Recommended thickness and density
| Location | Thickness | Density | Notes |
|---|---|---|---|
| Stair treads | 6-9 mm (0.24-0.35 in) | 8-10 lb/cu ft | Firm pad, prioritises stability over softness |
| Landing carpet | 9-11 mm (0.35-0.43 in) | 6-8 lb/cu ft | Can be slightly softer, flat floor only |
| Living room floor (comparison) | 10-12 mm (0.39-0.47 in) | 5-7 lb/cu ft | Softer feel acceptable, not for stairs |
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorWhy thinner and firmer wins on stairs
- Thick, soft pads compress unevenly on a narrow tread, reducing the effective nosing definition.
- Firm pads keep the runner from shifting sideways underfoot, aiding grip.
- Excess thickness at the nosing can visually and physically reduce usable tread depth.
- Higher density pads recover their shape better under the repeated point-loading of stair traffic.
Density in lbs explained
Underlay density is measured in pounds per cubic foot (or kg per cubic metre) and indicates how much the pad resists compression, not how thick it is. An 8lb pad at 7mm will outlast and outperform a 5lb pad at 12mm on stairs, even though the 5lb pad feels thicker underfoot initially.
| Density (lb/cu ft) | Approx kg/m3 | Suitability for stairs |
|---|---|---|
| 5-6 lb | 80-96 kg/m3 | Too soft, avoid on stairs |
| 8-10 lb | 128-160 kg/m3 | Recommended for stair treads |
| 12 lb+ | 192+ kg/m3 | Very firm, suited to heavy commercial traffic |
Match underlay to the runner, not the other way round
Ask your supplier to recommend a stair-specific underlay for the runner you have chosen; wool and synthetic runners can have different ideal underlay firmness.
Worked example with real numbers
To see how stair runner underlay thickness guide plays out on an actual staircase, take a common 13-step flight with a 10 in (25 cm) tread depth and a 7.5 in (19 cm) riser height. That gives 17.5 in (44 cm) per step, or 227.5 in (5.78 m) of raw length before any waste allowance is added, a figure worth keeping in mind whenever this topic is discussed in isolation from the rest of the job.
| Measurement | Imperial | Metric |
|---|---|---|
| Tread depth | 10 in | 25 cm |
| Riser height | 7.5 in | 19 cm |
| Per step total | 17.5 in | 44 cm |
| 13 steps, raw length | 227.5 in | 5.78 m |
| With 8% waste allowance | 245.7 in | 6.24 m |
When to call a professional instead
Plenty of homeowners handle stair runner underlay thickness guide themselves on a straightforward straight flight, but certain conditions tip the balance firmly towards hiring a fitter. Getting a second opinion before committing materials often costs nothing and can save an expensive mistake on a job that cannot easily be undone.
- The staircase has a turn, winders, a curve or a split level rather than one straight run.
- The existing substrate is damaged, damp, or its condition is genuinely unclear.
- The runner is an expensive natural fibre or bespoke pattern where a cutting mistake is costly.
- You have no experience with a staple gun, knee kicker or carpet seaming on stairs.
- The staircase is the main one in the home and downtime or an unfinished look is not acceptable.
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.
Frequently asked questions
What thickness underlay should I use for a stair runner?
6-9mm (0.24-0.35in) is typical for stair treads, noticeably thinner than living room underlay, to preserve nosing definition and underfoot stability.
What density underlay is best for stairs?
8-10lb per cubic foot (roughly 128-160 kg/m3) is recommended for stair treads, firmer than the 5-7lb pads often used on flat floors.
Can I use the same underlay on stairs as the rest of my house?
It is not recommended. Stair underlay should be firmer and thinner than living room underlay to avoid reducing tread definition and stability.
Does thicker underlay make a stair runner more comfortable?
Not necessarily an improvement on stairs; overly thick or soft underlay can reduce nosing definition and make the tread feel less stable underfoot, unlike on flat flooring.
What is the biggest mistake people make with this?
Rushing the measuring or prep stage. Most poor outcomes with stair runner underlay thickness guide trace back to a single measurement taken at the wrong point, or a substrate check skipped to save a few minutes, both far cheaper to fix before the runner is cut than after.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Plan my underlay and runner togetherEvery guide is written from hands-on measuring and fitting experience, checked against the calculator’s own maths, and re-reviewed whenever prices or methods change. Read our editorial standards.