Bags or a truck: the concrete decision that costs money
Nobody searching for a concrete calculator is stuck on length times width times depth. What they need to settle is how the concrete shows up. Either it is a stack of bags in the back of a pickup, or it is a mixer truck backing down the drive with a minimum charge attached.
That choice has a crossover point, and the folklore number is wrong about where it sits. You will read "under a cubic yard, use bags" everywhere. In some markets that is roughly right. In others it is badly off, because three inputs move it: the price of a bag, the plant's minimum billed load, and the delivery fee.
Run the numbers. At $6.50 for an 80 lb bag, a cubic yard built out of bags costs 45 × $6.50 = $292.50 in material. Now the truck. $175 per cubic yard looks cheaper per unit, but if the plant bills a four-yard minimum and adds a $120 delivery fee, the smallest delivery you can buy is $820. One yard or four, you pay $820. Bags stay cheaper all the way up to about 2.8 yards, a long way past "one yard."
Change one input and the answer flips. A supplier with a one-yard minimum and no fee charges $175 for that first yard, and bags lose right away. So the tool above works out the crossover from the numbers you type instead of hard-coding a threshold. Below it, buy bags. Above it, order the truck. Close to it, count the labor too: 45 bags is a ton and a half lifted, opened and mixed by hand, and no two batches come out quite the same.
Concrete bag yield is different from bag weight
The most common math mistake in concrete estimating is working out the bag count from weight. It sounds airtight. A cubic yard weighs about 4,050 lb, an 80 lb bag weighs 80 lb, so 51 bags. That is wrong, and it is wrong in the direction that costs you money.
Use the yield, which is printed on the bag:
| Bag | Yield | Per cubic yard | Per cubic meter |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 45 bags | 59 bags |
| 60 lb | 0.45 ft³ | 60 bags | 79 bags |
| 50 lb | 0.375 ft³ | 72 bags | 95 bags |
| 40 lb | 0.30 ft³ | 90 bags | 118 bags |
The gap between 45 and 51 is the mix water. You add roughly three quarts to an 80 lb bag, and that water doesn't evaporate out of the picture. It becomes part of the hardened paste and adds both mass and volume. The finished cubic yard weighs about 4,050 lb, and only 3,600 lb of it came out of bags.
Buy by weight and you have bought 13 percent more bags than the forms will take. On a small pour that is a leftover pallet you paid for, sitting in the garage until it sets in the bag. Buy by yield, round up to the next whole bag, and keep one or two spare. The spare bag is cheap. The trip back to the store while wet concrete stiffens in the forms is not.
Concrete volume for five common shapes
Most pours are not boxes, so a single length × width × depth field is not enough. These five come up all the time. Here is the formula behind each one:
| Shape | Volume | The part people miss |
|---|---|---|
| Slab, floor, driveway | L × W × T | Thickness is in inches, not feet, so divide by 12 before multiplying |
| Strip footing | total length × W × D | Adding a rectangle side by side counts each corner twice |
| Round column | π × (D ÷ 2)² × H | The form is sold by diameter; using it as the radius orders 4× too much |
| Stairs | steps + inclined slab | The slab underneath, which is usually the larger half |
| Retaining wall | stem + footing | The footing, and the fact that a battered stem uses its average thickness |
Watch strip footings. Your plan shows a 26 by 34 ft rectangle, you add the four sides, and you get 120 ft of footing. But every corner got counted twice. On a 20 inch wide, 20 inch deep footing those four corner blocks are 18.5 cubic feet, close to three quarters of a cubic yard of concrete that does not exist. Measure to the centerline of the trench, or take the outside perimeter and subtract four corner squares.
Concrete for stairs: two staircases, two very different numbers
Stairs are where concrete calculators go quietly wrong. "Stairs" covers two different structures.
A suspended flight is the ordinary indoor or deck staircase spanning between two levels. It is an inclined slab with steps cast on top, and its volume has two parts. The steps are triangular prisms, because the underside of each step is the sloping soffit and not a floor, so each one is rise × run ÷ 2 × width. Under them runs the inclined slab, called the "waist," for the full length of the flight. Its thickness is measured perpendicular to the slope. Its length is the hypotenuse, the number of steps × √(rise² + run²), and not the horizontal run.
