Paint Calculator
Buy 3 gallons of paint.
| Step | Value |
|---|---|
| Wall area (perimeter × height) | 416 ft² |
| Less 1 door and 2 windows | − 51 ft² |
| Paintable area, one coat | 365 ft² |
| × 2 coats, + 10% waste | 803 ft² |
| ÷ 350 ft² per gallon | 2.29 gal |
| What to buy | 3 × 1-gal cans |
Openings are deducted at 21 ft² per door (3 × 7 ft) and 15 ft² per window (3 × 5 ft) — adjust the counts if yours are much bigger or smaller. Coverage defaults to 350 ft² per gallon, which is the manufacturer figure for a smooth, primed, same-colour surface. Expect noticeably less on bare drywall, textured or porous walls, and over a big colour change — knock 20–25% off the coverage figure in those cases rather than relying on the waste factor. Buckets are suggested where five or more gallons are needed because they are almost always cheaper per gallon and give you one consistent batch; the cost line prices single cans, so treat it as the upper bound. Trim, doors and ceilings usually take a different product — run those separately.
Enter your room size or wall area to find how many gallons of paint you need — with doors and windows deducted, multiple coats, and what to actually buy.
How to use this calculator
- Choose room size if you can measure the space, or wall area if you already know the square footage.
- Enter length, width, and ceiling height. The calculator works out the perimeter for you — you do not need to measure each wall separately unless the room is an unusual shape.
- Count the doors and windows. These are deducted at standard sizes; adjust the counts if yours are much larger or smaller than a 3 × 7 ft door and a 3 × 5 ft window.
- Set the coats — two is right for almost any colour change — and the coverage from your paint can if it differs from 350 ft² per gallon.
- Add a price per gallon for a cost estimate. The "what to buy" row tells you whether buckets beat cans.
The formula
Wall area = 2 × (Length + Width) × Height · Paintable = Wall area − Openings + Ceiling · Gallons = (Paintable × Coats × Waste) ÷ Coverage
Door deduction: 21 ft² each · Window deduction: 15 ft² each · Default coverage: 350 ft² per gallon
Worked example — a 12 × 14 ft bedroom
Walls only, 8 ft ceilings, one door, two windows, two coats, standard paint at 350 ft² per gallon, 10% waste.
- Perimeter: 2 × (12 + 14) = 52 ft
- Wall area: 52 × 8 = 416 ft²
- Openings: 21 + (2 × 15) = 51 ft²
- Paintable: 416 − 51 = 365 ft²
- Two coats: 365 × 2 = 730 ft²
- With 10% waste: 730 × 1.10 = 803 ft²
- Gallons: 803 ÷ 350 = 2.29 → buy 3 gallons
Add the ceiling — another 168 ft² — and the same room needs about 3.4 gallons, so 4 cans. That is a third more paint for one surface, which is why the ceiling is worth deciding on before you shop rather than at the register.
Why the coverage number on the can is optimistic
350 to 400 square feet per gallon is a real figure, measured on a smooth, primed, previously painted wall in a similar colour. Most rooms are not that. Here is roughly what to expect:
| Surface | Realistic coverage | Why |
|---|---|---|
| Smooth, primed, similar colour | 350–400 ft²/gal | The manufacturer's test condition |
| Bare or patched drywall | 250–300 ft²/gal | Paper and joint compound absorb the first coat |
| Light texture (orange peel) | 275–325 ft²/gal | More actual surface than the measurement suggests |
| Heavy texture (knockdown, popcorn) | 200–250 ft²/gal | Considerably more surface area per square foot |
| Rough masonry or block | 100–200 ft²/gal | Porous, and the first coat fills as much as it covers |
The right move is to change the coverage field to match your surface rather than inflating the waste factor. Waste is meant to cover roller and tray loss and a little for touch-ups; using it to compensate for a porous wall hides what is actually going on and gets less accurate as the job grows.
What to buy: cans or buckets
A 5-gallon bucket typically costs 10 to 20% less per gallon than five separate cans, and it gives you one tint batch. Store tinting is accurate but not perfect, and on a long uninterrupted wall a slight variation between cans can show. If your job needs five gallons or more, the calculator suggests buckets automatically.
The exception is when you are painting several rooms in different colours, where you will never use a full bucket of any one. Buy cans then, and box them — pour all cans of one colour into a single container and remix before starting, which eliminates batch variation entirely.
