How Much Paint Do I Need? A Step-by-Step Calculation
Running out halfway along a wall is as frustrating as being left with three half-used tins. With a tape measure, a few multiplications and realistic coverage figures you can buy the right amount first time.
Paint quantities are one of those small calculations that cause large irritations. Buy too little and you end up making a second trip, possibly to find a different batch whose colour does not quite match. Buy too much and you pay for tins that sit in a shed until they go off. The good news is that the maths is simple. What trips most people up is not the multiplication but the assumptions: an optimistic coverage rate, forgetting the second coat, or ignoring how thirsty a bare surface can be.
This guide walks through the calculation step by step, gives realistic coverage figures for different surfaces, and finishes with a worked example for a typical room. If you would rather skip the arithmetic, the paint calculator does it for you, but it helps to understand what it is doing.
What you need before you start
- A tape measure (a laser measure is quicker for ceiling heights, but not essential).
- A notepad or phone to record each wall separately.
- The technical details of the paint you plan to use, especially its stated coverage per litre.
- Knowledge of the surface: smooth and previously painted, new plaster, textured, bare wood or a big colour change.
Measure in metres to two decimal places. Rounding up a little at each step is fine; rounding down is how shortages happen.
Step 1: Measure the wall area
For a simple rectangular room, add the lengths of all walls to get the perimeter, then multiply by the ceiling height. A room of 4.0 m × 3.5 m has a perimeter of 15.0 m; with a 2.5 m ceiling, the gross wall area is 15.0 × 2.5 = 37.5 m² (about 404 sq ft).
For rooms with chimney breasts, alcoves or sloping ceilings, measure each wall section as its own rectangle and add them up. For a gable or sloped wall, treat the triangular part as half of a rectangle: width × height ÷ 2.
Step 2: Deduct doors and windows
Subtract areas you will not paint with wall paint. Measure each opening, or use typical sizes as a guide:
| Opening | Typical size | Area to deduct |
|---|---|---|
| Standard internal door | 0.76–0.84 m × 1.98–2.04 m (30–33 × 78–80 in) | About 1.6 m² (17 sq ft) |
| Small window | 0.9 × 0.9 m (35 × 35 in) | About 0.8 m² (9 sq ft) |
| Medium window | 1.2 × 1.2 m (47 × 47 in) | About 1.4 m² (15 sq ft) |
| Patio or French doors | 1.5–1.8 × 2.1 m (59–71 × 83 in) | About 3.2–3.8 m² (34–41 sq ft) |
| Fitted wardrobe or kitchen wall units | Measure | Full covered area |
Do not bother deducting very small items such as sockets and switches. The paint "saved" is negligible, and the slight overestimate gives you a useful buffer.
Step 3: Multiply by the number of coats
Two coats is the standard for walls. The first coat seals and evens out the surface; the second gives full colour, an even sheen and a tougher film. Plan for:
- Two coats for most repaints, including a moderate colour change.
- Three coats, or a primer plus two, when painting light over dark, using deep or very bright colours, or covering patchy repairs.
- Mist coat plus two on new plaster. The mist coat is diluted emulsion that soaks into the plaster to seal it.
- One coat only when refreshing the same colour with a high-opacity paint over a sound, clean surface, and you accept that it may need a second in places.
Step 4: Divide by a realistic coverage rate
Every tin states a coverage rate, typically in square metres per litre per coat. These figures are measured under good conditions on smooth, sealed surfaces. In homes, rollers absorb paint, surfaces vary and some paint is lost in trays, so it is wise to use a slightly lower figure than the tin suggests.
| Surface or product | Planning coverage per litre per coat | Notes |
|---|---|---|
| Smooth, previously painted walls (emulsion) | 10–12 m² (about 400–490 sq ft per US gallon) | The most common case |
| New plaster, mist coat | 6–9 m² | Plaster is absorbent; diluted paint goes further per litre but soaks in |
| Lightly textured walls (woodchip, orange-peel) | 8–10 m² | Texture increases the real surface area |
| Heavy texture, rough render, bare brick | 5–8 m² | Use a long-pile roller; expect high absorption |
| Ceiling matt | 10–12 m² | Similar to walls; allow extra if stained or patchy |
| Woodwork (eggshell, satin, gloss) | 12–16 m² | Check the tin; thinner, more even coats are typical |
| Primers and undercoats | 8–12 m² | Varies widely by product |
The formula is then: litres needed = (net area × coats) ÷ coverage per litre.
