Risers and treads are not the same count
A flight that climbs from the ground to a landing has one more riser than it has treads. The last riser takes you up onto the landing, and the landing is not a tread you are buying concrete for. On a four riser stoop that means three treads, and estimating four gives you 33 percent too much concrete on the stepped part. It is the single most common error on small step jobs and it survives because a four step stoop obviously has four steps, which is true and is not the same statement.
The layout itself follows from the total rise. Measure from the finished ground level, not the dirt you are standing on today, to the top of the landing or the threshold. Divide by a target riser height, round to a whole number, and divide back to get the actual riser. That last division is the important one, because every riser in a flight must be the same. A quarter inch of variation between the top and bottom step is enough to catch a foot, and it is one of the first things anyone inspecting the work will check with a tape.
The volume is a staircase, not a wedge
People estimate steps as a triangular wedge and come out low, or as a rectangular block and come out high. Neither is right. The side profile of a set of poured steps is a staircase: a rectangle under each tread, each one deeper into the pile than the last. For three treads of 11 inches with 7 inch risers, the profile is 11 by 7, plus 11 by 14, plus 11 by 21, which is 462 square inches. At 48 inches wide that is 22,176 cubic inches, or 12.8 cubic feet, or just under half a cubic yard.
What surprises people is how quickly that grows. Add one riser and one tread to the same stair and the profile goes from 462 to 770 square inches, a two thirds increase, because the new rectangle is the tallest one. Steps are the part of a project where an extra step costs disproportionately more concrete than the last one, and it is worth knowing before the landing height gets adjusted on site.
Solid or shelled over fill
A small stoop is normally poured solid, and that is the simplest thing to form and the least that can go wrong. Larger flights are often formed as a concrete shell over a compacted gravel core, which saves a great deal of concrete once the mass gets big. The trade-off is entirely in the fill: it has to go in in lifts of a few inches and be compacted each time, because gravel that is tipped in and levelled will settle, and settlement under a step shows up as a crack across the back of the tread within a couple of years.
The calculator's shell estimate adds the skin over the stepped face and a skin on each side. Those overlap at the corners, so the figure runs slightly high, and on a narrow stoop the shell can add up to more than the solid volume, at which point the calculator caps it and tells you to pour solid. That is not a bug in the model, it is the honest answer: a 48 inch wide four step stoop with a 6 inch shell has almost no core left to fill.
Forming steps, which is the hard part
The concrete is the cheap half. Step forms are two side panels with the staircase profile cut into them, plus a riser board across the front of each step, and the whole assembly is trying to move outward and downward the whole time you are filling it. The bottom riser takes the most pressure because it has the most concrete above it.
| Form element | What it needs |
|---|---|
| Side panels | Sheet goods with the profile cut, staked hard and often braced back to the ground |
| Riser boards | One per riser, bevelled on the bottom edge so a trowel can reach the back of the tread |
| Stakes | Far more than feel necessary, especially at the bottom two risers |
| Cross bracing | Over the top of wide steps to stop the sides bowing |
The bevel on the riser boards is the detail that separates a decent job from one where the back inch of every tread is unfinishable. Without it the board sits square against the tread you are trying to trowel and you cannot get a tool into the corner.
What sits under and behind the steps
Steps that settle are steps that were built on something that moved. Depending on the site and the climate, that means a footing below frost depth, or a compacted base, or piers, and which of those applies is a question for the local building department rather than a web page: frost depth alone ranges from a few inches to well over five feet across the United States. Whether the steps should be tied into the house foundation or deliberately left free to move independently is likewise a local and site-specific decision, and both approaches are used for good reasons in different places.
If the steps land on a slab or a pad at the bottom, that is a separate pour and a separate calculation on the concrete slab calculator. If they need their own footing, the concrete footing calculator handles the strip, and any reinforcement that runs through the steps can be laid out on the rebar calculator. For a wooden flight instead of a poured one, the stair stringer calculator does the same rise and run arithmetic and then cuts it into a stringer.
Questions people ask
How many risers and treads do I have?
Risers are the total rise divided by your target riser height, rounded to a whole number. Treads are one fewer, because the top riser delivers you onto the landing rather than onto another tread. A 28 inch rise at a 7 inch target gives four risers of exactly 7 inches and three treads. A 30 inch rise at the same target gives four risers of 7.5 inches, because you cannot have four and a bit — every riser in a flight has to be identical, so the calculator divides the rise evenly rather than leaving a short step at one end.
What is a comfortable riser and tread?
Two proportion rules turn up constantly and both are guidelines rather than requirements. One is that rise plus run should land around 17 to 18 inches. The other is that twice the rise plus the run should be about 25 inches. A 7 inch riser with an 11 inch tread satisfies both. What is actually permitted for exterior steps where you live, including the maximum riser, the minimum tread and how much variation is allowed between steps, is set by your local building department, and it is worth asking before you form rather than after you pour.
Can I pour steps against the house without a footing?
Sometimes, and it depends entirely on the site. In climates with no meaningful frost, steps on a well compacted base perform fine. Where the ground freezes, steps sitting above frost depth will be lifted every winter and will either crack or pull away from the house. There is also a design choice about whether the steps should be tied to the foundation with dowels or deliberately kept independent so that any movement happens at a clean joint rather than through the concrete. Both are used and the right answer is local. Ask the building department for the frost depth and what they expect to see.
Should I fill the middle with gravel to save concrete?
On a large flight, yes, and it is standard practice. On a small stoop it often saves nothing, because a 6 inch shell on a 4 foot wide, four step stair is almost the whole volume anyway, and the calculator will tell you when that happens. Where you do use fill, the compaction is the whole job: place it in lifts of about four inches and compact each one. Fill that is tipped in and raked will consolidate under the weight of the concrete and the steps will crack at the back of a tread where the shell is thinnest.
Does the calculator include a nosing or a landing slab?
It has an optional top landing, which you enter as a depth and a thickness if the steps are delivering onto a pad you are pouring rather than onto an existing porch. It does not model a nosing, the small overhang some steps have at the front of each tread, because a nosing adds a fraction of a cubic foot in total and complicates the form work far more than it complicates the volume. If your steps have a substantial nosing, add its volume as a thin slab: overhang depth times width times tread thickness, times the number of treads.