Concrete Stairs Calculator estimates solid poured stair volume using rise × run × [steps × (steps + 1) ÷ 2] × width, then adds platform depth, waste, bag counts, cost, and concrete weight.
Most people overorder concrete for stairs — here’s why
When estimating concrete for a staircase, the instinct is to measure the bounding box: total height times total horizontal depth times width. That gives you the volume of a solid rectangular block, which is far more material than a staircase actually needs. A staircase is a triangular profile, not a rectangle, and the difference between those two shapes is where estimates go wrong — sometimes by a factor of two.
This calculator uses the correct method: it sums the stacked rectangular solids that make up a solid poured staircase sitting on flat grade. The first step needs one block of concrete; the second step sits on a block twice the height; the third, three times — and so on up. The total profile area works out to Rise × Run × N(N+1)/2, where N is the step count. Multiply that by stair width and you have the true raw volume before waste.
That formula changes non-linearly with step count. Going from 3 steps to 6 doesn’t double your concrete — it quadruples it, because the triangular stack grows with the square of the step count. Worth knowing before you call the ready-mix supplier.
What the calculator is actually computing
The stair body
Each step contributes a column of solid concrete below it, not just its own tread thickness. Step 1 sits on a column one riser tall. Step 2 sits on a column two risers tall. The total volume is the product of rise, run, width, and the triangular sum N(N+1)/2. For a standard 3-step entry stoop with 7 in. risers, 11 in. treads, and 3 ft. width, that’s roughly 9.6 cubic feet of raw material — less than you’d expect from measuring the outside dimensions.
Top platform depth
If your staircase terminates at a landing or threshold slab, enter its depth in the “Top Platform Depth” field. The volume added for the platform is: platform depth × total stair height × width. Note that it uses the full stacked rise height — the height of all steps combined — not just one riser. A shallow platform on a tall staircase can add substantial volume. Leave this at zero if your landing is a separate pour or already exists.
Waste and final volume
The default 10% waste factor covers spillage, form overfill, and the uneven ground surface that every poured stair contends with. The calculator displays both the raw volume (no waste) and the final ordered volume so you can see exactly what the waste allowance adds. For very small pours with tight form work, 5% may be reasonable. For rough or sloped grade, 15% is more defensible.
Bag counts
The 80 lb bag count divides final cubic footage by 0.60 — the standard stated yield for an 80 lb high-strength mix. The 60 lb bag count uses 0.45 cu ft per bag. Both are ceiling values (always rounded up), since you can’t buy a fraction of a bag. For any staircase larger than about 1.5 cubic yards, bagged concrete becomes expensive and labor-intensive; that’s where ready-mix makes financial sense.
Weight and delivery
The weight estimate uses 4,000 lbs per cubic yard — standard density for normal-weight concrete. This is the weight of the concrete you’re ordering, including the waste allowance, not just the finished stair. The ready-mix figure shows what fraction of a full truck your pour represents. A standard truck holds 9 cubic yards. Most suppliers enforce a minimum order of 3–4 cubic yards, so for small entry stairs, the economics of ready-mix often don’t work out even if the math says you need 0.5 yards.
Worked example: 5-step front stoop with landing
A contractor pricing a front entry replacement quoted a homeowner on 5 steps at 7 in. rise and 11 in. run, 4 ft. wide, with an 18-inch top platform leading to the front door. Here’s how the numbers work through the calculator:
- Stair body: 7 in. = 0.583 ft. rise; 11 in. = 0.917 ft. run; N(N+1)/2 = 5×6/2 = 15. Profile area = 0.583 × 0.917 × 15 = 8.02 sq ft. Stair volume = 8.02 × 4 ft. width = 32.1 cu ft.
- Platform: 1.5 ft. depth × (5 × 0.583 ft. total rise) × 4 ft. wide = 1.5 × 2.917 × 4 = 17.5 cu ft.
- Raw total: 49.6 cu ft. = 1.84 cu yd.
- With 10% waste: 54.6 cu ft. = 2.02 cu yd.
