Paint Calculator
Calculate how much paint you need for a room based on wall dimensions, number of coats, and standard tin sizes (1L, 2.5L, 5L).
Related tools
What Is a Paint Calculator?
Buying the right amount of paint saves money and prevents waste. Too little and you make extra trips to the store mid-project. Too much and you are left with tins taking up space in the garage. This calculator uses your room dimensions, door and window areas, and the paint's coverage rate to tell you exactly how many litres to buy, rounded up to standard tin sizes.
Paint coverage varies by product type and surface condition. Emulsion for interior walls typically covers 10-14 square metres per litre on smooth surfaces. A porous or textured wall may reduce coverage to 8-10 m²/L. Always check the coverage rate printed on the tin, as it varies between brands and finishes. Most walls require two coats for even colour, especially when changing from a dark to a light shade.
The calculator accounts for areas you will not paint, including doors and windows. Standard interior door dimensions are approximately 1.98m × 0.76m (1.5 m²) and standard window areas range from 1.0 to 1.5 m² depending on style. Enter the number of each and the calculator subtracts these from the total wall area. For ceilings, run a separate calculation using the floor area as the paintable surface.
How Do You Use This Paint Calculator?
Enter the room dimensions (length, width, height), number of doors and windows, the number of coats required, and the paint coverage rate. Click Calculate to see the total paint needed and the recommended tin sizes to buy.
- Enter the room length and width in metres (or feet).
- Enter the ceiling height — standard is 2.4m (8 ft) in most countries.
- Enter the number of doors and windows to subtract from the wall area.
- Select the number of coats required (typically 2 for a colour change, 1 for a refresh).
- Enter the paint coverage rate from the tin label (default: 12 m²/L).
- Review the total litres needed and the recommended combination of standard tin sizes.
How Does the Paint Calculator Formula Work?
The formula used: Paint Needed (litres) = (Total Wall Area − Unpainted Areas) × Number of Coats / Coverage Rate (m² per litre)
The paint calculation finds the total paintable wall area, subtracts openings, multiplies by the number of coats, and divides by the coverage rate.
Total Wall Area = 2 × (Length + Width) × Height
Paintable Area = Total Wall Area − (Doors × 1.5m²) − (Windows × 1.2m²)
Paint Needed = (Paintable Area × Number of Coats) ÷ Coverage Rate (m²/L)
Standard tin sizes are 1L, 2.5L, and 5L. The calculator recommends the most cost-effective combination that meets or slightly exceeds the required volume. Leftover paint is useful for touch-ups.
What Are Some Example Calculations?
Room 5m × 4m × 2.4m high, 1 door (1.9m²) and 2 windows (1.5m² each). Wall area = 43.2m². Minus openings: 43.2 − 4.9 = 38.3m². Two coats at 12m²/L: 38.3 × 2 / 12 = 6.38L. Buy: 1 × 5L + 1 × 2.5L tin.
A bedroom 4m × 3.5m × 2.4m with 1 door and 1 window, painting 2 coats with paint covering 12 m²/L
Wall area: 2 × (4 + 3.5) × 2.4 = 36.0 m². Subtract: 1 door (1.5 m²) + 1 window (1.2 m²) = 2.7 m². Paintable: 33.3 m². Paint = 33.3 × 2 ÷ 12 = 5.55 L
Buy 1 × 5L tin + 1 × 1L tin (6L total, with 0.45L spare for touch-ups).
A living room 6m × 5m × 2.7m with 2 doors and 3 windows, 2 coats at 10 m²/L (textured walls)
Wall area: 2 × (6 + 5) × 2.7 = 59.4 m². Subtract: 2 doors (3.0 m²) + 3 windows (3.6 m²) = 6.6 m². Paintable: 52.8 m². Paint = 52.8 × 2 ÷ 10 = 10.56 L
Buy 2 × 5L tins + 1 × 1L tin (11L total, with 0.44L spare).
