Wall area comes from perimeter, not floor area. Measure each surface, divide by the coverage printed on the technical data sheet, and count coats by the colour underneath.
Numbers to plan with
Wall area is perimeter × height, not floor area
A 12 x 15 ft bedroom contains 180 sq ft of floor, and that number tells you nothing about paint. Add the four wall lengths instead: 12 + 15 + 12 + 15 gives 54 linear feet of perimeter. Multiply by the 8 ft ceiling height and the room holds 432 sq ft of gross wall. From here the arithmetic is subtraction and division, and every number stays traceable when someone asks where it came from.
Run the calculation room by room instead of totalling the whole house first. Paint is bought by colour and sheen, and two rooms that share a wall do not share a colour. A 432 sq ft bedroom with 40 sq ft of doors and windows leaves 392 sq ft of paintable wall. At two coats that is 784 sq ft of coating, which divides by 350 sq ft per gallon to 2.24 gallons, or 2.46 gallons once you add a 10 percent allowance. Buy 3 gallons and keep the half gallon.
Rooms that are not rectangles get split into rectangles. An L-shaped family room becomes two boxes, a 14 x 18 ft main area and a 6 x 9 ft alcove, each measured on its own four sides. Measure the perimeter of the painted surface rather than the floor outline, because a wall that runs behind a cabinet still needs coating on the exposed face and on the returns beside it.
- Wall square feet = (length + width) x 2 x ceiling height. For 12 x 15 ft at 8 ft: 54 x 8 = 432 sq ft.
- Ceiling square feet = length x width, so the same room needs 180 sq ft of ceiling paint per coat.
- Sloped or vaulted ceilings: measure the slope length from plate to ridge, not the horizontal run.
- Closets count. A 6 x 3 ft walk-in adds 18 linear ft of wall, about 144 sq ft at 8 ft, and it usually needs the same two coats.
- Deduct nothing at this stage. Keep gross wall area on the sheet until the next step so the deduction stays visible.
Deduct openings by measured area, then stop deducting
Measure each window and door as width times height in feet, then subtract the total. A 3 ft x 6 ft 8 in interior door is 20 sq ft. A 3 x 5 ft single window is 15 sq ft. Do not subtract the casing. You still cut in around it, and the 2 to 4 in band of wall beside each opening gets brushed twice, so casing stays in the paintable area.
Pick one deduction rule and write it on the sketch so the number can be reproduced next year. The workable rule is to deduct every opening larger than 20 sq ft and leave smaller ones in the total as buffer. On a room with 432 sq ft of wall and 40 sq ft of openings, the deduction is 9 percent, already close to the allowance you would add back anyway. Deducting the big openings and keeping the small ones lands you at an honest 392 to 400 sq ft without counting the same saving twice.
Two situations break the rule. If a wall is going to be covered by cabinets, tile, or paneling, do not deduct it when you are painting the exposed strip above and beside it, because that strip still takes two coats and a lot of brush work. If a whole wall is glazing, such as a sunroom wall or a window wall, deduct it completely because no wall remains.
Nominal opening sizes and the area most estimators deduct
| Opening | Nominal size | Area |
|---|---|---|
| Closet door | 2 ft 6 in x 6 ft 8 in | 16.7 sq ft |
| Standard interior door | 3 ft 0 in x 6 ft 8 in | 20 sq ft |
| Single window | 3 ft 0 in x 5 ft 0 in | 15 sq ft |
| Double window | 6 ft 0 in x 5 ft 0 in | 30 sq ft |
| Picture window | 8 ft 0 in x 5 ft 0 in | 40 sq ft |
| Sliding glass door | 6 ft 0 in x 6 ft 8 in | 40 sq ft |
| French door pair | 6 ft 0 in x 6 ft 8 in | 40 sq ft |
Coverage per gallon is a product number, and it moves
Interior latex usually states 350 to 400 sq ft per gallon per coat. That figure assumes a smooth, sealed surface and a normal film thickness. New drywall, patched plaster, and light orange-peel texture all pull the real number down, sometimes to 300 sq ft. Read the technical data sheet for the exact product rather than the front of the can, and use the bottom of the range whenever the wall is not smooth.
Compute gallons in the order the paint goes on: wall area after deductions, times the number of coats, divided by coverage, then multiplied by 1.10. Using 392 sq ft, two coats, and 350 sq ft per gallon, the sum is 392 x 2 = 784, then 784 ÷ 350 = 2.24, then 2.24 x 1.10 = 2.46 gallons. Two gallons leaves you short in the last corner of the second coat. Three gallons leaves about half a gallon for touch-up, which is the outcome you want.
If the first gallon covers less than expected, stop estimating from the label and measure what actually happened. Suppose that gallon finished 290 sq ft of wall. Divide the remaining wall area by 290, add the allowance, and revise the order before you open the next can. Correcting mid-job is normal. Pretending the label is exact is how people end up rolling a second coat so thin that the old colour still reads through under a window.
- Use 350 sq ft per gallon for smooth painted drywall, 300 for light texture, and 270 or less for heavy texture or bare masonry.