- 80 lb bags: ⌈54.6 / 0.60⌉ = 91 bags — roughly 3,640 lbs of bags to haul and mix.
- At $155/cu yd: Material cost comes to about $313 for poured concrete, or $470+ in 80 lb bags at retail.
At 2.02 yards, a ready-mix order is borderline — some suppliers will deliver this quantity with a short-load fee, others won’t. The contractor in this case ordered 2.5 yards to meet the supplier’s minimum, used the excess for a small curb repair, and still came out ahead of bagged pricing.
Where this estimate breaks down
The formula assumes solid concrete poured on flat, undisturbed grade — no rebar cavity, no gravel sub-base profile, no existing foundation wall the stairs abut. In practice, many installations differ from this in ways that reduce actual concrete volume:
Stairs built over a compacted gravel base or existing slab require only the concrete above the sub-base, not a full solid triangular profile down to grade. If your stair form sits over 4 inches of compacted base, the volume per step column is shorter by that 4 inches for each column — across multiple steps that can amount to 10–20% less concrete than the calculator shows.
Hollow or form-filled stairs — where the interior is filled with rubble, brick, or compacted earth before pouring — use significantly less concrete. The calculator has no input for this; you’d need to estimate the fill volume and subtract it manually.
Pre-cast or modular stair units embedded in a poured surround use even less poured volume. This calculator is designed for full poured-in-place concrete stairs only.
If any of these conditions apply to your project, treat the calculator’s output as an upper bound and work downward from it based on your specific formwork setup.
Frequently asked questions
Why won’t the calculator accept a decimal step count like 2.5?
Steps must be whole numbers — you can’t have half a step in a staircase pour. The calculator rejects decimal step counts and shows a validation message. If you’re trying to model an unusual elevation change, adjust your riser height to fit a whole number of steps instead. For example, a 21-inch total rise divided by 3.5 doesn’t work; 3 steps at 7 in. or 4 steps at 5.25 in. both do.
The platform volume seems much larger than I expected. What is it multiplying?
The platform calculation uses total stair rise height as the platform’s vertical dimension, not just one riser. If you have 5 steps at 7 in. each, the total rise is 35 inches — and a 1 ft. platform depth gets multiplied by that 35-inch height. This correctly models a landing slab that extends from the top of the staircase down to grade level on the back face. If your platform is a thin slab rather than a solid block, reduce platform depth to 0 and account for it separately.
I’m working in metric. Which unit options should I use?
The Rise, Run, Width, and Platform fields all support centimeters (cm), meters (m), or a dual “Meters & cm” entry mode. You can mix units across fields — for example, enter rise in centimeters and width in meters — and the calculator converts everything internally before computing. The output is always in cubic yards and cubic feet, with an equivalent cubic meters figure shown in the Volume Metrics card.
What’s the difference between the “Raw Vol” shown in the card and the hero total?
The hero value is the ordered volume after applying your waste factor. The “Raw Vol (No Waste)” row in the Volume Metrics card is the pure calculated concrete volume with no safety margin added. Use raw volume if you’re comparing against an engineer’s takeoff or trying to understand the structural mass; use the hero value when placing the actual order.
The weight card shows a higher number than I expected. What density does it assume?
The calculator uses 4,000 lbs per cubic yard — the standard unit weight for normal-weight (stone aggregate) concrete. Lightweight concrete mixes run approximately 20–25% lighter; structural mixes with high-density aggregate run heavier. If your spec calls for a non-standard mix, the weight figure here will be off proportionally, though the volume calculation itself remains correct regardless of mix design.
References
- Quikrete 80 lb High Strength Concrete Mix (No. 1101): stated yield of 0.60 cu ft per bag — Quikrete Product Data Sheet
- Sakrete 60 lb High Strength Concrete Mix: stated yield of 0.45 cu ft per bag — Sakrete Product Page
- American Concrete Institute, ACI 318-19 Building Code Requirements for Structural Concrete: normal-weight concrete unit weight standard reference — ACI 318-19