Take a 16-step flight with a 7 inch rise, 11 inch run, 42 inch width and a 5 inch waist. The steps come to 16 × (7 × 11 ÷ 2) ÷ 144 × 3.5 = 14.97 cubic feet. The waist runs 16 × √(7² + 11²) = 208.6 inches, or 17.38 feet, so the slab is 17.38 × 3.5 × 0.417 = 25.35 cubic feet. Total: 40.3 cubic feet, 1.49 cubic yards. Forget the waist and you order 0.55 yards, barely a third of the pour. You find out when the truck runs dry with six steps still in formwork.
Steps cast solid on ground use a completely different sum. There is no slab and no void. Each step is filled to grade, so step k is k risers tall, and the total is width × run × rise × N(N+1)/2. That N² term grows fast. The same 16 steps solid on fill come to 254.5 cubic feet, or 9.4 cubic yards, more than six times the suspended flight, and the gap gets wider with every step you add. Never use one formula for the other. That is why the tool asks which staircase you are building before it asks anything else.
The section drawing above the results comes from the same geometry as the number, and its area is checked against the formula, not eyeballed. A diagram that disagrees with the math is worse than no diagram.
Round columns and the factor of four
A cardboard tube form is sold, labeled and specified by its diameter. The volume formula wants the radius. Everyone knows this. It still causes the biggest over-orders, because area scales with the square: forget the ÷ 2 and you order exactly four times too much concrete.
A 12 inch tube 10 feet tall holds π × 0.5² × 10 = 7.85 cubic feet, or 0.29 cubic yards. Treat 12 inches as the radius and you get 31.4 cubic feet, 1.16 yards. Four piers like that turn a 1.2 yard order into a 4.7 yard order. That lands right in the range where you have also triggered a full truck delivery you never needed.
A shortcut for deck footings: a 12 inch tube takes 0.79 cubic feet per foot of depth, a 10 inch takes 0.55, and an 8 inch takes 0.35. So a 12 inch tube uses about one and a third 80 lb bags per foot of depth. That is also the fastest way to check your calculator hasn't squared the wrong number.
How much waste to add to a concrete order
Every concrete order needs an allowance. Skipping it saves nothing. It moves the shortfall to the worst possible moment, halfway through screeding.
- Subgrade irregularity is the biggest source and the hardest to see. A slab formed for 4 inches over a base that averages half an inch low is a 4.5 inch slab: 12 percent more concrete than the drawing says, and none of it shows above the surface.
- Forms deflect. Wet concrete pushes hard, stakes flex, forms bow out between them, and the extra goes into the pour.
- Spillage adds up: what falls off the chute, what sticks in the wheelbarrow, what the pump line holds and can't give back.
- Trenches get over-excavated. They never come out the exact width on the drawing, and a trench 2 inches wider than spec across a 100 ft footing is real volume.
Use 5 percent for a large, well-prepared, machine-graded slab. Use 10 percent for footings dug by hand, small irregular pours, and anything on soft ground. Running short costs you a cold joint, a permanent plane of weakness that shows as a line on a slab forever. Running long costs a wheelbarrow of surplus, which goes into a spare form, a splash pad or a post hole. It isn't a close call.
What the concrete strength number buys you
Ready-mix is specified in psi at 28 days: 2,500, 3,000, 3,500, 4,000 and up. The price gap between them is small, typically ten to fifteen dollars a yard. The gap in how the concrete holds up is not.
On anything exposed to freeze-thaw (a driveway, a sidewalk, exterior steps), the strength class matters less than air entrainment. Air-entrained concrete carries millions of microscopic bubbles that give freezing water room to expand. Without them, exterior flatwork in a freezing climate scales and spalls within a few winters, no matter how many psi are on the ticket. Ask for 6 percent air on any exterior pour.
Slump measures how wet the mix is, and it is the other lever, the one most often abused on site. Adding water at the truck to make placing easier is the fastest way to ruin the concrete you just paid for. Every extra gallon per yard cuts strength by roughly 200 psi and multiplies shrinkage cracking. If the mix is too stiff to place, ask the plant for a plasticizer. Don't reach for the hose.
