How much paint do you need for a wall or room? Enter your measurements to calculate paint and optional primer in litres and gallons, with coats, openings, extra allowance and whole cans included.
How to use itThe calculationCommon questions
How to use the paint calculator
- Choose a rectangular room or individual surfaces. Enter the measurements in feet, inches, centimetres or metres. In room mode, ceiling area is optional.
- Add the doors, windows and other areas you will not paint. Use their actual sizes and quantities.
- Enter your paint's coverage per coat and the number of coats. Set an extra allowance and the size of the can you can buy.
- Turn on primer if needed. Enter its coverage, coats, share of the area and can size. Select Calculate paint to see separate quantities and can counts.
Decimal points and commas both work. Enter 8 ft 6 in as 8.5 ft or 102 in. Changing units converts existing values. Changing a setting hides the previous result until you calculate again.
How much paint do I need?
Start with the area you will actually paint. For one rectangular wall, multiply width by height. For all four walls of a rectangular room, use 2 × (room length + room width) × wall height. A flat ceiling is room length × room width. Subtract measured openings that will remain unpainted.
Paint needed = paintable area × number of coats ÷ coverage per coat. Then multiply by 1 + your extra allowance divided by 100. Finally, divide by the selected can size and round up to a whole number of cans.
Use matching units. With area in m² and coverage in m² per litre, the result is litres. With area in square feet and coverage in square feet per US gallon, the result is US gallons. The calculator handles these conversions.
A worked example with paint and primer
A room is 4 m long, 3 m wide and 2.5 m high. You are painting its four walls, without the ceiling. One door is 0.9 × 2 m and one window is 1.2 × 1.5 m.
- The four walls total 35 m². The door and window total 3.6 m².
- The paintable area is 31.4 m².
- Two coats of paint at 10 m² per litre need 6.28 L before extra allowance.
- With a 10% allowance, plan for 6.908 L of paint. Using only 2.5 L cans, buy three cans, totalling 7.5 L.
- If all the area needs one coat of primer at 8 m² per litre, the primer estimate is 4.3175 L including the same 10% allowance. Two 2.5 L cans provide 5 L.
Load example fills in these values. Displayed volumes are rounded for readability, but can counts use the unrounded calculation. The example's coverage values illustrate the method and are not a specification for every product.
Use the coverage stated on your paint
Coverage is the area a stated volume can cover for one coat under the product's specified conditions. A larger m²/L or sq ft/gallon value means less liquid is needed. If your label gives consumption in L/m² instead, use its reciprocal for m²/L. For example, 0.1 L/m² equals 10 m²/L. Do not enter a value in grams per square metre without a product-specific conversion.
Sherwin-Williams gives a general paint estimate of about 350 to 400 sq ft per US gallon. That is about 8.6 to 9.8 m²/L. Product values vary, so the calculator starts at 350 sq ft per US gallon and lets you replace it. Its primer starting point is 250 sq ft per US gallon, about 6.1 m²/L, within that manufacturer's general primer range.
The label and surface condition matter more than a general average. Textured or porous surfaces, application method and film thickness can change the amount needed. Do not spread paint beyond the stated coverage just to make a can last. For a stated range, the lower coverage value gives the larger quantity estimate.
Choose coats and extra allowance deliberately
The starting setting is two paint coats. It is editable because some products use a different coating system. A colour change, repaired areas or an uneven surface may need additional preparation or coats. Use the coating manufacturer's instructions, and check the result on a test area.
The 10% extra is an editable planning allowance, not a rule for every job. Set it to zero if you only want the theoretical amount. If you already lowered the coverage to account for the same application losses, consider that before adding another allowance. Spray losses can be different from brushing or rolling; use coverage and allowances appropriate to your method.
When should I calculate primer?
New or absorbent surfaces, repairs, stains and adhesion requirements can call for a suitable primer. A paint described as paint and primer does not replace every specialist primer. Follow the preparation and coating system for your chosen products.
The primer calculation is separate from the finish paint. Set 100% to prime the whole net area or a smaller percentage for spot priming. For example, 25% of a 40 m² paintable area is 10 m² of primer area. Primer uses its own coverage and coat count. It does not replace one of the paint coats entered above.
Common questions
Should I subtract doors and windows?
Subtract them if you want to calculate the actual unpainted area. Use the opening size within the wall and avoid overlapping deductions. Keeping small openings in the wall area adds an implicit reserve. If you do that, remember that the extra allowance adds another reserve. Frames, reveals and doors need their own measured areas and may use a different product.
Is a 10 × 10 room the same as 100 square feet of walls?
No. A 10 × 10 ft room has 100 sq ft of floor area. With 8 ft high walls, its four walls total 320 sq ft before openings. Adding its flat ceiling adds another 100 sq ft.
Does the calculator include the ceiling?
Only when you select it in room mode or add it as an individual surface. If the ceiling uses a different paint, colour, coverage or coat count, calculate it separately. The room mode assumes four rectangular walls of the same height and a flat ceiling.
What if the room has a sloped ceiling or an irregular wall?
Use the individual surfaces option for rectangular sections. This tool does not automatically work out triangles, curved walls or stepped ceiling shapes. Measure those areas separately before making a purchasing plan. Do not use the bounding rectangle as an exact area unless you deliberately want that extra allowance.
Are US gallons and Imperial gallons the same?
No. One US liquid gallon is 3.785411784 L. One Imperial gallon is 4.54609 L. The coverage and can size controls distinguish them, and the result labels both. Check which unit your product uses.
Can I mix small and large cans to reduce leftovers?
The can count uses one can size per product. It does not compare different pack combinations or prices. Use the liquid estimate to compare available packs, keeping the combined volume at least as large as the estimate. Different colours and finishes must be ordered separately.
Does the result include water added for dilution?
No. Treat the result as the amount of coating to buy, using coverage specified for that product. Dilution instructions and the basis of the stated coverage vary. Do not subtract added water from the paint estimate without checking the product's instructions. This tool does not calculate mist-coat mixtures.
Can I apply wall stickers after painting?
Let the coating cure fully according to its instructions, then check the sticker's surface requirements. Dry to the touch does not necessarily mean ready for adhesive. Test adhesion on a small area before decorating the whole wall.
Plan your room update
Considering another wall finish? Use our wallpaper calculator. For glass, try the window film calculator. For wall details, explore wall stickers after checking that your painted surface is suitable.
Coverage references
See Sherwin-Williams' paint and primer coverage guidance, Dulux's product-based paint calculator and Benjamin Moore's painting questions. The formula estimates quantities from your inputs. Your product label determines coverage, preparation and the required coating system.
