Calculator
Concrete Slab Calculator
A concrete slab's volume is length x width x thickness — straightforward geometry — but converting that volume into bag counts is where the real numbers matter, and they're not round. A common 60-pound bag of concrete mix yields about 0.45 cubic feet once mixed; an 80-pound bag yields about 0.6 cubic feet. Those aren't proportional-to-weight coincidences to round off — using the actual per-bag yield instead of a rough rule of thumb is the difference between an order that finishes the pour and one that runs short with wet concrete already in the forms.
This calculator converts your slab dimensions to cubic yards, then to bag count at real yield figures, and always rounds up to whole bags — see concrete bag yield: 60lb vs 80lb vs 90lb bags for the full comparison table and the point where ready-mix delivery becomes cheaper than bags.
Calculate your quantity
Extra for spillage and an uneven subgrade.
Concrete volume
1.87 cu yd
Concrete volume
50.4 cu ft
- 60lb bags
- 112 bags
- 80lb bags
- 84 bags
- 90lb bags
- 75 bags
Shopping summary
- • 84 x 80lb bags (or 112 x 60lb, or 75 x 90lb)
This is an estimate — confirm structural work with a professional.
How this calculation works
Cubic yards = (length x width x thickness in feet) / 27 (a cubic yard is 27 cubic feet). For bag counts: cubic feet needed / yield per bag (0.45 cu ft for 60lb, 0.6 cu ft for 80lb, roughly 0.675 cu ft for a 90lb bag), always rounded up — a partial bag still needs to be purchased whole, and running out mid-pour with concrete already curing in the rest of the slab is a real problem, not just a minor inconvenience.
Always add 5-10% to the raw volume for form spillage, an uneven subgrade, and general waste — subgrade is rarely perfectly flat, and low spots eat more concrete than the flat-plane math assumes.
The formula
cuft = length × width × (thickness/12) × (1+wastePct/100); bags = ceil(cuft / yieldPerBag)
- thickness/12
- Thickness is entered in inches but converted to feet before multiplying — a 4 inch slab uses 0.333, not 4, in the volume formula, since the other two dimensions are already in feet.
- yieldPerBag
- Cubic feet ONE mixed bag actually produces, not the bag's dry weight — 0.45 for a 60 lb bag, 0.6 for 80 lb, roughly 0.675 for 90 lb, figures that scale slightly less than linearly with bag weight because larger bags pack marginally denser mix.
- all three bag sizes returned at once
- The function computes 60, 80, and 90 lb bag counts from the same volume simultaneously rather than picking one — the page you're reading chooses which to feature, but all three exist for comparing weight-per-trip against total bag count.
Where these numbers come from
- 0.45 / 0.6 / 0.675 cu ft yield for 60 / 80 / 90 lb bags
- Manufacturer-published yield figures common across general-purpose concrete mix bags. These are the mixed, cured yield — not simply the bag weight divided by concrete's rough density — which is why 80 lb bags don't yield exactly a third more than 60 lb bags despite weighing a third more.
- 5-10% waste for spillage and an uneven subgrade
- Trade convention; the field defaults to 5% but is worth raising toward the top of that range whenever the subgrade hasn't been checked for level, since a low spot consumes volume the flat-plane thickness math never sees.
- 27 cu ft per cubic yard
- Standard unit conversion (3 ft × 3 ft × 3 ft), used only for the cubic-yard readout — the bag counts themselves are computed straight from cubic feet, so this conversion doesn't touch bag accuracy either way.
Worked examples
An 11 by 13 foot patio slab, 3.5 inches thick
| Slab length | 11 ft |
|---|---|
| Slab width | 13 ft |
| Thickness | 3.5 in |
| Waste factor | 6 % |
| Concrete volume | 1.64 cu yd |
|---|---|
| Concrete volume | 44.2 cu ft |
| 60lb bags | 99 bags |
| 80lb bags | 74 bags |
| 90lb bags | 66 bags |
143 sq ft at 3.5 inches thick, plus 6% for an unconfirmed subgrade, comes to 44.2 cu ft (1.64 cu yd) — 74 bags of 80 lb mix, or 99 of the 60 lb size, a 25-bag difference for the identical pour depending purely on which bag size gets loaded.