Common mistakes to avoid
- Painting a colour change in one coat. The single most common cause of a repaint. Any colour change needs two coats; dark to light may need primer plus two.
- Forgetting to deduct openings. A door and two windows is over 12% of a small room's wall area. That is most of a gallon.
- Deducting trim. Baseboards, casings, and crown get painted too — usually in a different sheen and a separate product. Do not subtract them from the wall area; calculate them separately.
- Using wall paint on the ceiling. Ceiling paint is formulated flat to hide imperfections and to not spatter. Calculate and buy it separately.
- Buying exactly the calculated amount. Running out mid-wall means a visible lap line where the fresh paint meets the dried edge. The 10% allowance exists for this.
- Ignoring lead paint in an older home. If the house predates 1978, sanding or scraping existing paint can release lead dust. Check EPA guidance before disturbing old finishes.
How we calculate this
Wall area is the room perimeter × ceiling height: 2 × (length + width) × height. Openings are deducted at 21 ft² per door (3 × 7 ft) and 15 ft² per window (3 × 5 ft), and the deduction cannot take the paintable area below zero. The ceiling, when included, is length × width. That paintable area is multiplied by the coat count and then by the waste factor, and divided by the coverage per gallon. Gallons round up to whole cans, and any five whole gallons are grouped into a 5-gallon bucket because buckets are cheaper per gallon and give one consistent tint batch. Coverage defaults to 350 ft² per gallon, the manufacturer figure for a smooth, primed, same-colour surface.
Sources
Frequently asked questions
How much paint do I need for a 12x14 room?
A 12 by 14 foot room with 8-foot ceilings has 416 square feet of wall. Deduct a door and two windows and you are painting 365 square feet. Two coats makes 730 square feet, plus 10% for touch-ups is 803 square feet, which at the usual 350 square feet per gallon works out to 2.3 gallons — so buy 3 gallons. Adding the ceiling pushes it to about 3.4 gallons.
How many square feet does a gallon of paint cover?
Manufacturers claim about 350 to 400 square feet per gallon, and that figure assumes a smooth, previously painted, primed surface in a similar colour. Real coverage is often lower: bare drywall soaks up the first coat, textured and porous walls have more surface area than their measurements suggest, and covering a dark colour with a light one takes more paint. Knock 20 to 25% off the stated coverage in any of those cases.
Do I need one coat or two?
Two, in almost every real situation. One coat only works when you are repainting the same colour on a sound, previously painted surface. Any colour change, any bare or patched drywall, and any move from dark to light needs two — and a dramatic colour change may need a tinted primer plus two finish coats. Paint that claims one-coat coverage is quoting a best case.
Should I deduct doors and windows?
Yes. A standard door is about 21 square feet and a typical window about 15, so a room with one door and two windows loses 51 square feet — over 12% of the wall in a small room. Ignoring them is how people end up with a spare gallon they never open. What you should not deduct is trim and baseboards, since those get painted too, usually with a different product.
Is a 5-gallon bucket cheaper than five 1-gallon cans?
Almost always, typically by 10 to 20%. It also gives you one consistent batch, which matters because tinted paint can vary very slightly between cans — a difference you will not notice in the store but might see on a long wall. This calculator suggests buckets once you need five gallons or more. The cost line prices individual cans, so treat it as the upper bound.
How much paint do I need for a ceiling?
A ceiling is simply the room's length times its width, so a 12 by 14 foot room has a 168 square foot ceiling. At 350 square feet per gallon that is about half a gallon per coat, or one gallon for two coats. Ceilings usually take a dedicated flat ceiling paint rather than your wall colour, so buy and calculate it separately — switch the ceiling toggle on and off to see the difference.
How much extra paint should I buy for touch-ups?
The 10% waste factor this calculator applies by default covers roller and tray loss and leaves a little for touch-ups. Beyond that, keep whatever is left over rather than buying extra — a sealed can of leftover paint stores for years and is worth far more later than a fresh can in the same colour, because store tinting drifts slightly between batches. Label the can with the room and the date.
Does primer count as a coat?
It counts as a coat of work but not a coat of paint, and it is a different product with its own coverage — usually 200 to 300 square feet per gallon, less than finish paint because it is designed to soak in. Calculate primer separately by setting coats to 1 and lowering the coverage figure. You need primer on bare drywall, over patches, over stains, and when making a drastic colour change.