Step 5: Add a buffer and choose tin sizes
Add around 10% for waste and touch-ups. Then round up to the tin sizes available, commonly 1 litre, 2.5 litres, 5 litres and 10 litres for wall emulsion, and 750 ml or 2.5 litres for woodwork. A 5 litre tin is usually cheaper per litre than two 2.5 litre tins, but a part-used 10 litre tin may go to waste if you only need six. Buy every tin of the same colour at the same time, and if they come from different batches, mix them together in a larger bucket ("boxing" the paint) so any small variation is spread evenly.
Worked example: a bedroom of 4.0 × 3.5 m
The room has a ceiling height of 2.5 m, one internal door and one window measuring 1.2 × 1.2 m. The walls are smooth and previously painted, and they are changing from a pale grey to a soft white. The ceiling and skirting are being refreshed at the same time.
Walls
- Perimeter: 4.0 + 3.5 + 4.0 + 3.5 = 15.0 m.
- Gross wall area: 15.0 × 2.5 = 37.5 m².
- Deductions: door 1.6 m² + window 1.44 m² = 3.04 m². Net area: 37.5 − 3.04 ≈ 34.5 m².
- Two coats: 34.5 × 2 = 69 m² of coverage.
- At 11 m² per litre: 69 ÷ 11 ≈ 6.3 litres.
- Add 10%: about 6.9 litres. Buy one 5 litre and one 2.5 litre tin (7.5 litres).
Ceiling
- Area: 4.0 × 3.5 = 14 m².
- Two coats: 28 m².
- At 11 m² per litre: about 2.5 litres. Add 10% and you need about 2.8 litres, so a 2.5 litre tin is tight; a 5 litre tin is safer if the ceiling is stained, otherwise buy 2.5 litres plus a 1 litre tin.
Skirting and door
- Skirting length: perimeter minus door width, about 15.0 − 0.8 = 14.2 m. At a height of 0.15 m (6 in), that is about 2.1 m².
- Door (both faces) plus frame and architrave: allow roughly 4.5 m².
- Total woodwork: about 6.6 m². Two coats: 13.2 m². At 14 m² per litre: just under 1 litre. A 750 ml tin may run short; a 1 litre or 2.5 litre tin is the practical choice, and the surplus covers the window board too.
Total shopping list: 7.5 litres of wall paint, 2.5–5 litres of ceiling paint, about 1 litre of woodwork paint, plus any primer or undercoat needed for bare wood or repairs.
Situations that change the numbers
Big colour changes
Covering a strong colour with a pale one is the most common reason for an unexpected third coat. A primer tinted towards the new colour reduces the number of topcoats and is usually cheaper than an extra coat of premium paint.
Feature walls
Calculate them separately. A single wall in a deep shade may need three coats and a different tin size from the rest of the room. Remember to deduct the feature wall from the main wall total.
Stairwells and tall spaces
Measure the height at the tallest point and split angled sections into triangles and rectangles. Stairwells often turn out larger than expected because the walls run up two storeys.
Absorbent or repaired surfaces
Heavily filled walls absorb paint unevenly, so the filled areas can look duller (an effect called flashing). Spot-priming repairs or adding a full first coat evens this out. Factor it into your quantities.
Choice of finish
Coverage can vary by finish and product line. Durable and premium paints often have higher opacity, which can save a coat, while cheaper "contract" matts may need more. The guide to paint finishes explains how sheen affects where each product should go.
Calculating in feet and gallons
If your tape measure or paint tins use imperial units, the method is the same. Multiply wall widths by height in feet to get square feet, deduct openings, multiply by coats, and divide by the coverage per gallon on the tin. For planning, allow roughly 400–490 sq ft per US gallon per coat, the equivalent of 10–12 m² per litre. To convert, 1 m² is about 10.8 sq ft, and 1 litre is about 0.26 US gallons. Avoid mixing units midway through the calculation, which is a common source of errors.
