Why symmetry is the 'fitters friend'
When you look up a staircase from the hallway, your brain immediately looks for patterns and lines. If the margin on the left is 8 cm and the margin on the right is 9 cm, the runner will look crooked, even if it is perfectly straight. This is why we measure the 'margin each side' rather than just the runner width. In an ideal world, you subtract your desired runner width from the total stair width and divide by two. For example, on an 80 cm wide stair with a 60 cm runner: (80 - 60) / 2 = 10 cm margin per side.
However, real-world stairs are rarely 'ideal'. Over decades, houses settle, and wood expands. You might find the bottom step is 81 cm wide while the top step is 79 cm. In this case, I usually keep the runner width constant at 60 cm and allow the margin to absorb the difference, meaning a 10.5 cm margin at the bottom and a 9.5 cm margin at the top. This is much less noticeable than having a runner that tapers in width. A professional fitter will 'split the difference' so the eye doesn't catch the variation.
The 10cm Rule
For most standard homes, a 10 cm (4 inch) margin is the 'sweet spot'. It's wide enough to show off the floor finish but narrow enough that the runner still feels substantial underfoot.
Measuring margins on winders and landings
Winders (steps that turn a corner) are where margin measurement gets tricky. Because the steps are triangular, the distance from the runner to the corner changes. The rule of thumb here is to maintain a constant margin along the 'straight' edges of the stairs and then follow the curve smoothly. For the inner 'point' of the winder, the margin will necessarily be very small, sometimes zero, while on the outer edge, it will be much wider. The goal is to keep the runner's path looking like a smooth, flowing ribbon rather than a series of jagged turns.
On landings, the margin should ideally match the stairs leading up to and away from it. If your stair runner carpet has a 10 cm margin on the steps, it should have a 10 cm margin where it crosses the landing. This creates a continuous 'track' that looks intentional and architecturally sound. If you're using a stair runner length calculator, remember that you need to measure the landing path exactly to ensure your margins stay consistent throughout the transition.
How much runner do I need?
Measurement for the margin doesn't change the length of carpet required, but it does define the width you should order. For a standard UK flight of 13 steps (25 cm tread, 19 cm riser), the base length calculation is: (19 + 25) x 13 = 572 cm. Add 5 cm for each of the 13 nosing wraps (65 cm) and 15 cm for the top and bottom floor extensions. Subtotal: 652 cm. A 10% waste margin is standard, bringing the final order to 7.17 metres.
In US units for the same 13-step flight (10 inch treads, 7.5 inch risers): (10 + 7.5) x 13 = 227.5 inches. Add 13 inches for the nosing wraps and 12 inches for the floor/landing extensions. Total: 252.5 inches. With a 10% waste margin, you should order 278 inches, or approximately 23.2 feet. Always double-check your 'margin each side' measurements before finalizing the order, especially if you are having the runner custom-bound to a specific width.
| Staircase Width | Target Margin | Required Runner Width |
|---|---|---|
| 75 cm | 7.5 cm | 60 cm |
| 85 cm | 10 cm | 65 cm |
| 95 cm | 10 cm | 75 cm |
| 105 cm | 12.5 cm | 80 cm |
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 calculatorTools for marking your margins
When I'm fitting, I don't just 'eyeball' the margin. I use a 'spacer block' - a small piece of wood cut exactly to the margin width (e.g., 10 cm). I place this block against the stringer on every step to ensure the runner is positioned perfectly. This is much faster and more accurate than using a tape measure for every single staple. For DIYers, marking the margin with a light pencil line on each riser will help you keep the runner centred as you work your way up the flight.
Don't forget to account for the thickness of the binding! If you order a 60 cm runner with a 5 mm thick leather binding, the finished width will be 61 cm. This will reduce your planned margin by 0.5 cm on each side. It might seem small, but in a precise installation, these millimetres matter. Always specify whether your measurements are for the 'carpet only' or the 'finished width including binding' when talking to your supplier.
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.
Still deciding on material, width or pattern? Our hub on stair runners compares weaves, widths, backings and fitting styles side by side, then hands the numbers straight to the calculator.
Frequently asked questions
What is the best margin for narrow stairs?
For stairs under 70 cm wide, a smaller margin of 4-5 cm is best. This ensures the runner is wide enough (60 cm) for safe walking while still showing a hint of the stairs underneath.
Should I measure the margin from the wall or the stringer?
Always measure from the stringer (the wooden side-board of the stairs) if you have one. If the stairs are open on one side, measure from the edge of the wood.
Is a 15 cm margin too big?
For a standard staircase, yes, it can make the runner look like a 'landing strip'. However, on very grand, wide staircases (over 120 cm), a 15 cm margin can look appropriately scaled and very luxurious.
How do I ensure the margin is the same on both sides?
Measure the total width of the step, subtract the runner width, and divide by two. Mark this distance on each side of the riser with a pencil before you begin fitting.
Can the margin vary between the tread and the riser?
No, the margin should be a straight vertical and horizontal line. If it varies, the runner will look like it's 'snaking' up the stairs.
Do I need a margin if I'm using stair rods?
Yes. Stair rods look best when they extend past the runner into the margin area, usually stopping about 2-3 cm before the stringer.
What if my stairs aren't perfectly square?
Keep the runner width constant and centred. Small variations in the margin (under 1 cm) are usually invisible to the casual observer, whereas a tapering runner is very obvious.
Should the margin match the hallway flooring?
Not necessarily, but the margin should 'flow' into the floor at the bottom of the stairs. If the runner ends in a bound return, the margin will surround the end of the runner, creating a framed look.
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.
Calculate runner width and margin for your stairsEvery 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.