- The 10 percent allowance covers roller loss, touch-up, and one patched area. Do not stack a second 20 percent reserve on top of it.
- A five-gallon pail makes sense above roughly 1,200 sq ft of single-coat coverage in one colour. Below that, gallons and quarts let you match the order to the estimate.
Coats are set by the colour underneath, not by habit
Two coats is the default for a colour change on a prepared wall, and it is not universal. The decision depends on how much light the old colour reflects, how much pigment the new colour carries, and whether the surface has been sealed. Write the coat count beside each room on the schedule before you price anything, because the coat count drives the gallon count more than any other single input.
- Same colour, same sheen, clean wall: one coat usually reads as uniform. Use two if the wall has been scrubbed, patched, or spot-primed.
- Light to light, or any move within one colour family: two coats.
- Mid-tone to deep, or deep to mid-tone: two coats, with a primer if the wall was patched.
- Deep to light, especially navy, red, or charcoal to white: three coats, or one tinted primer plus two.
- Bare drywall, bare plaster, or fresh compound: one primer plus two finish coats, and the primer is not a finish coat.
- Red, orange, or saturated yellow over white: a grey or tinted primer plus three coats is faster and cheaper than four coats of colour.
- Gloss or semi-gloss to flat: degloss with 120 grit, apply a bonding primer, then two coats. Paint does not grip a slick film.
- Water stains, smoke, marker, or ink: stain-blocking primer first. Finish paint lifts the stain back through within days.
Ceilings, trim, and doors are separate products and separate gallons
Ceiling flat, wall finish, and trim enamel have different sheens, different coverage, and sometimes different tint bases. Keep them on separate lines even when the colours look identical, because a gallon of wall eggshell cannot touch up semi-gloss trim without leaving a visible patch. The same reasoning applies to a feature wall in a different finish: it gets its own quantity even if it shares a tint.
Trim is measured along its length, not across its face, and then converted to area only when you need to price coverage. A 54 ft perimeter with three 3 ft doors leaves 45 linear ft of baseboard. At 5.5 in tall, that is 0.46 ft, so 45 linear ft covers about 21 sq ft per coat. Door slabs are the bigger item: two faces at 20 sq ft each plus edges comes to roughly 43 sq ft per door, and a bedroom with three doors carries 129 sq ft of enamel work that a wall-only estimate ignores. Casing adds about 17 linear ft per side of a standard opening, and window trim and sash add about 14 linear ft per single window.
Trim enamel covers less than wall paint, usually 300 to 350 sq ft per gallon, and brushing wastes more than rolling. A quart covers 75 to 90 sq ft on a flat surface and less on profiled moldings. For a typical bedroom with 45 ft of base, three doors, two windows, and crown, plan one gallon of trim enamel and expect to keep a third of it. Buying a second gallon you cannot return is the most common paint overspend on a small job.
Buy the sizes that exist, and keep one quart back
Paint arrives in quarts, gallons, and five-gallon pails. A pail is worth the price break when one colour covers more than about 1,200 sq ft in a single coat, which usually means an open-plan area or a whole-floor repaint in one colour. Everywhere else, gallons and quarts let you match the estimate without leaving a sealed pail in the garage for a decade.
Buy every container for one colour at the same time so the tint lot matches. Custom-tinted paint is normally non-returnable, and a second batch mixed a week later can read slightly cooler or warmer across a long wall. Mark each leftover can with the room, product, sheen, colour name, tint formula, and date, and keep one quart per colour for repairs. A labelled quart is worth more than a full can with no label on it.
Budget ranges for 2026, which shift by region and brand tier: mid-grade interior latex runs about $32 to $55 per gallon, premium lines $60 to $85, ceiling flat $25 to $40, and trim enamel $45 to $75. A third finish coat on 400 sq ft costs $40 to $70 in paint. Hiring out a repaint of that same wall at $2.50 to $5.50 per sq ft runs several hundred dollars, so the extra gallon is the cheaper decision every time.
Questions that come up on this job
Does one gallon really cover a whole room?
One gallon covers roughly one coat on a 10 x 12 ft room's walls at 8 ft, which is 352 sq ft gross before openings. Any colour change needs two coats, so the same room takes about 2 gallons. Rooms above 150 sq ft of floor area rarely finish in a single gallon.
Should I buy a five-gallon pail to save money?
Only when one colour covers more than about 1,200 sq ft per coat and you are confident about the colour. The price break is usually 10 to 20 percent, but custom tint cannot be returned, so a pail of the wrong colour is an expensive mistake.
How much paint should I keep for touch-ups?
One quart per colour, from the same tint lot as the walls, stored with the room and formula written on the lid. Touch-up paint that is two years and one tint batch away from the wall will not match, especially on a sunlit wall.
Do paint-and-primer products remove the need for primer?
They work on clean, previously painted walls in good condition. Bare drywall, bare wood, glossy trim, water stains, and repaired patches still need a dedicated primer for sealing, bonding, or stain blocking. On a 400 sq ft room a primer coat costs $35 to $65 in material and usually saves a finish coat.
Sources & further reading
References behind this guide
These sources support the safety, measurement, or industry conventions used above. Product instructions, approved plans, and local requirements control your project.