Other surfaces worth planning for
- Radiators: use a heat-resistant radiator paint or a suitable primer and topcoat, and allow roughly twice the visible face area because of the fins and edges.
- Kitchen cabinets: measure each door and drawer front on both sides, plus the visible carcass edges. Cabinet paints often need a specific primer on laminate or lacquered surfaces.
- Panelling and mouldings: profiled surfaces have more area than their flat dimensions suggest. Add 10–20% to the measured area.
- Floors: floor paints have their own coverage rates and usually need a primer on bare wood or concrete. Calculate from floor area.
Storing leftover paint
Seal tins tightly by tapping the lid down evenly with a mallet, wiping the rim first. Store them upright, indoors, away from frost and direct heat; water-based paint that has frozen often becomes unusable. Label each tin with the room, colour name, code, finish and date. Small amounts can be decanted into a clean, airtight jar so less air reaches the paint. Dispose of unwanted paint according to your local authority's guidance rather than pouring it down drains.
Common mistakes
- Using the tin's best-case coverage. Assume 10–12 m² per litre for walls unless you have used the product before.
- Forgetting the second coat. This alone halves the real coverage of a tin.
- Not deducting large openings, or deducting far too much for small ones. Deduct doors, windows and fitted units; ignore sockets.
- Measuring the floor, not the walls. Floor area is only useful for ceilings.
- Ignoring texture and porosity. New plaster, woodchip and brick drink paint.
- Buying in batches over time. Colours can vary slightly between batches. Buy all at once or box the paint.
- Throwing away the label details. Note the colour name, code, finish and batch for future touch-ups.
Quick reference checklist
- Measure each wall's width and the ceiling height; multiply and add together.
- Subtract doors, windows and fitted units.
- Multiply by the number of coats (normally two; more for strong colour changes).
- Divide by a realistic coverage rate for the surface (10–12 m² per litre for smooth walls).
- Add around 10% and round up to available tin sizes.
- Calculate ceilings, woodwork, primers and feature walls separately.
- Buy all tins of each colour at the same time and keep a little back, sealed and labelled.
With this approach you will rarely be more than a litre out, and a litre spare is exactly what you want for the inevitable scuff six months later. If you are planning more than a paint job, our step-by-step room makeover plan shows where decorating fits in the overall sequence of work.
Frequently asked questions
How many square metres does a litre of paint cover?
Most interior emulsions state around 10–14 m² per litre per coat on smooth, primed walls, which is roughly 400–570 sq ft per US gallon. In real conditions, with rollers, absorbent surfaces and some waste, planning on 10–12 m² per litre is a safer figure.
Do I always need two coats?
Two coats are the norm for an even, durable finish, especially with a colour change. One coat can be enough when repainting the same colour over a sound surface with a high-opacity paint, but uneven sheen and thin patches are common, so budget for two.
How much paint do I need for a 4 × 4 metre room?
With 2.4 m ceilings, the walls total about 38 m² before deductions. After removing a door and a window (around 3 m²) and applying two coats, you need about 70 m² of coverage, or roughly 6–7 litres at 10–12 m² per litre. A 5 litre and a 2.5 litre tin would cover it with a little to spare.
Should I buy extra paint for touch-ups?
Yes, keep a small amount back. Adding around 10% to your total covers waste and leaves enough for touch-ups. Store it sealed, labelled with the room, colour, finish and batch number, and protect it from frost.
Does dark paint need more coats?
Deep colours often need an extra coat, or a tinted primer, to reach full depth and evenness. Painting a light colour over a dark one also usually needs a primer or an additional coat, so plan for three coats in either case unless the manufacturer states otherwise.
Measurements and rules of thumb in this guide are starting points; always check your own room, local building rules and manufacturer instructions. Images are illustrative, from Unsplash. Read how we research and update our guides in our Editorial Policy, and tell us if you spot an error.
