.5 cu yd, ready-mix is cost-effective. Expect $180–$230 per cubic yard for 5,000 PSI mix delivered in most US markets (2026). A 10×6 ft balcony at 1.11 cu yd costs ~$200–$255 in concrete alone, plus $400–$700 in labor.
A fully installed concrete balcony slab by a licensed contractor typically costs $10–$14 per square foot for simple residential designs (2026 US average). A 10×6 ft balcony = 60 sq ft = $600–$840 total installed. Complex cantilevered designs run $15–$22/sq ft.
Use bagged concrete (Quikrete / Sakrete) for balconies under 1.5 cubic yards (approx. 10×6 ft at 5 in. thick). For anything larger, order ready-mix concrete — the per-yard cost is lower, the pour is faster, and you get consistent PSI strength across the entire slab. Most concrete plants have a 1-yard minimum delivery for residential orders.
All structural concrete balcony slabs in the USA require steel reinforcement per ACI 318-19 and the International Building Code (IBC 2021). Skipping rebar in a cantilevered balcony slab is a code violation and a serious safety hazard. According to the American Concrete Institute (ACI), cantilevered slabs must place primary reinforcement at the top of the slab to resist negative bending moments.
The US Consumer Product Safety Commission (CPSC) reports that balcony collapses cause dozens of fatalities annually in the USA. The primary cause is improper or missing reinforcement, inadequate concrete strength, and lack of waterproofing leading to rebar corrosion. Always obtain a building permit, use a licensed structural engineer, and ensure proper waterproofing on all exterior balcony slabs.
Trusted US standards, guides, and tools for planning your balcony slab pour.
The American Concrete Institute's ACI 318-19 is the primary US standard governing all structural concrete design including balcony slabs. Covers minimum PSI, rebar requirements, concrete cover, and load calculations for cantilevered members.
View ACI 318Quikrete 5000 is the recommended bagged concrete for structural balcony slabs. The official data sheet provides PSI strength curves, mix ratios, bag coverage, curing times, and cold-weather usage guidelines for USA contractors and DIYers.
Download Data SheetThe IBC 2021 governs live load requirements (60 psf residential, 100 psf public) for balconies and decks, permit requirements, and inspection stages for all structural concrete work in the United States.
View IBC 2021