To calculate how much paint you need for walls, multiply the total wall area by the number of coats, divide by the coverage printed on the paint tin, then round up to whole gallons or litres — and the Paint Calculator runs that entire formula live as you type.
The four numbers that drive the estimate are straightforward. Wall area is the sum of every wall's length times the ceiling height, kept in a single unit throughout. Doors and windows are subtracted because you do not paint the glass or the door faces. The number of coats is two by default, since most repaints need a second pass for an even, durable finish. Coverage is the area one gallon or one litre covers per coat, taken directly from the tin. Keep every measurement in the same unit and the result matches: square feet with US gallons, square metres with litres.
The result is always rounded up because paint comes in whole tins, never fractions. Everything below walks through each input, what numbers to plug in, and the small adjustments — textured walls, glossy finishes, porous drywall — that change real-world coverage away from the figure printed on the label.
Understanding Wall Area for a Paint Estimate
Wall area is the paintable surface of every wall, expressed in square feet or square metres. It is the sum of each wall's length multiplied by the room's ceiling height, then added together. A room with 60 linear feet of wall and an 8 ft ceiling is 480 sq ft of wall area, before any openings are removed.
Two rules keep the math clean. First, keep every measurement in the same unit — if you measure in feet, multiply by feet, and the result comes out in gallons; if you measure in metres, multiply by metres, and the result comes out in litres. Second, measure every wall separately and add them up. The shortcut of multiplying perimeter by height works for a perfectly rectangular room but breaks the moment a wall jogs in or out, has a sloped section, or meets an alcove.
Wall area is not the same as floor area. A 12 ft by 14 ft room with 8 ft ceilings is 168 sq ft of floor, but the four walls together run 52 linear feet, giving 416 sq ft of wall — roughly two and a half times the floor's footprint. That ratio is why a small room often needs more paint than a quick visual estimate suggests.
Measuring Each Wall Correctly
Walk the room with a tape measure, a notepad, and the same unit throughout. For each wall, record the length along the top where the wall meets the ceiling; you do not need to subtract for the baseboard, because the ceiling-to-floor height already covers the full wall surface. Multiply length by ceiling height to get that wall's area, then move to the next wall.
For odd-shaped rooms, break each wall into rectangles. A wall with a chimney breast is two rectangles — one on each side of the bump-out. A wall with a window seat or a knee wall can be treated the same way: split it at the change in depth, calculate each rectangle, and add them. Sloped ceilings under a roofline are best handled with an average between the tallest and shortest point, rather than the full floor-to-ceiling height.
Once every wall is on the page, add the wall areas together. That single total is the number to enter into the wall-area field. Subtract the doors and windows only after the total is summed, never wall by wall — otherwise a tall door on one wall and a wide window on another can offset each other in confusing ways.
How to Convert Wall Area Into Paint Tins
- Pick your units — square feet with US gallons, or square metres with litres — and enter the total wall area you just summed across every wall.
- Subtract the doors and windows you are not painting. Add up the rough area of each opening (a standard interior door is about 20 sq ft and an average window is 12 to 15 sq ft) and enter the total. Leave the field blank if you would rather ignore openings; the small over-estimate simply becomes spare paint.
- Set the number of coats. Two is the default and the most common choice, since most repaints need a second pass for an even, durable finish.
- Enter the coverage printed on the tin — the area one gallon or one litre covers in a single coat — and read the rounded-up tin count the calculator returns.
Coverage on the Tin — and Why Real Walls Cover Less
Coverage on the label is the area one tin covers in a single coat, quoted under ideal conditions: smooth, primed, evenly absorbent substrate. As a starting point, one US gallon covers about 350 to 400 sq ft per coat and one litre of emulsion covers about 10 to 12 sq m per coat on smooth primed walls.
