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Paint Calculator
Estimates wall paint from the wall area (room length, width and height, or a given area), subtracting 1.9 m² per door and 1.4 m² per window, multiplying by the number of coats and dividing by the coverage rate; reports litres, US gallons and cans.
When to use
You need to know how much paint to buy for the walls of a room or for a known wall area, in litres or gallons and cans.
Do not use when: You need ceiling, floor or exterior siding paint with different coverage rates (enter the area and the product's coverage directly), or you need the wall area only (use rectangle).
Formula
gross = 2 × (room_length + room_width) × wall_height (or wall_area); paintable = gross − 1.9 m² × doors − 1.4 m² × windows; litres = paintable_m² × coats / coverage_m²_per_L; gallons = litres / 3.785; cans = ceil(litres / can_size). Imperial coverage: ft²/gal × 0.09290304 / 3.785411784 = m²/L.
The door (1.9 m² ≈ 0.9 × 2.1 m) and window (1.4 m²) deductions are conventional averages; real coverage depends on surface texture, porosity, colour change and application method, so read the coverage figure from the product label.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
unit_system | enum: metric | imperial | default metric | Unit system of the length inputs (metric: m / cm / mm; imperial: ft / in). Outputs are reported in both systems. | |
room_length | number | m or ft | no | Room length (m or ft). Used with room_width and wall_height when wall_area is not given. Range: > 0, ≤ 1000 |
room_width | number | m or ft | no | Room width (m or ft). Range: > 0, ≤ 1000 |
wall_height | number | m or ft | no | Floor-to-ceiling height (m or ft). Range: > 0, ≤ 100 |
wall_area | number | m² or ft² | no | Gross wall area before openings (m² or ft²). When given, the room dimensions are ignored. Range: > 0 |
doors | integer | default 1 | Number of doors; 1.9 m² (20.5 ft²) is subtracted per door. Range: ≥ 0, ≤ 100 | |
windows | integer | default 1 | Number of windows; 1.4 m² (15.1 ft²) is subtracted per window. Range: ≥ 0, ≤ 100 | |
coats | integer | default 2 | Number of coats. Two coats are standard for colour changes; primer is not included. Range: ≥ 1, ≤ 5 | |
coverage | number | no | Coverage from the tin: m² per litre in metric (default 10) or ft² per US gallon in imperial (default 350). Typical emulsion covers 10–14 m²/L (350–400 ft²/gal). Range: > 0 | |
can_size | number | no | Litres per can (metric, default 5) or US gallons per can (imperial, default 1); used for the can count. Range: > 0 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
gross_wall_area_m2 | number | m² | 2 × (length + width) × height, or wall_area. |
gross_wall_area_ft2 | number | ft² | Gross wall area in square feet. |
paintable_area_m2 | number | m² | Gross area minus 1.9 m² per door and 1.4 m² per window. |
paintable_area_ft2 | number | ft² | Paintable area in square feet. |
paint_liters | number | L | paintable_area × coats / coverage, in litres. |
paint_gallons | number | US gal | Same amount in US gallons (1 gal = 3.785 L). |
cans_needed | integer | ceil(paint needed / can_size). | |
can_size_used | string | Can size assumed for the can count. |
Example
5 m × 4 m × 2.4 m, 1 door, 2 windows, 2 coats, 10 m²/L, 5 L cans: {"unit_system":"metric","room_length":5,"room_width":4,"wall_height":2.4,"doors":1,"windows":2,"coats":2,"coverage":10,"can_size":5} → {"gross_wall_area_m2":43.2,"gross_wall_area_ft2":465,"paintable_area_m2":38.5,"paintable_area_ft2":414.41,"paint_liters":7.7,"paint_gallons":2.03,"cans_needed":2,"can_size_used":"5 L"}
15 ft × 12 ft × 8 ft, 2 doors, 2 windows, 2 coats, 350 ft²/gal, 1 gal cans: {"unit_system":"imperial","room_length":15,"room_width":12,"wall_height":8,"doors":2,"windows":2,"coats":2} → {"gross_wall_area_ft2":432,"gross_wall_area_m2":40.13,"paintable_area_ft2":360.96,"paint_gallons":2.06,"paint_liters":7.81,"cans_needed":3,"can_size_used":"1 gal"}
GET https://tttkmbb.com/api/v1/calculate/paint?unit_system=metric&room_length=5&room_width=4&wall_height=2.4&doors=1&windows=2&coats=2&coverage=10&can_size=5
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/paint(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/paint · Markdown: https://tttkmbb.com/construction/paint.md · JSON definition: https://tttkmbb.com/construction/paint.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="paint" - OpenAPI operationId:
estimate_paint_needed - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
FAQ
Does this include primer or the ceiling?
No. Primer covers only 200–300 ft²/gal (5–7 m²/L) and is estimated separately; for the ceiling enter its area as wall_area with doors = 0 and windows = 0.
Why litres and gallons at the same time?
Paint is sold in litres in most countries and in US gallons/quarts in the USA; both are reported from the same paintable area so either label can be used.
What if coverage is given per coat on the tin?
Coverage rates on labels are always per coat; this calculator multiplies by the number of coats, so enter the label value unchanged.
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