Concrete steps are the calculation people most often get wrong, and the error is always in the same direction — too little. The reason is that steps are not a stack of identical slabs. Each one down the flight is longer than the one above it.
Why it is not simply steps times volume
Picture a flight of three steps poured against a bank. The top step is one tread deep. The second step is two treads deep, because it runs underneath the top one. The bottom step is three treads deep for the same reason.
So the volume is not three times one step. It is one plus two plus three, which is six times one step. For four steps it is ten times. For five it is fifteen.
That series — 1 + 2 + … + n — equals n times n plus one, divided by two. The calculator shows this step explicitly, because it is the whole reason a five-step flight takes three times as much concrete as most people estimate.
Riser and tread dimensions
Exterior steps are usually gentler than interior stairs. A seven inch riser with an eleven inch tread is a common exterior combination, and many codes cap exterior risers lower than interior ones.
Whatever you choose, every riser in the flight has to be the same height. The bottom step is where this goes wrong, because the landing pad it sits on adds its own thickness.
Uneven risers are the single most common cause of falls on exterior steps, and they are also the most common inspection failure.
What this calculator assumes
The formula here treats the steps as solid — poured full depth against soil, with no void inside. That is the conservative assumption and it is how most small residential flights are actually built.
If you are filling the core with compacted gravel, rubble, or a block stem wall and only pouring a shell, you will need considerably less. Work out the volume of whatever you are putting inside and subtract it.
Doing it the other way round — assuming a hollow core and discovering it is solid — is how a pour runs out halfway up.
Reinforcement and the base
Steps carry their own weight plus foot traffic, and they are attached to nothing. What they sit on matters. A compacted gravel base four to six inches deep under the whole footprint keeps water moving and stops the flight from settling on one side.
Steps that abut a house should not be tied rigidly to the foundation. They move differently through the seasons, and a rigid connection cracks one or the other.
One pour, no exceptions
Steps show every flaw. A slab can be floated and troweled until the seams disappear; a step face cannot, because it is vertical and the form comes off before you can work it.
Mix and stage everything you need before the first shovelful goes in. If the total is over a cubic yard, that means a delivery, not bags — and the truck will not wait while you mix.