Coving Calculator UK - How Many Lengths Do I Need?

How to Measure a Room for Coving

Coving is sold by length and fitted along the joint between wall and ceiling, so what you actually need is the total distance around the room at ceiling height, not the floor area. Here is the method:

1. Measure each wall separately at ceiling height (not floor level, since skirting boards and door frames can throw off a floor measurement). For a simple rectangular room, length and width is enough.
2. Add up the perimeter - for a rectangular room this is 2 × (length + width). A 4m × 3m room has a 14m perimeter.
3. Count your corners - a plain rectangular room has 4 internal corners. A chimney breast typically adds 2 external corners (where it juts into the room) plus 2 more internal corners where it meets the original wall. A bay window or alcove adds corners the same way.
4. Note any doorways coving stops at - if a doorway interrupts a wall, coving does not run across the opening, so deduct its width from that wall.
5. Choose your coving length and profile - 2m and 3m are the standard UK stock lengths, and profile size (100mm, 127mm, 135mm are common) affects how much adhesive you need and whether a standard mitre box can cut it.

For our 4m × 3m example with 4 internal corners and 2m lengths, that comes to 8 lengths once the corner-forced cuts and a 10% waste allowance are accounted for, even though the raw perimeter divided by piece length alone would suggest only 7.

Internal vs External Mitre Cuts, and Why You Cut the Pair Together

Coving cannot bend around a corner, so every corner in the room needs two pieces meeting at a 45 degree mitre cut, one cut on each piece. A plain rectangular room with 4 corners therefore needs 8 mitre cuts in total, not 4.

Internal corners (the inside of an "L", like the corner of a normal box-shaped room) need the coving cut so the two pieces form a concave joint, following the angle into the room.

External corners (the outside of an "L", like where a chimney breast juts out into the room) need the mirror-image cut, so the two pieces wrap around the outside of the corner instead of into it.

Always cut both pieces for one corner as a pair, working from the same mitre box setting, rather than cutting all your lengths first and hoping the angles line up later. Cut one piece, hold it up to the corner to check the angle sits flush, then use it as the reference for the second piece. The most common mistake is cutting both pieces of a corner the same way when they need to be mirror images of each other - one angled left, one angled right - which produces two pieces that simply do not meet.

How Much Coving Adhesive Do You Need Per Metre?

Coving adhesive coverage depends on whether you use powder adhesive (mixed with water, the standard choice for plaster and duropolymer coving) or a ready-mixed grab adhesive in a cartridge (more common for lightweight polystyrene coving).

These figures apply across the 100-135mm profile sizes this calculator supports. The calculator uses one flat coverage rate rather than scaling it by profile size: at this range of depths the difference is small enough to sit inside the buffer below, and we would rather use one figure we can source than invent a per-profile scaling factor with no manufacturer data behind it. Mitre joints use noticeably more adhesive than a straight run, so treat the calculator's estimate as a minimum and buy a little extra rather than running out mid-wall.

Cutting Coving: Mitre Box vs Cutting In Situ

Most DIY coving is cut with a plastic or wooden mitre box, which has slots pre-cut at 45 and 90 degrees to guide a hand saw. This works well for coving up to about 90-100mm deep, which covers most standard mitre boxes sold at Wickes or Screwfix. Wider profiles (127mm and above) often will not fit a retail mitre box at all, in which case you either need a specialist coving mitre box sized for the profile, or you cut in situ.

Cutting in situ means holding the coving up to the actual corner, marking the cut line directly against the wall and ceiling with a pencil, then cutting it on a bench. It takes longer but avoids buying a mitre box that will not fit your profile, and is often more accurate on rooms where the corners are not quite square (common in older UK houses).

Screwfix stock coving adhesive, mitre boxes and mitre saws if you do not already own the tools. A basic mitre box and fine-toothed saw is enough for most standard coving; a mitre saw is worth it if you are doing multiple rooms.

Plaster vs Polystyrene vs Duropolymer Coving

Which coving you choose affects both fixing method and cost:

• Plaster coving - the traditional choice, heavy, rigid and needs powder adhesive plus temporary support (nails or props) while it sets. Gives the crispest, most traditional profile and is what most plasterers fit. Stocked by Travis Perkins and Jewson for trade, and by most builders' merchants.
• Polystyrene coving - lightweight, cheap, and cuts easily with a sharp knife rather than a saw. Fixed with a ready-mixed grab adhesive, no temporary support needed. The lightest option and the easiest for a first-time DIYer, but the least durable and can dent easily. Widely stocked at B&Q and Wickes.
• Duropolymer coving - a dense polyurethane foam that looks like plaster but is much lighter and easier to cut and fix, using either grab adhesive or a caulk-style filler at the joints. A popular middle ground between plaster's finish and polystyrene's ease of fitting.

All three can use mitred joints or pre-formed corner blocks. Corner blocks are most common with polystyrene coving, since the material is soft enough that a slightly imperfect mitre cut looks worse than it would in plaster.

Common Room Sizes: Coving Lengths Needed

Figures below assume a rectangular room, 4 internal corners, 0 external corners, no doorway deductions and a 10% waste allowance, generated directly from the calculator above.

Use the calculator above for your exact room, especially if it has a chimney breast, bay window or doorways to deduct, since those change the corner count and cut list.

Frequently Asked Questions

How many lengths of coving do I need?

Measure your total room perimeter, then divide by your coving length (2m or 3m are standard UK sizes). But because coving cannot bend around a corner, you actually need to work out how many pieces fit each individual wall separately, since a corner forces a cut even if the stick had length to spare. This calculator does that wall by wall, reusing offcuts across walls where they fit, then adds your waste allowance on top.

How do I cut coving mitre corners?

Every internal or external corner needs a 45 degree mitre cut on each of the two pieces that meet there, so a simple rectangular room with 4 corners needs 8 mitre cuts in total. Cut both pieces for a corner from the same mitre box setting so the angles match, and cut a test piece first if you are working with an expensive profile. See our guide to internal vs external mitre cuts below.

How much coving adhesive do I need?

Standard powder coving adhesive covers roughly 3 metres of coving per kilogram, so a 5kg bag covers about 15 metres. Ready-mixed grab adhesive in a cartridge covers less, typically 3-5 metres per tube. Buy slightly more than the calculator suggests, since mitre joints use more adhesive than a straight run.

Do I need planning permission or building regs for coving?

No. Coving is a decorative interior finish, not a structural element, so it does not require planning permission or building regulations approval. There is no British Standard that specifically governs coving installation the way there is for structural work.

What is the difference between coving and cornice?

Coving is a simple curved or splayed profile that covers the joint between wall and ceiling, usually under 100mm deep. Cornice is a larger, more ornate moulding, often with multiple decorative steps, and can be 100-200mm deep or more. The terms are sometimes used interchangeably by UK merchants, but cornice generally means bigger and more decorative.

Can I use corner blocks instead of mitring coving?

Yes, for polystyrene coving. Pre-formed internal and external corner blocks glue in place and let you butt plain, square-cut coving lengths straight into them, avoiding mitre cuts entirely. They are a popular DIY shortcut, though most plaster and duropolymer coving is still mitred for a neater finish. Tick "corner blocks" in the calculator above to see how many you would need instead of mitre cuts.