Guide 02 · Walls
Paint & Wallpaper Calculator
Two tools for the walls: litres and tins of paint from net area, coats and coverage; and rolls of wallpaper from drops, roll size and pattern repeat.
How paint quantity is worked out
The formula is simple: net wall area multiplied by the number of coats, divided by the coverage rate. Net area is the total wall surface minus doors, windows and anything you are not painting, such as a tiled splashback. Coverage is the area one litre covers in one coat, and it is printed on the tin. Wall emulsions commonly quote somewhere around 10 to 14 m² per litre on smooth, prepared surfaces, but always use the manufacturer's figure for the exact product.
Real walls rarely match the ideal conditions behind that figure. Rough plaster, textured paper and bare masonry absorb and hold more paint. Rollers with a longer pile also load more paint onto the surface. If your walls are anything other than smooth and sealed, reduce the coverage figure in the calculator by a litre or two per square metre.
How many coats do you need?
- Refreshing the same colour: one coat can be enough if the existing finish is sound; two gives a more even result.
- Significant colour change: plan for at least two coats. Going from dark to light may need a third, or a tinted base coat.
- New plaster: it is porous and usually needs a diluted "mist coat" first, then two full coats. Follow the paint maker's guidance on dilution.
- Stains and repairs: water marks and filler patches may need a stain-blocking primer so they do not show through.
Choosing a finish
| Finish | Sheen | Pros | Cons |
|---|---|---|---|
| Matt / flat | Very low | Hides surface flaws, rich colour | Standard matt marks easily; look for "durable" versions in busy areas |
| Eggshell | Low | Wipeable, subtle glow, good on wood and walls | Shows more imperfections than matt |
| Silk | Medium | Easy to clean, reflects light | Highlights bumps and roller marks |
| Satin | Medium–high | Hard-wearing on doors and trim | Needs good preparation to look smooth |
| Gloss | High | Toughest, very easy to wipe | Unforgiving of poor surfaces; strong reflections |
How wallpaper rolls are calculated
Wallpaper is hung in vertical strips called drops. The number of drops is the room perimeter divided by the roll width, rounded up. Each drop needs the wall height plus a trimming allowance, plus one pattern repeat so the design lines up with its neighbour. Divide the roll length by that drop length, round down, and you know how many full drops each roll yields. Rolls needed equals drops divided by drops per roll, rounded up.
Many rolls sold in the UK and Europe measure about 10.05 m by 0.53 m, which is why the calculator starts there, but wide-width and non-standard rolls exist, so check the label. Designs with a large repeat can waste most of a drop's worth of paper per roll. Drop-match patterns — where alternate drops are offset by half a repeat — usually need extra paper compared with straight-match designs.
Wallpaper types compared
Non-woven
Often "paste the wall", so no soaking table is needed. It is dimensionally stable, which helps seams stay tight, and it usually strips off dry in whole sheets. On the downside, it tends to cost more than basic paper.
Vinyl
A washable, moisture-tolerant surface suits kitchens and hallways. Its heavier weight calls for a suitable adhesive, and it is less breathable than paper.
Traditional paper
Wide choice of printed designs, and usually economical. The trade-off: it can tear when wet, stretches as it soaks, and is not wipeable.
Lining paper
Smooths imperfect walls before painting or papering. It is an extra step and extra material, so budget time and rolls for it.
Batch numbers and spares
Both paint and wallpaper vary slightly between production runs. Buy everything for one room in a single order and check that wallpaper rolls share the same batch number. Keeping a spare roll or a small pot of paint makes future repairs invisible — a good reason not to trim your order too tightly.