A small bathroom 2.5m × 2m × 2.4m with 1 door and 1 small window, 2 coats at 12 m²/L
Wall area: 2 × (2.5 + 2) × 2.4 = 21.6 m². Subtract: 1 door (1.5 m²) + 1 window (0.8 m²) = 2.3 m². Paintable: 19.3 m². Paint = 19.3 × 2 ÷ 12 = 3.22 L
Buy 1 × 2.5L tin + 1 × 1L tin (3.5L total, with 0.28L spare for touch-ups).
When Should You Use a Paint Calculator?
Use this calculator before every painting project, whether you are refreshing a single room or redecorating an entire house. Run the calculation for each room separately, especially if you are using different colours. Knowing the exact quantity prevents the frustrating situation of running out of paint with one wall left to finish.
The calculator is equally useful for estimating project costs. Multiply the number of tins by the price per tin at your local supplier to get a materials budget. When comparing quotes from professional painters, you can verify that the paint quantities they have specified are reasonable for your room dimensions rather than inflated.
What Do These Terms Mean?
What Are the Best Tips to Know?
- Always buy slightly more than the calculated amount — keep a small tin for future touch-ups and repairs.
- Reduce the coverage rate by 20% for the first coat on new plasterboard or bare plaster, which absorbs more paint.
- Dark colours covering light walls often need 3 coats instead of 2 for full opacity.
- Measure each wall individually if the room is L-shaped or has alcoves rather than using the simple length × width formula.
- Store leftover paint upside down (to create an airtight seal) in a cool, dry place for touch-ups lasting up to 10 years.
What Mistakes Should You Avoid?
- Forgetting to subtract door and window areas, which leads to overbuying by 10-15%.
- Using the one-coat coverage rate when planning to apply two coats — you need to double the paint volume.
- Not accounting for surface porosity: bare plaster, new drywall, and textured surfaces absorb significantly more paint than sealed surfaces.
- Buying exactly the calculated amount with no margin for spills, roller absorption, and touch-ups.
Frequently Asked Questions
How many square metres does 1 litre of paint cover?
Standard interior emulsion paint covers approximately 10-14 m² per litre per coat on smooth, sealed surfaces. The exact coverage depends on the paint brand, finish type, surface texture, and application method. Always check the coverage figure printed on the tin.
Do I need to include the ceiling in the wall area calculation?
No. The wall area formula calculates only the four walls. For the ceiling, run a separate calculation using the floor dimensions (length × width) as the paintable area. Ceilings typically need the same number of coats as walls.
How many coats of paint do I need?
Two coats are standard for most colour changes. One coat may suffice for a same-colour refresh. Three coats may be needed when covering dark colours with light ones, when using bold or deep colours, or on very porous surfaces.
What standard tin sizes should I buy?
Paint is commonly sold in 1L, 2.5L, and 5L tins. Larger tins offer better value per litre. The calculator recommends the most efficient combination. Tester pots (usually 250 ml) are also available for colour sampling but are not cost-effective for full rooms.
Should I buy extra paint for touch-ups?
Yes. Keep at least 250-500 ml of each colour for future touch-ups from scuffs, nail holes, and furniture marks. Label the tin with the room name and date. Properly stored paint lasts 5-10 years.
How do I calculate paint for an irregularly shaped room?
Measure each wall section individually: multiply each wall's length by its height, then add up all the sections. Subtract doors and windows as usual. This approach works for L-shaped rooms, rooms with alcoves, and angled walls.
Does the type of roller or brush affect how much paint I need?
Slightly. Thick-nap rollers (for textured surfaces) hold more paint but also leave a thicker coat, which can increase consumption by 10-15%. Brushes use less paint per area but are much slower. Spray application typically uses 20-30% more paint due to overspray.
More Everyday Calculators
Category
← Everyday CalculatorsNext up
Percentage Calculator →