Concrete Calculator for Stairs: Estimating a Flight of Steps

Stairs are one of the hardest concrete shapes to estimate, which is exactly why they get underestimated so often. A slab is one simple block, but a flight of stairs is a stack of rectangular steps, and you have to account for both the tread area of each step and the vertical rise. Get the math right and you avoid both a cold joint and a truckload of wasted concrete. Here is the method, with a worked example you can apply to any set of stairs.

The Stair Volume Formula

A flight of concrete stairs can be modeled as a series of identical steps, each one a small rectangular block. Each step's volume is:

Step Volume = Tread Depth (ft) x Step Width (ft) x Step Height (ft)

Most residential stairs have a tread depth of about 11 inches (0.92 feet) and a step height, or riser, of about 7 inches (0.58 feet). If the stair width is 3 feet, each step is 0.92 x 3 x 0.58 = 1.60 cubic feet. For 10 steps, that is 16 cubic feet. But that only covers the steps themselves — you also need the volume of the concrete mass underneath them, which is a slab spanning the full run of the stairs. Add a 4-inch base slab under the whole flight.

Worked Example: A 10-Step Staircase

Let's build a concrete estimate for a 3-foot-wide staircase with 10 steps, each with an 11-inch tread and a 7-inch rise.

• Step volume: 10 steps x 1.60 cubic feet = 16 cubic feet
• Base slab: the run is 10 steps x 0.92 feet = 9.2 feet long, at 3 feet wide and 4 inches (0.333 feet) thick = 9.2 x 3 x 0.333 = 9.2 cubic feet
• Total volume: 16 + 9.2 = 25.2 cubic feet
• Convert to yards: 25.2 / 27 = 0.93 cubic yards
• Add 10% waste: about 1.02 cubic yards

So this flight needs roughly 1 cubic yard of concrete. In bags, that is 25.2 / 0.60 = 42 bags of 80 lb mix, or about 46 with waste — a brutal amount to mix by hand. Since the final order rounds to a full yard, this is a clear ready-mix job: 1 yard at $160 plus a short-load fee of about $100 for a sub-minimum order is roughly $260 delivered, versus 46 bags at $7 each ($322) plus hours of mixing. The truck wins.

Estimating Different Stair Dimensions

If your tread, riser, or width differs, just plug your numbers into the formula. A wider staircase scales directly — a 4-foot-wide version of the same flight is 10 x (0.92 x 4 x 0.58) = 21.3 cubic feet for the steps plus 9.2 x 4 x 0.333 = 12.3 for the base, totaling 33.6 cubic feet, or 1.24 yards (1.37 with waste). If your steps are steeper or shallower, recompute the per-step volume. And if the stairs are set into a slope with a taller foundation, add that concrete below the first step — a common omission that leads to running short.

Forming and Pouring Tips

Stairs are poured into forms, and the single biggest waste driver is an inaccurate form. Take the time to build the forms to your exact tread and riser dimensions, because every extra quarter-inch across ten steps adds up to meaningful volume — and every gap leaks concrete. Vibrate or rod the mix into the corners of each step to avoid honeycombing at the step edges, which are the weakest and most visible part of the pour. Work from the bottom step upward, and screed each tread level as you go. If you are using bags, mix small batches and place one or two steps at a time so the lower steps are still workable when you reach the top — otherwise you risk a cold joint running through the middle of the flight.

For your exact dimensions, use the concrete calculator at the top of this page. Enter the tread depth, riser height, step count, and width to get the cubic yardage and bag count for your specific staircase, including the waste factor.