That bag-count gap is mostly a trip-count difference, not a weight difference: 99 sixty-pound bags total 5,940 lb and 74 eighty-pound bags total 5,920 lb, within 20 lb of each other. The real choice is 99 lighter individual lifts against 74 heavier ones, not how much concrete ends up on the truck.
A 9 by 9 foot shed pad, 5 inches thick
| Slab length | 9 ft |
|---|---|
| Slab width | 9 ft |
| Thickness | 5 in |
| Waste factor | 6 % |
| Concrete volume | 1.33 cu yd |
|---|---|
| Concrete volume | 35.8 cu ft |
| 60lb bags | 80 bags |
| 80lb bags | 60 bags |
| 90lb bags | 53 bags |
This pad's 81 sq ft footprint is a little over half the patio's 143, but its 5 inch thickness (against 3.5) closes most of that gap in volume — 35.8 cu ft here versus 44.2 cu ft for the patio, only 19% less despite 43% less footprint.
Thickness genuinely outweighs footprint on a small pour like this: a shed pad rated to carry real point loads from stored equipment needs the thicker slab, and the concrete order reflects that far more than it reflects the smaller area alone would suggest.
Common mistakes
- Rounding bag yield to a flat 'half a cubic foot per bag' regardless of bag size — the real yield differs meaningfully between 60lb, 80lb, and 90lb bags.
- Not adding a spillage/subgrade allowance, which nearly always leaves the pour short by a bag or two.
- Underestimating labor for large bag-mix pours — mixing and placing more than about 1-1.5 cubic yards by hand/bag is a full-day, multi-person job; beyond that, ready-mix delivery is usually the better call (see concrete bags vs ready-mix).
- Forgetting reinforcement (wire mesh or rebar) and a compacted gravel base, which aren't part of the concrete volume itself but are standard for a durable slab.
- Pouring without control joints planned in advance — a slab that isn't scored or jointed at regular intervals tends to crack in a random, less controlled location instead of at the intentional joint line.
- Finishing (troweling/floating) too early or too late relative to the concrete's set — this is a technique issue independent of bag count, but it affects the slab's final surface quality just as much as getting the volume right.
Shopping summary
Buy whole bags totaling cubic feet needed (raw volume plus 5-10% spillage allowance) at your chosen bag's real yield, or switch to ready-mix delivery once the pour exceeds roughly 1-1.5 cubic yards.
Worked example: a 10x10 foot slab at 4 inches thick is (10 x 10 x 4/12)/27 ≈ 1.23 cubic yards, or about 33.3 cubic feet. At 0.6 cu ft per 80lb bag, that's roughly 56 bags before waste; adding 8% for spillage and an uneven subgrade brings it to about 60 bags — right at the point where ready-mix delivery starts to look more attractive than a full day of hand-mixing 60 individual bags.
FAQ
How much extra should I add for spillage and an uneven base?
5-10% on top of the calculated volume is standard — more if your subgrade has visible low spots, since concrete fills to level and low spots consume disproportionately more volume than the flat-plane calculation assumes.
At what point should I switch from bags to a ready-mix truck?
Once a pour approaches roughly 1-1.5 cubic yards (around 60-100 bags), the labor of mixing that many bags by hand usually costs more in time than a ready-mix delivery, which also cures more consistently as one continuous pour.
Do I need wire mesh, rebar, or both for a patio slab?
A typical residential patio slab is often reinforced with wire mesh alone; rebar is more common on slabs bearing heavier loads or spanning larger areas, and some jobs use both — this is a structural decision worth confirming against your local code and slab's intended use rather than defaulting to whichever material happens to be on hand.
How thick should a patio slab be compared to a driveway slab?
A pedestrian patio is commonly poured around 4 inches thick, while a driveway bearing vehicle weight is typically poured thicker (5-6 inches or more) with correspondingly more reinforcement — using a patio's thickness spec under a driveway is a real structural under-build, not just a minor difference in bag count.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.