Those are the best-case figures. Real-world coverage runs 10 to 20 percent lower on rough, textured, or unprimed surfaces, and thinner semi-gloss and high-gloss finishes also cover less per coat because they lay down a thinner film. Premium paints with more titanium dioxide hide better in fewer coats, so the tin figure is closer to what you actually use.
| Surface or finish | How it affects the area one tin covers |
|---|---|
| Smooth, primed drywall | Matches the coverage printed on the tin |
| Premium paint (high titanium dioxide) | Closer to the tin figure; covers well in fewer coats |
| Textured, rough, or unprimed walls | Lower than the tin figure (the surface drinks more paint) |
| Bare, patchy, or porous drywall | Lower than the tin figure; often needs an extra coat |
| Semi-gloss or high-gloss finish | Lower than the tin figure (thinner film per coat) |
If your walls fall into one of the lower-coverage rows above, enter a more conservative coverage number than what is printed on the tin. Lowering the coverage figure is how the calculator lands on a tin count that has slack built in for real-world losses, instead of producing a number that only works on a perfectly flat, primed wall.
Doors, Windows, and Other Openings to Subtract
Glass is not painted, and most door faces are painted as trim with a different product — so subtracting the openings tightens the estimate. A standard interior door is roughly 20 sq ft; a typical window runs 12 to 15 sq ft. A large picture window or sliding glass door can be 30 sq ft or more. Walk the walls and total the rough area of every opening before entering it into the openings field.
The calculator will not let the openings equal or exceed the wall area, because then there is nothing left to paint. If you would rather skip the subtraction entirely, leave the field blank — the small over-estimate simply becomes spare paint kept aside for touch-ups, which is rarely a problem on a wall repaint and may even save a last-minute trip back to the store.
Coats, Paint Type, and Other Multipliers
Two coats is the standard for an even, durable finish, which is why the calculator defaults to two. A fresh, well-matched colour over primer can manage one coat. A light colour going over a dark wall — or bare, patchy, or porous drywall — can need three, because the underlying colour keeps telegraphing through until the second or third film is fully opaque.
Each extra coat multiplies the total area the paint has to cover, which in turn multiplies the tins. If your net wall area (after openings) is 440 sq ft and coverage is 350 sq ft per gallon, one coat is just over one gallon, two coats is 880 sq ft or 3 gallons after rounding up, and three coats is 4 gallons. The Paint Calculator handles this multiplication automatically as you change the coats field, so the tin count updates live.
Worked Example: A Standard Bedroom Repaint
Consider a bedroom with 480 sq ft of wall area, two doors (about 20 sq ft each), and two windows (about 15 sq ft each). The openings total 40 + 30 = 70 sq ft, leaving 480 − 70 = 410 sq ft of net wall area. At two coats and a coverage rate of 350 sq ft per gallon, the paint has to cover 410 × 2 = 820 sq ft. Dividing by coverage gives 820 ÷ 350 = 2.34 gallons. Rounding up to whole tins gives 3 gallons to buy.
Common Mistakes That Throw Off the Estimate
The most common error is mixing units — measuring some walls in feet and others in metres — which silently halves or doubles the wall area and the paint count with it. The second most common is reading the coverage figure as the total for the whole job rather than per coat; coverage is always per coat, and the calculator multiplies by the number of coats. The third is forgetting that ceilings, trim, doors, and a separate primer coat are their own calculations; this estimator covers walls only and is not a full paint schedule.
It is always worth buying a little extra and keeping one spare tin from the same batch for future touch-ups, because colours drift slightly between production runs and a fresh tin from the store rarely matches a wall painted a year ago. Real-world coverage running 10 to 20 percent below the tin's figure on textured or thirsty walls is another reason to round up — or to enter a more conservative coverage number from the start so the calculator builds the slack in for you.
Putting It All Together
For a quick live calculation, the Paint Calculator accepts wall area, opening deductions, coat count, and tin coverage in a single form and rounds the result up to whole gallons or litres as you type. All calculations run entirely in your browser, so measurements stay on your device. Once you know the tin count, the natural next step is to estimate the cost — see the guide on how to calculate paint cost per square foot for any room to turn the wall estimate into a budget.
If you're weighing options, Parallelogram Area With Vectors: Cross Product Method covers this in detail.