Paint Calculator
Measure the room in metres and get the area to paint, litres of paint, a suggested mix of local tin sizes (0.946 L (quart), 3.78 L (gallon) and 18.9 L (5-gallon pail) cans) and an estimated cost in CAD.
Formulas shown on this page · Last reviewed October 2026
Tins are chosen largest first, then the smallest tin that covers the remainder, so it may not be the cheapest mix. Cost assumes a flat price per litre.
How the paint calculator works
wall area (m²) = 2 × (length + width) × height − doors − windows
litres = (wall area + ceiling) × coats ÷ coverage (m²/L)
The calculator deducts 1.9 m² per door and 1.5 m² per window by default – both are editable. Coverage defaults to 10 m² per litre per coat. A 3.78 L can usually covers roughly 35–40 m² (about 400 sq ft) per coat, which works out at around 10 m² per litre.
Worked example
| Step | Result |
|---|---|
| Room 4 × 3.5 m, 2.44 m walls | 36.6 m² of wall |
| Less 1 door and 1 window | 33.2 m² |
| Two coats at 10 m²/L | 6.64 L |
| Tins | 1 × 3.78 L + 1 × 3.78 L |
Choosing tin sizes
Paint is sold in 0.946 L (quart), 3.78 L (gallon) and 18.9 L (5-gallon pail) cans from home centres and paint stores. Larger tins are normally cheaper per litre, so the calculator fills the need with the biggest tins first and finishes with the smallest tin that covers what is left. This is a simple rule, not a price optimiser – always compare the price per litre of the sizes in store.
Ceiling paint is normally sold in the same quart, gallon and pail sizes as wall paint. Tick “Paint the ceiling too” to add the floor area of the room. Dark colours, bare plaster and textured walls can need an extra coat or a primer, and prices here are typical – check local prices and enter your own.
Frequently asked questions
How much paint do I need for a 4 × 3.5 m room?
With 2.44 m walls, one door and one window, the walls come to 33.2 m². Two coats at 10 m² per litre need about 6.6 L. Painting the ceiling as well raises that to about 9.4 L.
What tins should I buy for that room?
For 6.6 L the simple rule suggests 1 × 3.78 L + 1 × 3.78 L, which is 7.56 L in total and costs about $105.84 at $14.00 per litre.
How far does one 3.78 litre tin go?
At 10 m² per litre, a 3.78 L tin covers about 38 m² for a single coat, or about 19 m² with two coats.
Do I need to subtract doors and windows?
Yes, for an accurate figure. A typical door is about 1.9 m² and a window about 1.5 m². Small windows are often ignored to leave a little spare paint for touching up.