How Much Paint Do I Need?Complete Guide
Everything you need to know to calculate the right amount of paint for walls, ceilings, and trim — avoid buying too much or running out mid-project.
Quick Answer
One gallon (3.78 litres) of interior latex paint covers approximately 32 to 37 square metres (350 to 400 square feet) with one coat. For a standard room (3.7×3.7 m, 2.4 m ceilings), you need about 2 gallons (7.6 L) for two coats on the walls. Add 1 gallon for the ceiling and 1 quart (946 mL) for trim and doors. Always round up — leftover paint is essential for future touch-ups.
How to Measure Your Walls
Accurate wall measurements are the foundation of any paint estimate. Here is how to measure like a professional painter:
- Measure the length of each wall in metres (or feet) and multiply by the ceiling height. A 4.3 m wall with 2.4 m ceilings = 10.3 m² (112 sq ft). Repeat for all four walls and add them together.
- For quick whole-room calculations, measure the room perimeter (sum of all wall lengths) and multiply by ceiling height. A 3.7×4.3 m room has a 16 m perimeter, times 2.4 m ceilings = 38.4 m² of wall area.
- Subtract openings: a standard door is approximately 2 m² (21 sq ft, 0.9×2.1 m) and a standard window is approximately 1.4 m² (15 sq ft, 0.9×1.5 m). For a room with one door and two windows, subtract about 4.8 m².
- For rooms with vaulted or cathedral ceilings, measure the wall height at the tallest point and the shortest point, average them, and multiply by the wall length.
- Include closet interiors if you plan to paint them — a standard 0.6×1.8 m closet adds about 5.2 m² of wall area.
- Record all measurements before going to the paint store. Take photos of the rooms for reference when choosing colours.
A typical 140 m² (1,500 sq ft) house has 370 to 465 square metres (4,000 to 5,000 sq ft) of paintable wall area, depending on ceiling height, number of rooms, and window/door count.
Coverage Rates by Paint Type
Not all paints cover the same area. Coverage rates vary by finish, quality, and the surface you are painting.
Flat and matte finishes have the highest coverage rates at 32 to 37 m² (350 to 400 sq ft) per gallon because they have lower binder content and spread easily. They are ideal for ceilings and low-traffic rooms but show marks and are difficult to clean. Eggshell finish covers 28 to 32 m² (300 to 350 sq ft) per gallon and is the most popular choice for living rooms, bedrooms, and hallways — it has a subtle sheen that is easy to clean without being too shiny.
Satin finish covers 28 to 32 m² (300 to 350 sq ft) per gallon and is excellent for kitchens, bathrooms, and high-traffic areas due to its moisture resistance and washability. Semi-gloss covers 28 to 32 m² per gallon and is the standard choice for trim, baseboards, doors, and window frames — its reflective surface is highly durable and easy to wipe clean. High-gloss covers 23 to 28 m² (250 to 300 sq ft) per gallon and is used for accent areas, cabinets, and furniture where maximum durability and reflectivity are desired.
Premium paints from brands like Benjamin Moore, Sherwin-Williams, and Behr (all widely available at Canadian Tire, RONA, and Home Depot Canada) typically achieve the higher end of these coverage ranges due to higher pigment concentration and better levelling properties. Economy paints often fall at the lower end and may require an additional coat to achieve full coverage. Textured, porous, or unpainted surfaces reduce coverage by 20 to 30 percent regardless of paint quality — always prime these surfaces first.
How Many Coats of Paint Do You Need?
The number of coats directly affects how much paint you need to buy. Here is when to apply one, two, or three coats:
- One coat is sufficient only for touch-ups using the exact same paint colour and sheen on a previously painted surface in good condition. Some premium paints advertise one-coat coverage, but professional painters still recommend two coats for best results.
- Two coats is the standard recommendation for virtually all painting projects. The first coat seals and primes the surface, while the second coat provides the final colour depth, uniform sheen, and maximum durability. Two coats of quality paint can last 7 to 10 years on interior walls.
- Three coats are necessary when making a dramatic colour change — especially going from a dark colour (red, navy, forest green) to a light colour (white, cream, pastel). The first coat of tinted primer neutralises the old colour, followed by two topcoats for full coverage.
- New, unpainted drywall always requires a coat of primer plus two topcoats (three coats total). The primer seals the porous surface and provides a uniform base for the paint. Skipping primer on new drywall causes flashing — visible differences in sheen where the joint compound and bare paper absorb paint at different rates.
- Textured surfaces like knockdown, orange peel, and popcorn ceilings may require an extra coat because the texture creates peaks and valleys that are difficult to cover evenly with a single pass.
