What this calculator actually computes
Three numbers go in: the slab’s length and width in feet, and its thickness in inches. The volume is length × width × (thickness ÷ 12) cubic feet, because dividing the thickness by 12 turns inches into a fraction of a foot. Divide the cubic feet by 27 and you have cubic yards, the unit ready-mix concrete is sold in. In a single step: cubic yards = length × width × thickness ÷ 324.
The second half of the job is turning volume into bags. Mix is sold by weight, not by volume, so the calculator divides your cubic feet by the yield of one bag and rounds up — you cannot buy a third of a bag, and rounding down leaves the pour short. The yield and the “add ~10% for waste” note both sit on the result line, so you can check the arithmetic yourself.
The one number worth memorising: a cubic yard is 45 of the 80-lb bags, 60 of the 60-lb bags, or 90 of the 40-lb bags.
The three bag yields, and what they are worth
Every bagged product states a yield, and the three common sizes share one ratio — the same concrete in different packages:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Per pound, all three are 0.0075 cubic feet (0.60 ÷ 80 = 0.45 ÷ 60 = 0.30 ÷ 40). That is why the bag counts on the result line move together: 56 of the 80-lb, 75 of the 60-lb and 112 of the 40-lb bags all describe the same 33.33 cubic feet. If a count looks one higher than your own long division, that is the round-up working — 33.33 ÷ 0.6 is 55.6, and 56 is what you carry home.
Worked example 1 — a 10 × 10 ft patio at 4 inches
Enter 10, 10 and 4. The volume is 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, which is 1.23 cubic yards, and the calculator reports ≈ 56 × 80-lb bags — the same volume is 75 of the 60-lb bags or 112 of the 40-lb bags.
Waste is a separate step the calculator does not bake in. A 5% allowance lifts 33.33 to 35.00 cubic feet, or 59 bags; a 10% allowance reaches 36.67 cubic feet and 62 bags. On a patio that small the extra bags cost little, but on a larger pour the same percentage is real money.
Worked example 2 — a 20 × 24 ft driveway at 5 inches
Enter 20, 24 and 5: 20 × 24 × 5 ÷ 12 = 200 cubic feet = 7.41 cubic yards, and the calculator returns ≈ 334 × 80-lb bags. At 80 lb each that is 26,720 lb — 13.4 short tons of bagged mix to carry, open and mix. The finished slab is heavier still: at a normal-weight 150 lb per cubic foot, 200 cubic feet is 30,000 lb, or 15 tons of concrete in place.
Very few people hand-mix 334 bags competently, so a 7.4-cubic-yard driveway belongs on a ready-mix truck. The bag number is still worth reading, because it tells you the true size of the job you are organising.
Worked example 3 — a 12 × 16 ft shed pad at 4 inches
Enter 12, 16 and 4: 12 × 16 × 4 ÷ 12 = 64 cubic feet = 2.37 cubic yards → ≈ 107 × 80-lb bags. Three shapes, one rule:
| Slab | Size | Thickness | Volume | Cubic yards | 80-lb bags |
|---|---|---|---|---|---|
| Patio | 10 × 10 ft | 4 in | 33.33 cu ft | 1.23 | 56 |
| Shed pad | 12 × 16 ft | 4 in | 64 cu ft | 2.37 | 107 |
| Driveway | 20 × 24 ft | 5 in | 200 cu ft | 7.41 | 334 |
The driveway is about six times the patio’s volume from roughly five times the area plus one extra inch of thickness — which is why the next section is about thickness.
How thickness changes the answer
Thickness is the lever most people under-think, because it multiplies every square foot you pour. On the same 100 square feet, moving from 4 to 6 inches adds half again as much concrete:
| Thickness (100 sq ft) | Volume | Cubic yards | 80-lb bags |
|---|---|---|---|
| 4 in | 33.33 cu ft | 1.23 | 56 |
| 5 in | 41.67 cu ft | 1.54 | 70 |
| 6 in | 50.00 cu ft | 1.85 | 84 |
Those are the everyday ranges. Four inches is the common minimum for footpaths, patios and shed pads; driveways that carry cars generally run 5 to 6 inches, deeper where soil is weak. A slab carrying vehicles also wants reinforcement — size the steel with the rebar material calculator, and for pad footings start from the concrete footing calculator. For anything structural, thickness is a design decision set by local code, not a guess.
Bags or a ready-mix truck?
The crossover sits around one to two cubic yards. Below a yard — the 10 × 10 patio — bagged mix is the only practical choice and the bags fit in a car; at a yard you commit to 45 bags and 3,600 lb. Past a couple of yards a ready-mix truck delivers the same concrete in minutes, cheaper per yard and with none of the lifting, in exchange for a minimum load, a short-load charge below it, and a pour you must place before it sets.
Where this estimate goes wrong
- One rectangle only. An L-shaped slab, or a footing that turns a corner, must be split into rectangles and added. Rounding then happens per piece, so four small pours can need several bags more than one large one.
- Waste is your decision. The bag count is exact volume with no allowance; add about 5% for a straightforward pour and up to 10% on rough ground, where concrete settles into hollows.
- Over-excavated subgrade. If the dug depth wanders, the real average thickness exceeds the plan and the volume follows it up.
- Rounded shapes. This tool takes a rectangle. A circle is only about 0.785 of the square around it, so a rectangular estimate overstates a round footing by roughly a quarter.
- Weight, not just volume. A yard is 45 bags and 3,600 lb to move — loading a volume figure alone ignores.
Sources and standards
Where these figures come from
The volume and yardage formulas are elementary geometry. The bag yields (0.60, 0.45 and 0.30 cubic feet per 80-, 60- and 40-lb bag) are the packaged-mix yields published by the producers, and every bag count here was produced by running this calculator rather than by hand.
- Portland Cement Association — Concrete Construction (checked 8 Oct 2026) — placing, finishing, jointing and curing of slab-on-ground concrete.
- Portland Cement Association — Curing In Construction (checked 8 Oct 2026) — why concrete is kept damp and how curing affects 28-day strength.
- ACI 302 — Guide to Concrete Floor and Slab Construction (checked 8 Oct 2026) — slab thickness, subgrade preparation and jointing.
- ACI 318 — Building Code Requirements for Structural Concrete (checked 8 Oct 2026) — the code governing thickness and reinforcement of load-bearing slabs.
- National Ready Mixed Concrete Association (checked 8 Oct 2026) — ordering, batching and delivering ready-mixed concrete.
- Federal Highway Administration — Concrete Pavement (checked 8 Oct 2026) — federal guidance on concrete pavement.
Figures, formulas and reference tables last checked: 8 October 2026. Bag yields are the producers’ published figures and vary slightly by product.