Bags or a truck is the decision, not the volume
Almost every concrete calculator answers a question nobody is stuck on. Multiplying length by width by depth is not why people are searching. The real question underneath is how the concrete arrives: a stack of bags in the back of a pickup, or a mixer truck backing down the drive with a minimum charge attached to it.
That decision has a crossover point, and it is not the folklore number. The rule of thumb you will read everywhere is "under a cubic yard, use bags." It is roughly right in some markets and badly wrong in others, because three things move it: the price of a bag, the plant's minimum billed load, and the delivery fee.
Work an example. 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 sounds cheaper per unit, but if the plant bills a four-yard minimum and adds a $120 delivery fee, the smallest possible delivery is $820. That $820 buys one yard or four — the price is identical. Bags stay cheaper all the way up to about 2.8 yards, which is a long way past "one yard."
Change one input and the answer flips. A supplier with a one-yard minimum and no fee costs $175 for that first yard, and bags lose immediately. That is why the tool above solves the crossover from the numbers you type instead of hard-coding a threshold. Below the turning point, bags. Above it, the truck. And near it, remember what bags actually cost in labour: 45 bags is a ton and a half lifted, opened and mixed, and a batch that is never quite the same twice.
A bag's yield is not its weight
The single most common arithmetic failure in concrete estimating is deriving 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. It is wrong, and it is wrong in the direction that costs you money.
The correct figure is the yield, printed on the bag itself:
| Bag | Yield | Per cubic yard | Per cubic metre |
|---|---|---|---|
| 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 is not a solvent that disappears — it becomes part of the hardened paste, contributing both mass and volume. The finished cubic yard weighs about 4,050 lb while 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 and will store until it sets in the bag. Buy by yield, round up to the next whole bag, and keep one or two spare — nobody has ever regretted the spare bag, and everyone has regretted the trip back to the store with wet concrete stiffening in the forms.
Five shapes and what each one actually needs
The reason a single length × width × depth box is not enough is that most pours are not boxes. These are the five that come up constantly, with the formula each one runs on:
| Shape | Volume | The part people miss |
|---|---|---|
| Slab, floor, driveway | L × W × T | Thickness is in inches, not feet — divide by 12 before multiplying |
| Strip footing | total length × W × D | Measuring 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 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 that a battered stem uses average thickness |
Strip footings deserve a note. If your plan shows a 26 by 34 ft rectangle and you add the four sides, you get 120 ft of footing — but each corner has been counted twice. With 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 centreline of the trench, or take the outside perimeter and subtract four corner squares.
Stairs: two staircases, two completely different numbers
Stairs are where concrete calculators go quietly wrong, and the reason is that "stairs" is two different structures wearing one word.
A suspended flight — the ordinary indoor or deck staircase spanning between two levels — is an inclined slab with steps cast on top of it. Its volume has two terms. The steps are triangular prisms, because the underside of each step is the sloping soffit, not a floor: each one is rise × run ÷ 2 × width. Then the inclined slab, the "waist", runs the full length of the flight underneath. Its thickness is measured perpendicular to the slope and its length is the hypotenuse — the number of steps × √(rise² + run²), not the horizontal run.
Take a 16-step flight, 7 inch rise, 11 inch run, 42 inches wide, on 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. Leave the waist out and you order 0.55 yards — barely a third of the pour, discovered at the moment the truck runs dry with six steps still in formwork.
Steps cast solid on ground are 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 is brutal. 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 widens with every step you add. Which is exactly why you never use one formula for the other, and why the tool asks which staircase you are building before it asks anything else about it.
The section drawing above the results is generated from the same geometry the number comes from, and its area is checked against the formula rather than eyeballed. A diagram that disagrees with the arithmetic is worse than no diagram at all.
Round columns and the factor of four
A cardboard tube form is sold, labelled and specified by its diameter. The volume formula wants the radius. Everyone knows this, and it is still the error that produces the most spectacular 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, which happens to be right in the range where you have also now triggered a full truck delivery you did not need.
Useful 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 eats about one and a third 80 lb bags per foot of depth — which is also the quickest sanity check that your calculator has not squared the wrong number.
Waste is a quantity, not an attitude
Every concrete order needs an allowance, and pretending otherwise does not save money — it just moves the shortfall to the worst possible moment, which is halfway through screeding.
