Concrete stair calculator

A set of solid concrete steps is really a stack of shrinking rectangular prisms: the bottom step is the tallest and widest block of concrete, and each step above it sits on top, contributing its own riser height and tread depth to the total.

Below the visible steps there's usually a throat slab, a sloped or stepped layer of concrete connecting the underside of the stair to the ground or footing, which also needs to be counted in the volume.

For n risers of height h and tread depth t at width w, the stepped portion's volume is the sum of each step's prism: step i contributes w × h × t × i (since each lower step carries the height of every step above it). Add the throat slab volume, roughly slab thickness × width × total run, and you get the full pour volume.

This is an approximation that assumes rectangular steps without decorative returns or curves; a mason forming an elaborate stair with radiused nosings or side walls will need to add that volume separately.

1. Multiply riser height × tread depth × width to get one 'unit step' volume
2. Multiply that unit volume by the sum 1 + 2 + 3 + ... + n (n = number of risers) for the stepped portion
3. Calculate total run: (risers − 1) × tread depth, or risers × tread depth depending on your landing detail
4. Multiply slab thickness × width × total run for the throat slab volume
5. Add the stepped volume and throat slab volume, then convert cubic inches to cubic feet (÷ 1,728) and cubic yards (÷ 27)

Pre-mixed bagged concrete is sold by weight, and each bag yields a fairly consistent volume once mixed with water: a 60 lb bag yields about 0.45 cu ft, and an 80 lb bag yields about 0.6 cu ft. Divide your total cubic feet by the bag yield to estimate how many bags to buy.

For anything larger than a small set of 3 or 4 steps, ready-mix delivered by the cubic yard is usually cheaper and far less labor than mixing dozens of bags by hand; bagged concrete tends to make sense mainly for small repairs or a single short flight.

Formed stairs almost always use a bit more concrete than the pure geometric volume, because of form irregularities, spillage, and slightly over-thick edges. Add 5 to 10 percent to your calculated volume before ordering, and round bag counts up rather than down.

If you're ordering ready-mix by the truck, most suppliers sell in quarter- or half-yard increments, so round up to the nearest practical delivery size rather than trying to order an exact fractional yard.

Solid concrete stairs are commonly reinforced with rebar, both to control cracking and to tie the steps into the footing or foundation they bear against. Bar size, spacing, and whether dowels into an existing foundation are needed depend on the stair's span, soil conditions, and local structural requirements.

This calculator estimates concrete volume only; reinforcement design should be confirmed with a local contractor, engineer, or your building department, since requirements vary by jurisdiction and site conditions.

If your concrete stair ties into a landing or porch slab, that landing's own thickness and area should be estimated and ordered separately unless you're pouring it in the same monolithic pour, in which case add its volume to the stair total.

Formwork for concrete steps needs to hold its shape under the weight of wet concrete, which is heavier than most people expect: figure roughly 150 lb per cubic foot, so bracing the outer stringers of the form matters as much as getting the riser and tread dimensions right.

If you're deciding on riser and tread dimensions before you build the forms, the outdoor stair calculator or stair rise and run calculator can help you land on comfortable, code-referenced proportions first.

Building freestanding wood or composite steps instead of solid concrete? The deck stair calculator and outdoor stair calculator are built around lumber-based construction instead of poured material.