Paint Calculator

How the paint estimate works

• Wall area = 2 × (length + width) × ceiling height. A 12 × 14 room at 8 ft = 416 sq ft.
• × coats — two is standard, so 416 × 2 = 832 sq ft to cover.
• ÷ coverage — a gallon covers ~350 sq ft of smooth drywall, so 832 ÷ 350 = 2.38 → 3 gallons, rounded up.

Floor area is not in that calculation anywhere, which is why a long narrow room takes more paint than a square one of the same square footage. Painting the ceiling adds length × width on top.

Every common room size, worked out

All 12 sizes below assume an 8 ft ceiling, two coats and smooth previously-painted walls. Hours are hands-on time for one person at an unhurried pace — prep, cutting in and rolling — and exclude drying between coats.

Room	Wall area	Gallons (2 coats)	Est. paint cost	Hours
8×10 room	288 sq ft	2	≈ $70	≈ 5.4
10×10 room	320 sq ft	2	≈ $70	≈ 6.1
10×12 room	352 sq ft	3	≈ $105	≈ 6.7
11×12 room	368 sq ft	3	≈ $105	≈ 7.1
12×12 room	384 sq ft	3	≈ $105	≈ 7.5
12×14 room	416 sq ft	3	≈ $105	≈ 8.2
12×16 room	448 sq ft	3	≈ $105	≈ 8.9
14×14 room	448 sq ft	3	≈ $105	≈ 9
14×16 room	480 sq ft	3	≈ $105	≈ 9.7
15×20 room	560 sq ft	4	≈ $140	≈ 11.6
16×20 room	576 sq ft	4	≈ $140	≈ 12.1
20×20 room	640 sq ft	4	≈ $140	≈ 13.8

Follow any row for that room's full supplies list, schedule and cost. The largest 3 sizes cross 4 gallons, where a 5-gallon pail (≈ $145) beats buying singles — cheaper per gallon, and it removes batch-to-batch colour variation across a big surface. For what the whole job costs, DIY and professionally, see what it costs to paint a room.

Coverage: the number everything else hangs off

Manufacturers quote 350–400 sq ft per gallon per coat on smooth drywall. This site uses 350, the conservative end, because that figure assumes a smooth, sealed, previously painted wall and a roller in good condition — and most walls are not quite that. Bare drywall, new plaster and heavy texture drop it to about 250: more surface area in the texture, and a porous substrate that drinks the first coat. On the 12 × 14 reference room:

Surface	Coverage	Gallons (2 coats)	Est. cost
Smooth drywall, premium paint (best case)	400 sq ft/gal	3	≈ $105
Smooth, previously painted drywall — this site’s default	350 sq ft/gal	3	≈ $105
Bare drywall, new plaster, or heavy texture	250 sq ft/gal	4	≈ $140

Texture is the single biggest swing in the whole estimate: it adds a gallon on 10 of the 12 room sizes above. If you are painting bare or textured walls, switch the surface setting in the calculator before you shop. For small jobs, a quart covers roughly 100 sq ft by manufacturer guidance — about 87.5 sq ft at the conservative rate used here, which is one accent wall or a set of trim.

Why two coats — and when you genuinely need three

Two coats is the default because one coat almost never looks finished. The first coat goes onto a surface with a different porosity than paint, so it dries unevenly; the second coat goes onto itself and builds a uniform film. One coat also leaves the previous colour faintly readable and gives you none of the washability the paint was formulated for.

Coats	Area to cover	Gallons	Est. cost
1	416 sq ft	2	≈ $70
2	832 sq ft	3	≈ $105
3	1,248 sq ft	4	≈ $140

Three coats is genuinely needed in two situations: light over dark, where the old colour keeps bleeding through, and saturated reds, oranges and yellows, whose pigments are naturally translucent and simply do not hide in two. Reaching for a third finish coat is the expensive way to solve either one. On the reference room it adds $35 of paint and about 3.4 more hours of cutting in and rolling — and it may still not cover.

Prime instead. A coat of primer tinted toward your finish colour is 2 gallons on this room, roughly $56 — less than the $70 a coat of finish paint costs — and primer is formulated to hide and seal rather than to build a durable film, so it does the covering work far more reliably than a third topcoat. Ask the paint desk to tint it toward your colour; most will, at no extra charge.

Why door and window deductions are off by default

Most calculators invite you to subtract your openings. This one leaves them in, and the arithmetic explains why. A standard door is about 20 sq ft and a window about 15, so a typical bedroom with one door and two windows is 50 sq ft of non-wall — a fraction of a gallon that usually disappears in the round-up to whole cans.

