Concrete calculator
Adds up a whole pour of mixed shapes in one go, then tells you when the job has outgrown bags. A cubic yard is 45 bags of eighty, and no calculator funded by bag sales is going to mention it.
The number every bag calculator leaves out
A cubic yard of concrete is 45 bags of eighty pounds. That is 3,600 lb of material to buy, transport, carry, open and mix. Every concrete calculator on the internet is published by somebody who sells bags, so none of them will ever tell you that the job in front of you has outgrown them.
This one leads with it. Past roughly one cubic yard the result stops giving you a bag count as the headline and tells you to phone a ready-mix supplier instead, with the yardage they will ask for.
Why one yard is the line
It is a rule of thumb rather than a rule, and it comes from the mixing rather than the money. A portable mixer takes two or three bags at a time. A cubic yard is therefore fifteen to twenty-three separate batches, each one needing to be measured, mixed, barrowed, placed and screeded before the previous one starts to stiffen.
The risk is not effort, it is the cold joint. Concrete placed against concrete that has already begun to set never bonds into one mass. It leaves a plane of weakness through the pour, and that is where it cracks and where water gets in. Fifteen batches by hand is fifteen chances to lose that race on a warm day.
Add the whole job, not one shape at a time
A real pour is rarely one rectangle. A shed base is a slab with a thickened edge. A deck is six or eight pier holes. A set of steps sits on its own footing. Manufacturer calculators do one shape per visit, so the usual method is running the thing four times and adding the results up on a phone.
That is where the arithmetic goes wrong, and it goes wrong in the direction that costs you: a forgotten footing means an under-order. So this takes rows. Add every part of the same pour and order against one total.
An unreadable row is never quietly skipped
If a row is missing a dimension, this tool will not drop it and carry on. It says so, it names what is missing, and it marks the total as short until you fix it.
That is deliberate, and it is the most important behaviour in the tool. A silently dropped row makes the total smaller than the pour. You would order from a number that looks perfectly reasonable, and find out it was wrong when the truck left and the forms were not full.
Inches where inches belong
Slabs are specified as a thickness in inches over a footprint in feet, footings as a width and depth in inches over a run in feet. Every dimension here carries its own unit selector, so you enter the number you actually have rather than converting it in your head first. Metric units are in the same list.
What it does not do
It computes plain volume. It does not size reinforcement, work out a mix design, or tell you what strength to specify, all of which are structural questions that depend on the load and on local code.
The stair option is a solid stair, formed and poured full. A suspended stair on a waist slab holds considerably less concrete and is not modelled here.
Questions people actually ask
How many bags of concrete are in a cubic yard?
45 bags of 80 lb, and that number is the reason this tool exists.
A cubic yard is 27 cubic feet. An 80 lb bag of Quikrete Concrete Mix yields approximately 0.60 cubic feet, so 27 divided by 0.60 is exactly 45. In the other sizes it is 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb.
45 bags is 3,600 lb of material to buy, get home, carry to the pour, open, and mix in batches. It is not a realistic afternoon. Somewhere around one cubic yard, ready-mix delivery stops being the expensive option and starts being the only sensible one.
How much concrete do I need for a 10 by 10 slab?
For a 4 inch slab, 1.36 cubic yards with a 10% waste allowance.
The arithmetic: 10 ft by 10 ft is 100 square feet, and 4 inches is one third of a foot, so 33.3 cubic feet. Add 10% for spillage and uneven subgrade and it is 36.7 cubic feet, which is 1.36 cubic yards. At 6 inches thick the same slab is over 2 cubic yards.
Both are past the bag threshold, which is worth knowing before you load a trolley at the builders merchant.
Why does the thickness field default to inches and the length to feet?
Because that is how a slab is actually specified, and forcing one unit is how mistakes happen.
Nobody says "a nought point three three foot slab". They say a four inch slab, over a length and width in feet. A calculator that demands decimal feet throughout invites somebody to type 0.3 instead of 0.333, which is a 10% under-order on its own before any other error.
So every dimension here carries its own unit and you can mix them freely in the same row. Metric is in the same list if you want it.
How much waste should I allow?
10% is the usual allowance for a formed pour on prepared ground, and that is the default here.
It covers what spills, what stays stuck in the mixer or the truck, and the fact that a subgrade is never perfectly flat, so the slab is a little thicker in places than the number you calculated.
Go higher, 15% or more, for a footing trench dug in soft or crumbly ground, where the sides slump inward and the trench quietly becomes wider than the one you measured. Going lower than 10% is a false economy: running short means a second trip, and concrete that has already started to set does not join cleanly to fresh concrete.
What is a cold joint, and why does running short cause one?
A cold joint is the plane where fresh concrete was placed against concrete that had already begun to set, so the two never bonded into one mass.
It is a weak line through the pour. It cracks there, and water gets in there.
This is why under-ordering is worse than over-ordering by the same amount. Surplus concrete costs you a little money and some disposal effort. Running out partway through means the first load stiffens while you fetch more, and the finished slab has a permanent weakness through it. When in doubt, order the next increment up.
Does this work for a suspended staircase?
No, and that is worth being clear about rather than quietly returning the wrong number.
The stair option computes a solid stair: formed on both sides and poured full, the way exterior steps onto a porch usually are. The formula is the staircase cross-section, which is the big triangle under the nosing line plus one half-step triangle per step, multiplied by the width.
A suspended stair is a sloping structural slab, the waist, with steps formed on top of it. It contains materially less concrete than a solid one of the same dimensions, and the volume depends on the waist thickness, which this tool does not ask for. Work that one from the structural drawing.
Are these bag yields right for brands other than Quikrete?
Close enough to plan with, but check the bag.
The figures used here are the published yields from the QUIKRETE Concrete Mix data sheet, product number 1101: 0.30, 0.375, 0.45, 0.60 and 0.675 cubic feet for the 40, 50, 60, 80 and 90 lb bags respectively.
Other manufacturers of standard concrete mix are within a few percent of these, because the mix design is broadly the same. Specialty products are not: fast-setting, high-strength and countertop mixes have their own yields, and lightweight mixes differ more. The yield is printed on the bag, so it takes one look to confirm.
Why does it round every bag count up?
Because you cannot buy two thirds of a bag.
More to the point, the waste allowance exists precisely so you are not sitting exactly on the line. Rounding a bag count down would spend that allowance immediately and put you back where you started, with a job that needs one more bag than you own.
Notes and sources
Bag yields are the published figures from the QUIKRETE Concrete Mix data sheet, product number 1101: a 40 lb bag yields approximately 0.30 cu ft, 50 lb 0.375 cu ft, 60 lb 0.45 cu ft, 80 lb 0.60 cu ft, and 90 lb 0.675 cu ft. The 90 lb bag is marked regional availability on the same sheet and is flagged here accordingly. Other brands of standard concrete mix are within a few percent; specialty and lightweight mixes are not, and the yield is printed on every bag.
Geometry is ordinary solid geometry and nothing is licensed. The one formula worth stating is the solid stair, because it is the only one that is not obvious: the cross-section is the triangle under the nosing line plus one half-step triangle per step, which works out as rise times run times steps times steps-plus-one, halved, times the width. It is checked against hand cases in the tests, including the single step, which has to come out as one plain rise-by-run rectangle.
The one cubic yard threshold is a judgement, not a standard. It comes from batch count and working time rather than from any code, and it is stated as a rule of thumb on purpose. Your mixer, your helpers and the weather all move it.
Everything runs in your browser and no dimension you type is sent anywhere. Tell me if a figure here disagrees with the bag in front of you, and say which product it is.