- Subgrade irregularity. The biggest one and the least visible. 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.
- Form deflection. Wet concrete pushes hard. Stakes flex, forms bow out between them, and the extra goes in the pour.
- Spillage and the chute. What falls off the chute, what sticks in the wheelbarrow, what the pump line holds and cannot recover.
- Over-excavation. Trenches never come out the exact width of the drawing, and a trench 2 inches wide of spec across a 100 ft footing is real volume.
Five percent is right for a large, well-prepared, machine-graded slab. Ten percent is right for footings dug by hand, for small irregular pours, and for anything on soft ground. Running short costs a cold joint, which is a permanent plane of weakness and, on a slab, a visible line forever. Running long costs a wheelbarrow of surplus — put it in a spare form, a splash pad, a post hole. That trade is not close.
What the strength number buys you
Ready-mix is specified in psi at 28 days: 2,500, 3,000, 3,500, 4,000 and up. The difference in price between them is small, typically ten to fifteen dollars a yard, and the difference in outcome is not.
For 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 somewhere to expand into. Without them, exterior flatwork in a freezing climate scales and spalls within a few winters regardless of how many psi are on the ticket. Ask for 6 percent air on any exterior pour.
Slump, the measure of how wet the mix is, is the other lever and the one most often abused on site. Adding water at the truck to make placement easier is the single fastest way to destroy the concrete you just paid for: every extra gallon per yard drops strength by roughly 200 psi and multiplies shrinkage cracking. If the mix is too stiff to place, the answer is a plasticiser from the plant, not a hose.
Ordering the load without paying for air
A standard mixer truck holds 8 to 11 cubic yards, and it is legal weight, not drum volume, that usually sets the limit. Beyond capacity, these are the charges that appear on the invoice and never on the quote:
- Minimum load / 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 is the number that decides bags versus truck, and it is the first thing to ask 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 it. A crew that is not ready when the truck arrives is paying 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 somewhere on site for the truck to rinse the chute that is not the storm drain or the neighbour's lawn.
When the calculated volume lands just under a truckload or just under the minimum, round up. A little surplus is a splash pad. A little shortfall is a second delivery with its own minimum charge, and a cold joint through the middle of the slab.
Check the ticket against the pour
Every delivery arrives with a batch ticket giving the volume, the mix design, the batch time and usually the water added. Keep it. It is the only evidence of what you received, and it lets you back-calculate the thickness you actually got:
inches placed = yards delivered × 324 ÷ area in square feet
The 324 is not magic — 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 it shows up first.
Run the check in advance too. Divide the volume this calculator gives you by 324 per inch of thickness, compare it against the area you measured, and you will catch a decimal-point error before it becomes a truck sitting in the street.
Everything stays 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 it takes 60 of those; a 40 lb bag yields 0.30 cubic feet, so it takes 90. Note that the yield is not the bag weight divided by concrete density — the mix water you add contributes 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 the number is not universal — it depends on 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. Change the minimum to one yard and the truck wins almost immediately. The calculator solves that crossover from the numbers you enter rather than repeating a rule of thumb.
How do you calculate concrete for stairs?
For a normal suspended flight the volume is the steps plus the inclined slab underneath, and the steps are triangular prisms: rise × run ÷ 2 × width, per step. The inclined slab is its thickness × the width × the hypotenuse of the step line, which is the number of steps × the square root of (rise² + run²). Using the horizontal run instead of the hypotenuse underestimates the slab by 15 to 25 percent, and leaving the slab out altogether — the most common error — loses more than half the pour. Stairs cast solid on ground are a different calculation: 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, which is 1.23 cubic yards. Add a 10 percent waste allowance for subgrade irregularity and spillage and you are ordering about 1.36 cubic yards, or 62 bags of 80 lb mix. At that size you are almost certainly below the plant's minimum load, so compare the bag total against the minimum charge before calling anyone.
What is a short load fee and how do I avoid it?
It is the charge for taking less concrete than the plant's minimum billable load, typically applied below 3 to 5 cubic yards and priced either 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 that cost has to land somewhere. You 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 finding somewhere useful to put the surplus, or by staying in bags. Discovering the fee after the concrete is on the ground is the expensive way to learn it exists.