For budgeting purposes, always calculate paint quantity based on two coats. If you need primer, calculate that separately at the primer’s coverage rate.
Surface Preparation
Proper surface preparation is the single most important factor in a long-lasting paint job. No amount of expensive paint can compensate for poor prep work.
- Clean all surfaces before painting. Dust, grease, and cobwebs prevent paint adhesion. Wipe walls with a damp cloth or use TSP (trisodium phosphate) solution for greasy kitchen walls. Allow surfaces to dry completely before painting.
- Fill nail holes, dents, and cracks with lightweight spackling compound. Apply with a putty knife, let it dry, and sand smooth with 150-grit sandpaper. For larger holes (over 12 mm), use mesh tape and joint compound, applied in two thin coats.
- Sand glossy surfaces lightly with 150-grit sandpaper to create tooth for the new paint to grip. This is especially important when painting over semi-gloss or high-gloss finishes.
- Remove or sand any loose, peeling, or flaking paint down to a sound surface. Feather the edges of remaining paint with sandpaper to create a smooth transition.
- Apply painter’s tape to trim, baseboards, ceiling lines, and any areas you want to protect. Press the tape firmly with a putty knife or credit card to prevent paint from bleeding underneath.
- Protect floors and furniture with drop cloths. Canvas drop cloths grip the floor better than plastic and absorb drips. Move furniture to the centre of the room and cover with plastic sheeting.
Calculating Ceiling Paint
Ceilings require their own calculation since they use a different paint formulation than walls.
Ceiling paint is specifically formulated with higher solids content for better spatter resistance when rolling overhead, and it dries to a perfectly flat finish to minimise the appearance of imperfections. Standard ceiling paint covers 32 to 37 m² (350 to 400 sq ft) per gallon. To calculate the amount needed, multiply the room length by the room width — a 3.7×4.6 m room has 17 m² of ceiling. At 37 m² per gallon, that is about 0.46 gallons — buy 1 gallon for that room.
Most ceilings only need one coat if they are being painted the same white colour. If you are changing the ceiling colour or painting over a stain, apply primer first and then two topcoats. For popcorn or textured ceilings, reduce the coverage rate to 23 to 28 m² (250 to 300 sq ft) per gallon because the texture absorbs more paint. A whole-house ceiling (140 m² / 1,500 sq ft) typically requires 4 to 5 gallons of ceiling paint.
Calculating Trim and Door Paint
Trim, baseboards, doors, and window frames are typically painted with a semi-gloss or satin finish for durability and easy cleaning.
Baseboards: Measure the total linear metres (or feet) of baseboard in the room. Standard baseboards are 90 to 140 mm tall. A room with 16 linear metres of baseboard has approximately 2.2 m² of paintable surface. One quart (946 mL) of trim paint covers about 7 to 9 m² — enough for an average room’s baseboards.
Doors: A standard interior door (both sides) has approximately 3.7 m² of paintable surface. Window trim (frame and casing) averages 0.9 m² per window. Crown moulding adds about 0.09 m² per linear foot (both face and edge). For a whole house, plan on 1 to 2 gallons of trim paint for all baseboards, door frames, window trim, and crown moulding. Paint doors with a foam roller for the smoothest finish — brush marks are very visible on semi-gloss paint on large flat surfaces.
Paint Cost Breakdown
Understanding the full cost of a painting project helps you budget accurately and choose the right products for your needs.
Wall paint is the largest cost component. Budget $35 to $95 CAD per gallon depending on quality tier. For a typical room requiring 2 gallons at 2 coats, wall paint costs $70 to $190. For a whole 140 m² (1,500 sq ft) house (approximately 25 gallons needed for walls), paint costs $875 to $2,375 CAD. Buying five-gallon buckets (18.9 L pails) saves 15 to 20 percent per gallon compared to individual cans.
Primer costs $18 to $35 CAD per gallon. New construction (unpainted drywall) requires about 1 gallon per 28 to 37 m². For a 140 m² house, that is 10 to 15 gallons of primer ($180 to $525 CAD). Self-priming paints (available in premium lines) eliminate this cost but are more expensive per gallon. For repaints over similar colours, primer is usually unnecessary.
Supplies for a whole-house interior paint job cost $140 to $280 CAD total: roller frames and covers ($25 to $40), extension pole for ceilings ($20 to $35), angled brushes in 50 mm and 65 mm widths ($25 to $40), painter’s tape ($30 to $55 for multiple rolls), drop cloths ($20 to $40), paint trays and liners ($12 to $20), sandpaper and spackling ($12 to $20), and rags and cleanup supplies ($8 to $15). Quality tools make a significant difference in the final result — do not skimp on roller covers and brushes.
Calculate Your Paint Needs
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