Show the formula and working
This carpet underlay calculator works out how many rolls of underlay you need for a room, how to cut the strips, how much seam tape to buy, and how many lengths of gripper rod and door bars go round the edge. It is built for UK and European rooms measured in metres, with a feet and inches switch, and it lays the strips whichever way round uses less underlay.
Quick answer: Divide the room width by the roll width and round up for the strips, multiply by the room length, then divide by the roll length and round up. A 4 m by 3.5 m room needs 3 strips of 3.5 m, 10.5 m in all, so one 1.37 m by 11 m roll. The carpet underlay calculator adds gripper rod.
What Is a Carpet Underlay Calculator?
A carpet underlay calculator is a tool that turns a room’s size into the underlay, gripper rod and door bars needed before a carpet is fitted. Underlay is the cushioning layer laid under carpet, described in Wikipedia’s underlay article. It comes in rolls narrower than the room, so it is laid in strips side by side and taped at the joins.
The formula in plain words: strips = room width / roll width, rounded up; underlay length = strips x room length; rolls = underlay length / roll length, rounded up. The calculator checks both directions and uses the one needing less underlay. Gripper rod = room perimeter minus doorways, divided by the length of one gripper strip, rounded up.
How Do You Use the Carpet Underlay Calculator?
- Enter the Room length and Room width.
- Enter the Underlay roll width and Underlay roll length from the roll label.
- Add any Extra allowance for bays, alcoves or awkward shapes.
- Enter the number of Doorways and the Doorway width.
- Enter the Gripper rod length your supplier sells.
- Read the rolls, strips and gripper lengths, then open “Show the formula and working” to see each step.
How Much Underlay Do I Need for a Room?
You need enough underlay to cover the whole floor, laid in strips the width of the roll, so the amount depends on how the strips fit as well as the floor area. With 1.37 m by 11 m rolls, a 4 m by 3.5 m room needs one roll, while a 5 m by 4 m room needs two.
| Room size | Strips | Underlay length | Rolls of 1.37 m x 11 m | Gripper lengths of 1.5 m (one 762 mm door) |
|---|---|---|---|---|
| 3 m x 3 m | 3 x 3 m | 9 m | 1 | 8 |
| 4 m x 3.5 m | 3 x 3.5 m | 10.5 m | 1 | 10 |
| 5 m x 4 m | 3 x 5 m | 15 m | 2 | 12 |
| 6 m x 4 m | 3 x 6 m | 18 m | 2 | 13 |
| 5 m x 5 m | 4 x 5 m | 20 m | 2 | 13 |
Roll sizes vary by product, so the 1.37 m by 11 m roll in the table is an example. For the carpet itself, use the carpet calculator, which works with 4 m and 5 m roll widths.
Does Underlay Go Under the Gripper Rod?
No. Gripper rod is nailed or glued round the edge of the room first, with a small gap from the skirting, and the underlay is then laid inside it, butting up to the gripper but not over it. That way the carpet hooks onto the pins and the underlay stays flat. Strips of underlay are butted together, never overlapped, and taped at the joins so they do not move.
How Many Lengths of Gripper Rod Do I Need?
Measure the room perimeter, subtract each doorway, and divide by the length of one gripper strip, then round up. A 4 m by 3.5 m room has a 15 m perimeter; with one 762 mm doorway that leaves 14.24 m, which is 10 lengths of 1.5 m. Each doorway needs a door bar instead, chosen to suit the floor on the other side. On stairs, the stair carpet calculator works out carpet and underlay per step.
Which Way Should Underlay Strips Run?
Underlay strips can run either way, so lay them in the direction that needs the fewest joins and the least underlay. Many fitters run underlay at right angles to the carpet seams so the joins never line up. The calculator compares both directions and tells you which way it used. Carpet roll widths of 4 m and 5 m in Europe are listed in Wikipedia’s carpet article.
Worked Example: Underlay for a 4 m by 3.5 m Bedroom
Example: a bedroom 4 m long and 3.5 m wide gets new underlay from example rolls of 1.37 m by 11 m. There is one 762 mm doorway, and gripper rod comes in 1.5 m lengths. No extra allowance is added. These are the calculator’s default inputs.
- Strips: 4 m / 1.37 m = 2.92, so 3 strips of 3.5 m (laid along the 3.5 m width, which uses less underlay than 3 strips of 4 m).
- Underlay length: 3 x 3.5 m = 10.5 m.
- Rolls: 10.5 m / 11 m = 0.95, so 1 roll.
- Seam tape: 2 joins x 3.5 m = 7 m.
- Gripper: 15 m perimeter minus 0.762 m doorway = 14.24 m, so 10 lengths of 1.5 m, plus 1 door bar.
One 15.07 m² roll covers the 14 m² floor with a little left for trimming.
Carpet Underlay Calculator FAQs
How many square metres are in a roll of underlay?
It depends on the product. Multiply the roll width by its length, both printed on the label. An example roll of 1.37 m by 11 m covers about 15 m². The calculator works out the coverage from the two figures you enter.
Should I buy extra underlay?
The calculator already rounds up to whole strips, so extra is only needed for awkward rooms with bays, alcoves or a chimney breast. Add 5% to 10% in the extra allowance for those. Short offcuts can be butted and taped to finish a strip.
Do I need underlay tape?
Yes. Tape the joins between strips so they do not move or ride up under the carpet. The calculator shows the length of the joins, which is the minimum length of tape to buy for the seams.
Can I reuse old underlay?
Reusing old underlay is not usually advised, because it compresses and breaks down with age, which shortens the life of a new carpet. If it is in good condition and the right type for the new carpet, ask your fitter.
What gap should gripper rod leave from the wall?
Gripper rod is fixed with a small gap from the skirting so the carpet edge can be tucked in, with the pins pointing towards the wall. Check the gripper maker’s instructions or ask a carpet fitter for the gap to suit your carpet thickness.
Does the calculator work in feet?
Yes. Switch to feet and inches and the room size, roll size, lengths and areas are shown in imperial units. The strip and roll counts are worked out the same way, so you can use it with rolls labelled in feet.
Checked October 2026. Sources: Wikipedia: Underlay, Wikipedia: Carpet (4 m and 5 m roll widths). Roll size, gripper length and door width defaults are editable examples; 762 mm door from Howarth Timber UK door sizes.