Concrete Slab Calculator (Bags)
Length × width × thickness → bags. Use 4" for patios and shed bases, 5–6" where vehicles drive. The chart below shows where bags stop making sense.
Bag counts are rounded up. Yields per bag: 80 lb → 0.60 ft³ · 60 lb → 0.45 ft³ · 50 lb → 0.375 ft³ · 40 lb → 0.30 ft³ (check your bag's printed yield).
Bags needed for common slabs
| Slab | Volume | 80 lb bags (incl. 5% waste) | Verdict |
|---|---|---|---|
| 4×4 pad (AC unit) @ 4" | 0.21 yd³ | 10 | Bags |
| 6×6 pad @ 4" | 0.47 yd³ | 22 | Bags |
| 8×8 shed base @ 4" | 0.83 yd³ | 38 | Either (rent a mixer) |
| 10×10 patio @ 4" | 1.30 yd³ | 59 | Ready-mix |
| 12×12 patio @ 4" | 1.87 yd³ | 85 | Ready-mix |
| 10×20 parking pad @ 5" | 3.24 yd³ | 146 | Ready-mix |
| 24×24 garage @ 5" | 9.33 yd³ | 420 | Ready-mix |
The pattern is obvious: past roughly 45 bags (one cubic yard), get a ready-mix quote; the truck often costs less than the bags, before you even price your weekend. Turn a yardage quote into bag counts with the bags-per-yard converter. Bag yields and coverage by thickness are in the bag yields chart.
Doing it right
- Base: 4" compacted gravel, level and firm. Most cracked DIY slabs failed underneath, not in the mix. Size the base with the gravel under slab calculator.
- Forms: 2×4s (for 3.5" slabs) or 2×6s, staked every 3–4 ft, checked square with a diagonal tape measure.
- Reinforcement: welded wire mesh or fiber mix for patios; #3/#4 rebar grid where vehicles drive. Count bars, sticks and mesh sheets with the rebar and wire-mesh calculator.
- Control joints: cut or tool joints at spacing (ft) ≈ 2–3× slab thickness (in), so a 4" slab wants joints every 8–10 ft.
- Cure: keep it damp for 7 days; concrete gaining strength slowly is concrete not cracking early.
FAQ
How thick should a concrete slab be?
Rules of thumb: 4 inches for patios, walkways and shed bases; 5–6 inches for driveways and anything a vehicle parks on; 6+ inches with reinforcement for heavy equipment. Thickness is the wrong place to save money: a 3" driveway slab will crack under the first SUV.
Do I need gravel under a concrete slab?
Almost always yes: 4 inches of compacted gravel (¾" crushed stone or road base) gives uniform support and drainage, which prevents most slab cracking. Pour on undisturbed or compacted soil, never on loose fill or topsoil. Size the base with the gravel under concrete calculator.
Can I pour a big slab in stages with bags?
Only if you plan real joints. Concrete poured against yesterday’s concrete makes a cold joint: fine at a planned control joint with a divider, a random crack magnet anywhere else. For anything over roughly a yard, one continuous ready-mix pour gives a stronger, flatter slab.
How many bags can one person actually mix in a day?
Hand-mixing in a wheelbarrow, 10–15 bags of 80 lb is a hard day for most people. A rented drum mixer (~$60–90/day) realistically gets you through 30–50. Past that, the labor math almost always favors the ready-mix truck.