Ordering a ready-mix load without paying for air
A standard mixer truck holds 8 to 11 cubic yards, and the legal weight limit usually caps the load before drum volume does. Past capacity, these are the charges that show up on the invoice and never on the quote:
- The minimum load or short load fee. Below the plant's minimum, commonly 3 to 5 yards, you pay for the minimum anyway or a flat surcharge on top. This number decides bags versus truck, so ask for it first on the phone.
- Standby time. Most plants allow a set unloading window, often 5 to 7 minutes per yard, and bill by the minute after that. A crew that isn't ready when the truck arrives pays for its own delay.
- Saturday, after-hours and winter charges. Hot water and accelerator in cold weather are usually billed separately.
- Wash-out. Some suppliers charge for it. Either way you need a spot on site where the truck can rinse the chute that isn't the storm drain or the neighbor's lawn.
When the calculated volume lands just under a truckload or just under the minimum, round up. A little surplus becomes a splash pad. A little shortfall becomes a second delivery with its own minimum charge and a cold joint through the middle of the slab.
Check the batch ticket against the pour
Every delivery comes with a batch ticket showing the volume, the mix design, the batch time and usually the water added. Keep it. It is your only proof of what you received, and it lets you work back to the thickness you actually got:
inches placed = yards delivered × 324 ÷ area in square feet
The 324 comes from simple geometry: a cubic yard spread one inch deep covers 324 square feet. So 5 yards on a 1,200 square foot slab is 5 × 324 ÷ 1,200 = 1.35 inches. If that slab was specified at 4 inches, something is very wrong, and the ticket total is where you see it first.
Run the check before the pour too. Divide the volume this calculator gives you by 324 per inch of thickness and compare it with the area you measured. You will catch a decimal-point error before it turns into a truck idling in the street.
Your numbers stay in your browser
The whole calculation is arithmetic running on your device. Nothing is uploaded, nothing is stored, and there is no account. Your dimensions, your supplier's price per yard and your margins are not our business.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. An 80 lb bag of concrete mix yields 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags exactly. A 60 lb bag yields 0.45 cubic feet, so you need 60 of those. A 40 lb bag yields 0.30 cubic feet, so you need 90. Don't get the yield by dividing bag weight by concrete density. The mix water adds both mass and volume, which is why 45 bags of 80 lb (3,600 lb of dry mix) make a cubic yard that finishes at about 4,050 lb.
At what point is ready-mix cheaper than bags?
Usually somewhere between half a cubic yard and one cubic yard, but there is no universal number. It moves with your bag price, the plant's minimum billed load and the delivery fee. At $6.50 per 80 lb bag, a cubic yard in bags is about $293. If the plant charges $175 per yard with a 4 yard minimum and a $120 delivery fee, the truck costs $820 whether you take one yard or four, so bags win until roughly 2.8 yards. Drop the minimum to one yard and the truck wins almost right away. The calculator finds that crossover from the numbers you enter instead of repeating a rule of thumb.
How do you calculate concrete for stairs?
A normal suspended flight is the steps plus the inclined slab underneath. Each step is a triangular prism: rise × run ÷ 2 × width. The inclined slab is its thickness × the width × the hypotenuse of the step line, and that hypotenuse is the number of steps × the square root of (rise² + run²). Use the horizontal run instead and you underestimate the slab by 15 to 25 percent. Leave the slab out altogether, which is the most common error, and you lose more than half the pour. Stairs cast solid on ground use a different formula: width × run × rise × N(N+1)/2.
How much concrete do I need for a 10x10 slab at 4 inches?
A 10 by 10 ft slab at 4 inches is 100 sq ft × 0.333 ft = 33.3 cubic feet, or 1.23 cubic yards. Add 10 percent for subgrade irregularity and spillage and you order about 1.36 cubic yards, which is 62 bags of 80 lb mix. At that size you are almost certainly under the plant's minimum load, so put the bag total next to the minimum charge before you call anyone.
What is a short load fee and how do I avoid it?
It is what the plant charges when you take less than its minimum billable load. It usually kicks in below 3 to 5 cubic yards and is priced as a flat fee or per yard short. The truck, the driver and the round trip cost the same whether the drum is full or a quarter full, and somebody pays for that. Avoid it by combining pours (do the footing, the pad and the post holes on the same day), by rounding the order up to the minimum and having a use ready for the surplus, or by staying in bags. Finding out about the fee after the concrete is on the ground is the expensive way to learn it exists.