Concrete for 3 steps: 42″ wide, 6″ rise, 10″ run
| Ready-mix | 0.36yd³ |
|---|---|
| 80 lb bags | 17bags |
| 60 lb bags | 22bags |
| Volume (order this)+10% waste | 9.63ft³ |
| Total rise3 × 6″ | 1.5ft |
| Total run3 × 10″ — how far it projects | 2.5ft |
| Gravel base (6″) | 4.4ft³ |
| Side formstwo stepped cheeks | 5ft² |
| Riser boards3 risers, 42″ wide | 5.3ft² |
| Rebar #4 @ 12″ grid | 28linear ft |
| Code check — rise6″ against the 7¾″ IRC maximum | OK |
| Code check — run10″ against the 10″ IRC minimum | OK |
| Comfort (2R + T)outside the 24–25″ range carpenters work to | 22in |
Notes
- Concrete goes up with the square of the step count, not in a straight line. The bottom step carries the full depth of everything above it, so 3 steps is 6 step-blocks of material, not 3. Multiplying one step's volume by 3 — which is what most step calculators do — would give 4.4 ft³ instead of the real 8.8.
- 6″ rise and 10″ run are both inside the IRC limits (7¾″ maximum rise, 10″ minimum run). The rule that catches people out is the third one: no more than ⅜″ variation between the tallest and shortest riser in a flight. With concrete you get one attempt at that.
- 2R + T comes to 22″, outside the comfortable 24–25″ band. These will feel cramped and quick. Adjusting the run by an inch usually fixes it without changing the total rise.
- Get the total rise right first, then divide. Measure from the finished landing down to the finished grade — 1.5 ft here — and divide by a whole number of steps. Fudging the bottom riser to absorb an error is exactly what the ⅜″ rule exists to prevent.
- They project 2.5 ft from the wall. That is the measurement that collides with paths, driveways and property lines, and it is usually discovered after the forms are built.
- Compacted gravel under the whole footprint, below the frost line at the front edge. Steps that heave separate from the house at the top and open a gap you can watch grow every winter.
Volume grows with the square, not in a line
This is where step calculators go wrong. The bottom step has to carry the full depth of every step above it, so 3 steps is 6 step-blocks of concrete, not 3.
Multiplying one step's volume by 3 would give 4.4 ft³. The real figure is 8.8 ft³, or 0.36 cubic yards once 10% waste is added — 17 bags of 80 lb mix.
What it takes up
1.5 ft of total rise, projecting 2.5 ft from the wall. That projection is the measurement that collides with paths, driveways and property lines, and it is normally discovered after the forms are built.
Two different checks
Code. The IRC allows a maximum 7¾″ rise and requires at least a 10″ run. There is a third rule that catches more people: no more than ⅜″ variation between the tallest and shortest riser in one flight. With concrete you get exactly one attempt at that.
Comfort. Carpenters check 2 × rise + run, and want it between 24 and 25 inches. These come to 22″. Steps can satisfy code and still feel wrong underfoot, and this is the check that catches it.
Order of work
Measure total rise from the finished landing to the finished grade, then divide by a whole number of steps. Adjusting the bottom riser to absorb an error is precisely what the ⅜″ rule exists to prevent.
Compacted gravel under the whole footprint, below the frost line at the front edge. Steps that heave separate from the house at the top and open a gap that grows every winter.
Nearby sizes
Read more
- Concrete Steps: The Rise That Divides Evenly, and the Code That Limits ItConcrete steps are one of the few pours where the geometry has to be resolved before the volume, and where the code sets a tolerance measured in eighths of an inch.
- Bags or a Truck — The Crossover Is About One Cubic YardThe arithmetic is not really about price. It is about how long a slab can sit half-poured before the joint between the halves becomes permanent.
- Rebar in a Slab — Spacing, Cover, and Why Position Matters MoreRebar laid on the ground and left there does nothing. It has to be held up into the concrete, and that is the step that gets skipped.
- Concrete Slab Cost by State — 400 ft², All 51The figures price a 4-inch slab on grade. A driveway wants six inches and rebar, and that is not a small adjustment.