Run it across every size on this page and deducting one door and two windows leaves the gallon count completely unchanged on 8 of the 12 sizes. On the few where it does drop a gallon, it drops you to exactly enough — with nothing left for the wall you decide to redo. The surplus is not waste: it becomes cut-in buffer — the brushed band at the ceiling, corners and baseboard where you use more paint per square foot than the roller does — and then labelled touch-up paint from the exact batch, which is worth far more than the few dollars it cost. Store it somewhere that never freezes; latex that has frozen and thawed is ruined.

Primer: when you actually need it

Primer is not a cheaper first coat, and modern paint-and-primer-in-one products do not replace it where it is genuinely required. Prime when:

• The drywall is bare. Board face and joint compound absorb paint at completely different rates, so finish paint over unprimed drywall dries to a patchwork of dull and shiny — the effect painters call flashing. Every taped seam shows.
• You have patched or skim-coated. Same problem, in spots. Spot-prime the repairs at minimum.
• There are water stains, smoke or marker. These bleed through latex indefinitely. You need a stain-blocking primer — shellac or oil-based — not another coat of wall paint.
• You are making a big colour change, especially light over dark. A tinted primer does the hiding so your finish coats only have to do the colour.
• The old surface is glossy or oil-based. Sand it dull, then use a bonding primer, or the new paint will peel off in sheets.

Primer covers less than finish paint — about 300 sq ft per gallon against 350 — because it is formulated to seal rather than to build film, so budget 2 gallons for a room like the reference one. Previously painted walls in sound condition, going a similar colour, do not need it at all.

Ceiling height changes more than you expect

Wall area scales directly with ceiling height, so an 8 ft room becomes 12.5% more wall at 9 ft and 25% more at 10 ft. Whether that costs you another gallon depends entirely on where your room sits relative to the round-up:

Ceiling	10×10	12×14	16×20
8 ft	320 sq ft · 2 gal	416 sq ft · 3 gal	576 sq ft · 4 gal
9 ft	360 sq ft · 3 gal	468 sq ft · 3 gal	648 sq ft · 4 gal
10 ft	400 sq ft · 3 gal	520 sq ft · 3 gal	720 sq ft · 5 gal

Going from 8 ft to 9 ft pushes 1 of these 3 rooms into another gallon; at 10 ft it is 2. Measure your actual ceiling rather than assuming — 8 ft is standard in older housing but 9 ft is common in newer builds, and a room you assumed was 8 ft can leave you a gallon short on the second coat. Above roughly 10 ft, or on a stairwell, the labour cost rises faster than the area does, because the work now needs a ladder you have to keep repositioning or proper scaffolding.

Which sheen goes where

Sheen is the choice people regret most, because it is the one you cannot change without repainting. The rule of thumb: the shinier the paint, the tougher and more washable it is — and the more it shows every flaw in the wall underneath.

Sheen	Where it belongs	Why
Flat / matte	Ceilings, low-traffic bedrooms, and any wall with visible flaws	Hides bumps and patches better than anything else; scuffs and will not wash clean
Eggshell	Living rooms, dining rooms, bedrooms — the default interior wall sheen	A slight lustre that wipes clean without spotlighting every dent
Satin	Hallways, kids’ rooms, laundry, family bathrooms	Genuinely washable; shows roller lap marks if you let an edge dry
Semi-gloss	Trim, doors, cabinets, and wet bathroom walls	Hard, moisture-resistant film that takes repeated cleaning
Gloss	Front doors, feature trim, furniture	Toughest and most reflective — it magnifies every imperfection underneath

For most interiors the answer is eggshell on the walls, semi-gloss on the trim and doors, flat on the ceiling. That combination hides ceiling imperfections, wipes clean where hands actually touch, and gives the trim a deliberate contrast without needing a second colour. Buy trim paint by the quart, not the gallon: at 100 sq ft per quart, the reference room’s 52 ft of baseboard plus its door and window casings come to only 51.5 sq ft — 2 quarts for two coats.

Work out your room

Start with the calculator at the top, or a common size from the table. For the method behind every number see how much paint do I need?, for sheen (which does not change the gallon count) see which paint sheen, for the constants and their sources see how we calculate, and for the money side — supplies, trim, professional rates and minimum job charges — see the cost to paint a room. More project guides are in the guides index.