BuildCalc Guides · Concrete
How Many Bags of Concrete for a 16×20 Slab?
A 16 by 20 foot pad is detached-garage / workshop / small-RV-pad territory — the size at which bagged concrete stops being a real option and the conversation becomes about yards delivered.
The math
- Volume in cubic feet = 16 × 20 × (4 ÷ 12) = 106.67 ft³ at 4 inches, or 160 ft³ at 6 inches.
- Cubic yards = ft³ ÷ 27. 106.67 ÷ 27 = 3.95 yd³ net at 4 inches.
- Add waste. 3.95 × 1.05 = 4.15 yd³ to order. At 6 inches you are at 6.22 yd³.
- Bags (theoretical) at 0.6 ft³ per 80 lb: ~187 bags. At 0.45 ft³ per 60 lb: ~249 bags. Realistically nobody bags a slab this size.
By slab thickness
Thickness drives the order, and on a 16×20 the difference between 4 and 6 inches is over 2 cubic yards.
| Thickness | ft³ | yd³ | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 3" | 80.0 | 2.96 | 140 | 187 |
| 4" | 106.7 | 3.95 | 187 | 249 |
| 6" | 160.0 | 5.93 | 280 | 374 |
Includes a 5 percent waste factor. Bag columns are theoretical — order yards.
Order ready-mix, here is how
- Round up the with-waste number. A 4 inch slab at 4.15 yd³ usually gets ordered as 4.5 yd³ to be safe.
- Short-load fee. Most suppliers charge $50 to $150 for any order under 5 cubic yards. A 6 inch slab at 6.22 yd³ avoids the fee entirely — worth knowing if you are on the edge.
- Get the right mix. 3,000 PSI is standard for patios and sheds. 4,000 PSI for driveways, garages and anywhere vehicles park.
- Have a crew. At 4+ yards, you need at least three people: one to screed, one to float, one to manage the chute or wheelbarrow.
Calculate for your own slab
The numbers above are for a 16′ × 20′ × 4″ pad. For any other size, depth or shape — including thickened edges and round columns — BuildCalc's concrete calculator handles fractional input, waste factor and live material cost.
Open the concrete calculator →FAQ
How many cubic yards of concrete is a 16x20 slab?+
A 16 by 20 foot slab at 4 inches thick is about 3.95 cubic yards of net concrete. Add a 5 percent waste allowance and you order about 4.15 cubic yards. At 6 inches thick (garage-floor depth) you are at about 6.22 cubic yards.
Should a 16x20 garage slab be 4 or 6 inches?+
6 inches is the standard call for a garage or workshop floor — anywhere a vehicle parks. 4 inches is enough only for a covered patio, shed pad or other space that never sees a wheel. The IBC and most local codes are at 4 inches minimum residential, 6 inches if vehicles are involved.
Is bagged concrete realistic for a 16x20 slab?+
No, not really. A 4 inch 16×20 needs 187 bags of 80 lb, which is 15,000 lbs of dry mix. Mixing and pouring that takes a small crew several days and the cold joints make the slab weaker than one continuous pour. Order ready-mix.
How much does 16x20 of concrete cost?+
Ready-mix runs about $150 to $200 per cubic yard delivered in most US markets, so a 4 inch 16×20 (4.15 yd³) is roughly $625 to $830 for the material — plus a short-load fee if under 5 yards. A 6 inch garage slab (6.22 yd³) is $930 to $1,250 in material. Forms, rebar and finishing are separate.
How much rebar does a 16x20 slab need?+
For a 4 inch patio: 16 in OC #3 rebar both directions, or a roll of 6×6 W2.9×W2.9 welded wire mesh. For a 6 inch garage floor: 12 in OC #4 rebar both directions, with the bars supported on chairs at slab mid-depth. That is roughly 280 ft of #3 for the patio or 580 ft of #4 for the garage.
Do I need a thickened edge on a free-standing slab?+
Yes — a free-standing 16×20 pad benefits from a turn-down edge or footing around the perimeter. The standard detail is 12 inches deep by 6 inches wide. That adds roughly 0.7 cubic yards to the order. Skip the thickened edge only when the slab pours up against an existing foundation.
Related guides
- How many bags of concrete for a 12×12 slab? — the common patio size below this one.
- How many bags of concrete for a 20×20 slab? — full two-car garage footprint.
- How many bags of concrete for a driveway? — typical sizes and yards needed.