Paint Coverage Calculator Guide

How Much Paint Do I Need?

Buying the right amount of paint saves you money and avoids wasted trips to the merchant. One tin short means a half-finished room. Three tins too many means clutter in the garage and paint that goes off before you use it. Here's how to get the quantity right every time.

Step 1: Measure Your Walls

For each wall, measure:

Wall area (m²) = Width (m) × Height (m)

Do each wall separately and add them up. For a standard rectangular room, that's:

2 × (Length × Height) + 2 × (Width × Height)

Example: A 4m × 3m Room with 2.4m Ceilings

| Surface | Calculation | Area (m²) | |---|---|---| | Wall 1 (long) | 4.0 × 2.4 | 9.6 | | Wall 2 (short) | 3.0 × 2.4 | 7.2 | | Wall 3 (long) | 4.0 × 2.4 | 9.6 | | Wall 4 (short) | 3.0 × 2.4 | 7.2 | | Total walls | | 33.6 |

Subtract Doors and Windows

Deduct any areas you won't be painting:

| Opening | Typical Size | Area to Deduct | |---|---|---| | Standard internal door | 0.76 × 2.0m | 1.5 m² | | Standard window | 1.2 × 1.2m | 1.4 m² | | Patio/French doors | 1.8 × 2.1m | 3.8 m² |

For our example room with one door and one window:

33.6 - 1.5 - 1.4 = 30.7 m² of wall to paint

Step 2: Measure Your Ceiling

The ceiling area is simply:

Ceiling area (m²) = Room length (m) × Room width (m)

4.0 × 3.0 = 12.0 m²

Step 3: Check the Coverage Rate

Every tin of paint has a coverage rate printed on the label, usually quoted in m² per litre. Here are typical figures:

| Paint Type | Coverage (m² per litre) | Typical Use | |---|---|---| | Matt emulsion | 12–14 | Ceilings, living spaces | | Silk/satin emulsion | 12–14 | Kitchens, bathrooms, hallways | | Vinyl matt | 12–14 | General walls | | Masonry paint | 8–10 | External walls | | Primer/undercoat | 10–12 | New plaster, bare wood | | Gloss/satinwood (oil) | 14–16 | Woodwork, skirting, doors | | Gloss/satinwood (water) | 12–14 | Woodwork, skirting, doors |

Important: These coverage rates assume a smooth, sealed surface with a roller. Textured surfaces, bare plaster, and porous surfaces will soak up more paint and reduce coverage by 20–30%.

Step 4: Account for Number of Coats

| Situation | Coats Needed | |---|---| | Same colour over existing paint | 2 coats | | Light colour over light colour | 2 coats | | Dark colour over light colour | 2–3 coats | | Light colour over dark colour | 3 coats (or use a tinted primer first) | | New plaster (mist coat + finish) | 1 mist coat + 2 finish coats | | Bare plasterboard | 1 primer + 2 coats |

The Mist Coat

New plaster must have a mist coat first — emulsion thinned with water (typically 70% paint, 30% water). The Painting and Decorating Association recommends allowing plaster to dry for at least four weeks before applying a mist coat. This seals the plaster and gives the top coats something to grip. Without it, the paint peels off in sheets.

Apply the mist coat, let it dry overnight, then apply two full coats of your chosen emulsion.

Step 5: Calculate the Total Paint Needed

Paint needed (litres) = (Total area × Number of coats) ÷ Coverage rate

For our example room (walls and ceiling, matt emulsion, 2 coats):

Walls: (30.7 × 2) ÷ 13 = 4.7 litres

Ceiling: (12.0 × 2) ÷ 13 = 1.8 litres

Total: 6.5 litres

Standard Tin Sizes

| Tin Size | Best For | |---|---| | 1 litre | Woodwork, test pots for large areas | | 2.5 litres | Single room ceilings, small rooms | | 5 litres | Standard rooms (most popular size) | | 10 litres | Multiple rooms, whole houses |

For our example, you'd buy a 5-litre tin for the walls and a 2.5-litre tin for the ceiling — or a single 10-litre tin if you're doing the same colour throughout.

The Paint Calculator does all this arithmetic for you — enter the room dimensions, number of doors and windows, and the number of coats, and it tells you exactly how many tins to buy.

Coverage Killers: When You'll Need More

The manufacturer's coverage rate assumes ideal conditions. In practice, you'll use more paint when:

• Painting over filler patches. Filler is more absite than the surrounding surface. Spot-prime filled areas with a thin coat first.
• Textured surfaces. Artex, woodchip, or heavy textures can double your paint consumption. The peaks absorb paint and the valleys need filling.
• Using deep or bright colours. Reds, oranges, dark blues, and bright yellows have lower opacity. Budget for an extra coat — or use a grey-tinted primer underneath deep colours to build coverage faster.
• Spraying vs rolling. Airless sprayers are faster but use 20–30% more paint due to overspray and atomisation losses.
• Windy or hot conditions (external). Paint dries too fast and doesn't spread as far. External coverage rates are always lower than internal.

Tips for Buying Paint

• Buy all your paint at once from the same batch. Batch-to-batch colour differences are real, especially with tinted colours. If you run out and buy another tin later, it might not match.
• Keep the tin label. Note the colour code and batch number. If you need to touch up later, this is essential for getting a match.
• Don't buy cheap trade paint for feature walls. Budget matt is fine for ceilings and landlord whites, but feature colours need better-quality paint with higher pigment loading. Brands like Dulux Trade and Crown Trade publish detailed coverage data for each product.
• Check the shelf life. Water-based emulsion lasts 2–3 years unopened. Once opened, use within 12 months and store with the lid sealed tight in a frost-free spot.

Estimating for a Whole House

For a quick whole-house estimate before doing room-by-room calculations, use these rough figures:

| House Size | Approximate Wall + Ceiling Area | Paint Needed (2 coats) | |---|---|---| | 1-bed flat | 80–100 m² | 15–18 litres | | 2-bed terrace | 120–160 m² | 20–25 litres | | 3-bed semi | 180–220 m² | 28–35 litres | | 4-bed detached | 250–320 m² | 40–50 litres |

These figures are for walls and ceilings only. Woodwork (doors, skirting, architraves, window frames) adds another 15–25% on top.

If you're decorating over fresh plaster, you'll also want to check the plaster has had time to cure. The Plaster & Render Calculator can help with plastering quantities if you're doing the full job from scratch.

Common Mistakes

• Skipping the mist coat on new plaster. The top coat will peel. Every time.
• Not stirring the paint. Pigment settles in the tin. Give it a thorough stir before each use, especially with coloured paints.
• Overloading the roller. More paint per pass doesn't mean fewer coats. It means drips, roller marks, and an uneven finish. Two thin coats always beats one thick one.
• Painting in direct sunlight (external). The paint dries before it flows out, leaving brush marks and lap lines. Work in the shade or paint early morning/late afternoon.
• Forgetting the woodwork. Doors, frames, skirting boards, and window frames all need painting too. Budget separately for gloss or satinwood.

Quick Summary

1. Measure each wall (width × height) and the ceiling (length × width)
2. Subtract doors and windows
3. Choose your paint and check the coverage rate on the tin
4. Multiply total area by number of coats, then divide by coverage rate
5. Round up to the nearest standard tin size
6. Use the Paint Calculator to get exact tin quantities instantly