Calculate the litres (and US gallons) of paint needed for walls and ceiling — with automatic deductions for standard doors (about 2 m²) and windows (about 1.5 m²), and a coat multiplier for two-coat coverage.
What Your Results Mean
Measure the longest wall first for length, then the perpendicular wall for width, and the floor-to-ceiling height. For rooms with vaulted or sloped ceilings, use the average height for the wall measurement and calculate the ceiling separately if you're painting it. The calculator multiplies perimeter by height to get total wall area, then subtracts your door and window counts automatically.
The door and window deductions use industry-standard sizes: about 2 m² per interior door (a 0.9 × 2.0 m slab minus a small frame allowance; the classic 20 sq ft) and about 1.5 m² per window (roughly 0.9 × 1.5 m; 15 sq ft). If you have oversized features — sliding glass doors, bay windows, picture windows — count them as multiple standard windows or measure and subtract the area manually. French doors count as two standard doors.
Coat count matters more than most homeowners realize. A single coat of the same color refreshes an existing wall but rarely achieves full coverage. Two coats are required for any color change, for painting over primer, for deep or saturated colors (reds, yellows, dark blues), and for any premium paint job. The calculator defaults to two coats for this reason — one coat will leave streaks and uneven coverage that's visible in natural light.
Reading the Coverage Number
Premium interior paint covers about 9–10 m² per litre (350–400 sq ft per US gallon) on smooth, primed surfaces, and typical emulsions often rate 10–12 m² per litre. Builder-grade paint covers roughly 6–7 m² per litre (250–300 sq ft per US gallon). Coverage drops on rough surfaces (stucco, brick, textured drywall), on porous surfaces (new drywall without primer, bare wood), and on dark-to-light color changes. The calculator's coverage field opens at 8.6 m² per litre — the classic 350 sq ft per US gallon planning figure — so check your paint can's label and adjust the input if it differs.
Primer vs Paint
Primer is a different product with a different purpose. It seals porous surfaces, blocks stains, provides adhesion for topcoat paint, and creates a uniform base color. Use primer when painting over new drywall, repaired walls, dark existing colors, oil-based paint (with a bonding primer), water stains, or wood paneling. One coat of primer plus two coats of finish paint is the gold standard for any color change. Primer covers slightly more area than paint — about 10 m² per litre (400 sq ft per US gallon) — but buy it separately and don't try to use it as a paint substitute.
Pro Tip
Always buy paint from the same batch. Paint color varies subtly between manufacturing runs, even with the same formula. If you must buy multiple batches, mix them together in a 20 L (5 US gallon) bucket ("boxing") to ensure uniform color across the room.
The Math Behind the Calculator
Paint estimating follows a clear sequence: compute wall area, add ceiling if applicable, subtract openings, multiply by coats, and divide by coverage per litre. The engine then rounds up to whole US gallons — a habit from its gallon-can origins — and reports the litre equivalent, so metric shoppers should round to a real can size (2.5 L, 5 L, 10 L) instead:
wall_area = (2 × (length + width)) × height
ceiling_area = length × width (if included)
deductions = (doors × 1.86 m²) + (windows × 1.39 m²)
paintable_area = wall_area + ceiling_area − deductions
total_coverage_needed = paintable_area × coats
litres = total_coverage_needed ÷ coverage_per_litre
gallons = ceil(litres ÷ 3.785) — rounded up to whole US gallons
Standard Deductions
The 1.86 m² per door and 1.39 m² per window deductions come from typical interior dimensions (the classic 20 and 15 sq ft values, metricised):
- Interior door: 0.91 × 2.03 m ≈ 1.86 m² (20 sq ft), subtracting frame
- Standard window: 0.91 × 1.52 m ≈ 1.39 m² (15 sq ft)
- Closet door (double): 1.52 × 2.03 m ≈ 3.1 m² (33 sq ft) — count as 1.5 standard doors
- Sliding glass door: 1.83 × 2.03 m ≈ 3.7 m² (40 sq ft) — count as 2 standard doors
- Picture window: 1.52 × 1.22 m ≈ 1.9 m² (20 sq ft) — count as 1.3 standard windows
Why Litres and US Gallons?
The calculator reports litres alongside US gallons because paint in most of the world is sold in litre cans — 1 L, 2.5 L, 5 L and 10 L are the common sizes — while US paint comes in gallons (1 US gallon = 3.785 L). The conversion factor is exact, so the same paint job needs 3.785 litres or 1 US gallon — round up to the nearest can size you can actually buy.
Coverage Variations
Manufacturer coverage ratings assume ideal conditions: smooth surface, primed, same-color repaint, a 10 mm (3/8 in) nap roller, 21°C (70°F), 50% humidity. Real-world coverage is typically 10–15% lower due to surface absorption, roller waste, and cutting-in losses. For new construction or dark-to-light repaints, drop the coverage input to about 7.4 m² per litre (300 sq ft per US gallon) to be safe.
Assumptions & Limitations
Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.
- Coverage: 8.6 m² per litre per coat (the field default — typical premium latex, 350 sq ft per US gallon)
- Deductions: 1.86 m² per door, 1.39 m² per window (the classic 20 and 15 sq ft)
- Coats: your number multiplies the paint needed; primer counted separately
- Textured or porous walls can take 10–20% more paint
Worked Example: 4 × 3 m Bedroom with a 2.4 m Ceiling
A standard small bedroom: 4 metres by 3 metres, a 2.4 m ceiling, two doors, three windows, two coats of premium paint, ceiling not included, coverage left at the default 8.6 m² per litre. Let's walk through the math:
- Perimeter: 2 × (4 + 3) = 14 m.
- Wall area: 14 × 2.4 = 33.6 m².
- Ceiling area: 0 (not included in this example).
- Deductions: (2 × 1.86) + (3 × 1.39) ≈ 7.9 m² (the engine's 20 and 15 sq ft standards, metricised).
- Paintable area: 33.6 − 7.9 = 25.7 m².
- Two coats: 25.7 × 2 = 51.4 m² total coverage needed.
- Litres at 8.6 m²/L: 51.4 ÷ 8.6 ≈ 6.0 litres.
- US gallons: 6.0 ÷ 3.785 = 1.58 → the engine rounds up to 2 gallons = 7.6 litres reported.
- Order summary: a 5 L can plus a 2.5 L can (7.5 L), or a single 10 L can, of finish paint covers two coats on the walls, with wash-out margin.
If you also painted the ceiling (12 m²), the paintable area would jump to 37.7 m², total coverage to 75.4 m², and you'd need 11.4 L (3 US gallons) instead of 7.6 L (2 US gallons). The calculator updates the moment you toggle the ceiling checkbox.
For comparison, if you used paint covering only 6.7 m² per litre (275 sq ft per US gallon), you'd need 51.4 ÷ 6.7 = 7.7 litres — 2.03 US gallons, so the engine's round-up takes it to 11.4 L (3 US gallons) instead of 7.6 L (2 US gallons). Lower-coverage paint often needs a third coat on dark-to-light colour changes, pushing the total higher still. Coverage per litre is printed on the can: check it before comparing brands.
Reminder
The estimate excludes primer (add about 4 L / 1 US gallon if priming new drywall or making a drastic colour change), brushes, rollers, drop cloths, painter's tape, sandpaper and spackle. List these consumables before you start — they are the classic forgotten trip to the store.
