Carpet Stair Calculator: How Much Carpet for Your Stairs

Why full stair carpet is measured differently to a runner

A stair runner is generally a fixed width narrower than the stair, so runner calculations use the runner's own width. Full carpet instead uses your actual stair width, wall to wall (minus a small trim allowance), and needs enough length to wrap every tread and riser edge to edge with no exposed timber margin, which changes both the total yardage and how forgiving the job is of small measuring errors.

The core carpet calculation

1. Measure stair width wall-to-wall (or string-to-string, minus trim allowance).
2. Measure the wrap depth per step: tread going plus riser height plus a little extra to dress into the angle, commonly totalling roughly 500-600mm (20-24in) per step depending on riser and going.
3. Multiply wrap depth per step by the number of steps (treads plus risers together as one continuous wrap, or per-step if fitted in pieces) to get total running length.
4. Multiply total running length by stair width to get area, then convert to your supplier's selling units (square metres or square yards).
5. Add a waste allowance for pattern matching, cutting and winders.

These figures are close estimates for planning purposes. Fitters and suppliers commonly do their own detailed take-off from a site visit, since actual carpet consumption depends heavily on how the fitter chooses to wrap and join pieces, and on whether the roll width allows full-width pieces or requires seaming.

Waste and pattern allowance

Winders and landings

Winder treads are wider at one end than the other, so they consume more carpet width per step than a straight tread, and pattern matching around a winder is one of the hardest parts of a stair-fitting job. Landings are typically carpeted separately from the stair flight itself, often from the same roll, and should be added to your quantity as their own area calculation rather than folded into the per-step figure.

Underlay and gripper for full-width carpet

Full-width carpet needs full-width underlay and gripper, unlike a runner which only needs gripper the width of the runner itself. Order underlay to the same area as the carpet itself, and gripper as a running length equal to roughly the total wrap length calculated above, since gripper is fixed along both the tread and riser edges.

When to get a professional measure

• Any staircase with winders, as pattern and quantity estimates get considerably harder to do accurately from a tape measure alone.
• Bold, large-repeat or directional patterns, where a small quantity error can leave the flight unmatched partway up.
• Very wide staircases where roll width forces seaming, which changes how much waste a fitter needs to allow.
• Any staircase where the treads or risers are not uniform in size.
• Whenever the calculator estimate is close to a full roll or a supplier's minimum order quantity, since a professional measure can avoid over-ordering.

Common carpet stair calculation mistakes

• Using a stair runner's narrower-width calculation method for a full wall-to-wall carpet job.
• Forgetting to add pattern repeat waste on top of standard cutting waste.
• Treating winder treads as standard width when they consume noticeably more carpet.
• Not ordering matching underlay and gripper quantities alongside the carpet itself.
• Skipping a professional measure on a patterned or winder staircase to save a small fee, then discovering a shortfall mid-job.

Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.