Paint Calculator
Getting a second can of paint home halfway through the job is the project-day equivalent of a flat tyre. Enter the room’s length, width, height and the number of doors and windows, pick how many coats you’re doing, and the calculator returns the square footage of paintable surface, the gallons needed per coat, and the total accounting for doors, windows and a 10% safety margin.
How the paint estimate is calculated
-
1
Measure the room
Length, width and ceiling height in feet or metres.
-
2
Count and size openings
Standard door 21 sq ft, standard window 15 sq ft, or enter custom sizes.
-
3
Pick surfaces
Walls, ceiling, trim — or any combination.
-
4
Set coats
Two coats over a primed surface is standard. Bold colours often need three.
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5
Read gallons needed
The tool factors a 350 sq ft per gallon spread rate and adds 10% safety stock.
Coverage rates
| Paint type | Coverage per gallon |
|---|---|
| Flat / matt | 400 sq ft |
| Eggshell | 375 sq ft |
| Satin | 350 sq ft |
| Semi-gloss | 350 sq ft |
| Gloss | 325 sq ft |
| Primer | 300 sq ft |
| Textured / stucco | 200-250 sq ft |
Manufacturers list coverage on the can; 350 sq ft per gallon for wall paint is a safe average.
Opening sizes (deduct from wall area)
| Opening | Typical area |
|---|---|
| Interior door (frame in) | 21 sq ft |
| Exterior door | 21 sq ft |
| Standard window | 15 sq ft |
| Large window | 24 sq ft |
| Sliding patio door | 42 sq ft |
Worked example
A 12 ft × 15 ft room with 9 ft ceilings, one door, two standard windows, walls only, two coats.
- Wall perimeter: 2 × (12 + 15) = 54 ft
- Wall area: 54 × 9 = 486 sq ft
- Deduct openings: 486 − 21 − 30 = 435 sq ft
- Per coat at 350 sq ft/gal: 435 ÷ 350 = 1.24 gal
- Two coats: 2.49 gal
- +10% margin: 2.74 gal → round up to 3 gallons
Tips for an accurate estimate
- Always round up. A quart of leftover paint is cheap; a half-painted wall at 9 pm is not.
- Buy the same batch. Paint can vary batch to batch. Get all of it at once from the same retailer.
- Primer changes everything. New drywall, bare wood or dramatic colour changes need primer, which is usually sold by the gallon too.
- Ceilings use less. Flat white ceiling paint typically covers 400 sq ft per gallon — add a bit less than for walls.
- Trim is cheap on area but expensive on quality. Semi-gloss or gloss paint for trim, usually in quart containers. A quart covers about 100 linear feet of baseboard.
When to hire a pro
If you’re painting a 20-foot stairwell, working off a ladder with drop cloths, or changing dramatic colours (dark to light) that need multiple coats and primer, the maths can justify the $400-800 for a pro. DIY cost tends to break even above roughly 1,200 sq ft of wall area.
Frequently Asked Questions
Yes for bare drywall, new plaster, bare wood, or going from dark to light. Skip for re-coating a similar colour over existing washable paint in good condition. Primer adds a whole extra coat of coverage and cost but makes top coats go on cleaner.
Two as standard. Three for: bold colours (deep red, navy, orange), going light-over-dark, or when the paint label itself recommends it. Self-priming “paint and primer in one” products may genuinely cover in one coat on well-prepared surfaces.
Touch-ups, dripped paint, thin spots that need a third pass, and matching future nail-hole repairs. Paint left unopened and stored in a cool dry place stays usable for years.
For wall paint, yes — subtract the baseboard height times the wall perimeter. For a 4-inch baseboard on 54 ft of wall, that’s 18 sq ft less. For most rooms this rounds out the 10% margin you already added.
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