Concrete Calculator
Concrete comes two ways: a ready-mix truck priced by the cubic yard, or bags you mix yourself. Which one makes sense depends entirely on how much you need, and that's the number this tool nails down. Enter the slab's length, width, and thickness. You'll get cubic yards for the truck, a bag count in your chosen bag size, the total weight, and a cost estimate at your local per-yard price. Pads, patios, walkways, footings, it covers them all.
Concrete in the two units it actually sells in
Ready-mix plants quote cubic yards. The home center sells bags. Same material, two completely different orders, and the crossover point between them decides how your Saturday goes. This tool gives you both at once: yards for the truck call, bags in whichever size your store stocks, and the weight so you know what you're asking your wheelbarrow to do.
Thickness is the number to get right before anything else. Going from 4 inches to 6 adds 50% to the order, and unlike gravel you can't come back tomorrow for the rest. A pour is one continuous event.
The math, out in the open
Length times width gives area. Area times thickness in feet (inches divided by 12) gives cubic feet. Divide by 27 for cubic yards. Bags work on yield, and the figure printed on the bag is the one that matters: an 80-lb bag yields 0.60 cubic feet, a 60-lb bag 0.45, a 50-lb bag 0.375, a 40-lb bag 0.30. Those are the manufacturers' own numbers, and they already include the mixing water.
Normal-weight concrete comes in around 150 pounds per cubic foot, the baseline figure in ACI guidance. That's just over 2 tons per cubic yard, which is why the weight cell exists: a "small" 1-yard pour is a 4,000-pound object.
How many bags for common slabs
These counts use 80-lb bags with 10% waste. Swap the bag size in the tool and it recalculates.
| Slab | Thickness | Cubic yards | 80-lb bags |
|---|---|---|---|
| 4 x 4 (mailbox pad) | 4 in | 0.18 | 9 |
| 6 x 6 (AC pad) | 4 in | 0.41 | 19 |
| 10 x 10 (shed) | 4 in | 1.36 | 62 |
| 12 x 16 (patio) | 4 in | 2.61 | 118 |
| 20 x 20 (garage) | 6 in | 8.15 | 367 |
| 24 x 24 (2-car drive) | 5 in | 9.78 | 440 |
Read that last column honestly. Nobody mixes 367 bags. Past about 2 yards, the truck isn't a convenience, it's the plan.
Bags or the truck: where the line sits
An 80-lb bag runs $6 to $9 at the big-box stores, so a yard of bagged concrete costs $270 to $400 before your labor. Ready-mix runs $125 to $180 a yard, but trucks carry minimums, and loads under 4 or 5 yards usually pick up a short-load fee of $50 to $150. That leaves three zones: under half a yard, bags win outright. From half a yard to about 2, it's close, and the fee decides it. Past 2 yards, the truck wins even with the fee, and it wins by more every yard after that.
There's a third option between them: mobile mix trucks that batch on site and charge only for what they pour. If a local outfit runs one, it's often the answer for the awkward 1-to-3-yard jobs.
A worked pour
Say you're pouring a 12 by 16 patio at 4 inches. That's 192 square feet times a third of a foot, 64 cubic feet, or 2.37 yards. With 10% waste you order 2.61. As bags that's 118 at 80 lb, almost 4.5 tons to cart and mix one wheelbarrow at a time. As a truck order it's a 2.5-yard short load. Get both prices; around this size the truck usually costs slightly more and is worth every dollar.
Where pours go wrong
- Ordering the exact figure. Subgrades dip, forms bow, and a pump line holds concrete too. The 10% default exists because running short mid-pour creates a cold joint you'll see forever.
- Forgetting the base. A slab wants 4 inches of compacted stone under it, which is its own order. The crushed stone calculator sizes it.
- Skipping reinforcement. Anything a vehicle touches wants rebar or mesh. The rebar calculator lays out the grid.
- Pouring in the wrong weather. Below 40°F and above 90°F, concrete needs special handling. If the forecast argues with you, reschedule the truck, not the finishing.
Quick answers
How much concrete for a 24x24 slab? At 4 inches: 7.9 yards, call it 8.7 with waste. At 6 inches: 11.9 yards. That size is firmly ready-mix territory; the truck beats roughly 390 bags on price and sanity.
How many 80-lb bags on a pallet? Forty-two at most suppliers, which conveniently makes almost exactly one cubic yard per pallet with a couple of bags to spare.
What's the strongest mix for a driveway? Order 4,000 PSI with air entrainment in freeze climates. It adds $10 to $20 a yard over basic mix and buys decades of surface life.
How long does concrete take to cure? Walkable in 24 to 48 hours, cars after 7 days, full design strength at 28. Keeping it damp the first week adds real strength, not superstition.
Common questions
At 4 inches thick, about 1.23 cubic yards, call it 1.4 with waste. Mixing it yourself, that's around 62 bags at 80 lb. The calculator adjusts all of it the moment you change a dimension.
Forty-five 80-lb bags, or sixty 60-lb bags. Each 80-lb bag yields 0.6 cubic feet and a yard is 27 cubic feet. Past a yard or so, bags cost more in money and your back than a short-load delivery.
Under half a yard, bags win. Between half a yard and 2 yards, compare bag cost against the truck's short-load fee. Past 2 yards, ready-mix wins on price, consistency, and your Saturday.
Four inches handles walkways, patios, and shed pads. Driveways want 5, garage and shop floors 6. Anything carrying vehicle weight also wants rebar or mesh, which the rebar calculator sizes.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.