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.

Comparing full carpet against a stair runner on the same flight

A lot of homeowners weigh up full wall-to-wall carpet against stair runners on the same staircase before ordering, and the deciding factors are usually cost, look and how much of the timber string and tread edge they want left on show. On a 13-step flight roughly 900mm (35.4in) wide, full carpet needs the whole width covered, while a typical runner is ordered somewhere around 600-700mm (24-28in) wide, leaving a timber margin either side. A stair runner calculator works from the same tread going and riser height as the full-carpet method above, but multiplies by the narrower runner width instead of the full stair width, so material quantity - and cost - drops substantially even before you account for the cheaper timber finish either side needing no covering at all.

If you are still deciding, it is worth running both calculations side by side - the full-carpet method on this page, and a separate stair runner calculator pass at your chosen runner width - since the material saving on a runner is often significant enough to justify a higher-quality carpet at a narrower width for a similar total spend.

Roll width and seaming

Broadloom carpet commonly comes in roll widths around 3.66m or 4m (12ft or 13ft), and most domestic staircases at 900mm-1,000mm (35-39in) wide fit within that roll width with the pile running up the flight, avoiding a seam altogether. Wider stairs, or carpet ordered in a narrower roll width (common with some wool and patterned ranges, often 68-80cm/27-32in), may force a seam somewhere along the run, and a seam on a stair is far more visible and more prone to lifting than a seam on a flat floor, so it is worth checking your chosen carpet's roll width against your stair width before ordering.

This is another reason narrow-roll wool carpets are so often used as stair runners rather than full-width carpet: a 68-80cm roll comfortably covers a typical runner width with no seam, whereas the same roll would need joining to cover a full 900mm+ stair width.

Fibre choice and how it affects yardage and durability

Fibre choice affects both how much carpet you need and how it performs on a stair, which is one of the highest-wear areas in most homes. Wool and wool-blend carpets are the most common choice for stairs because they hold their pile and resist crushing on the nosing edge better than most synthetics, but they are usually sold in narrower roll widths, which is exactly why wool is so often chosen for stair runners rather than full-width carpet. Synthetic fibres such as nylon or polypropylene are typically cheaper and come in wider broadloom rolls suited to full-width stair carpet, but tend to crush and flatten on the nosing edge faster under heavy daily use.

Whichever fibre you choose, the nosing edge of every tread wears faster than the flat of the tread or the riser, so many fitters recommend moving the carpet or runner slightly (sometimes called 'move up' or periodic repositioning) every few years on a high-traffic flight, which is only practical if the original quantity ordered included a little extra length for that adjustment - another reason to round up rather than order the bare calculated minimum.

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.