Paint Calculator
Gallons from paintable area at the honest 375-per-gallon law — two coats, ceiling separated, trim as a quart.
Paint Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Rectangular rooms; windows at a fixed 15 ft².
- No primer gallons — named, not modeled.
- No linear-foot trim take-off.
In short: A 12 × 12 room at 9 ft, door + 2 windows: 381 ft² paintable × 2 coats ÷ 375 = 3 gal of wall paint; the 144-ft² ceiling takes 1. Trim: one quart.
Formula
walls = 2(L+W)×H − 21/door − 15/window
gallons = ⌈walls × coats ÷ coverage⌉
ceiling = ⌈L × W ÷ 400⌉
primer covers 250-350 — a different can
[('375', 'ft² per gallon-coat, the 350-400 mid-band'), ('2 coats', 'the color-quality standard, not a luxury'), ('400', 'ceiling flat’s one-coat coverage'), ('100 ft²', 'what a trim quart spreads')]
Worked Example
- Take the walls, out the openings.
- Double for the second coat — the honest coat.
- Divide by your paint’s real coverage.
- Ceiling separate: area ÷ 400, once.
- Budget the trim quart and the primer truth.
The 12 × 12 room at 9 ft: 381 ft² paintable × 2 ÷ 375 = 3 gallons ($135.00 at $45); ceiling 144 ft² → 1 gal ($32.00). Total $167.00 — plus the trim quart the bins always need.
Strengths & Limits Of This Model
Where this engine is strong
- The two-coat standard enforced on the math
- Ceiling as its own honest line
- Coverage bands by paint tier
Where it stops
- Fixed opening sizes
- No primer or texture line
Practical Use Cases
Room repaints
Two-coat gallons, priced.
Whole floors
Room-by-room orders that add up.
Color changes
Primer plus paint, budgeted.
Ceiling refreshes
Flat white by its own law.
Bid checks
Gallons against the painter’s number.
Methodology & Editorial Standards
Paintable walls are perimeter × height minus 21/15 ft² per door/window. Gallons ceiling the area × 2 coats over entered coverage; the ceiling lines up separately at 400 ft² one-coat flat. Trim is a fixed quart card, not a field. Coverage outside 250-450 is refused — no paint covers 500.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Paint Calculator — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many gallons to paint a 12x12 room?
381 ft² of paintable wall × 2 coats ÷ 375 = 3 gallons; add one for the ceiling. The second coat is the difference between painted and covered.
How much area does a gallon of paint cover?
350-400 ft² per coat on sealed interior walls — 375 is the honest mid-band. Rough, bare or very dark surfaces drop it; premium solids raise it.
Is one coat enough?
For same-color refreshes on sealed walls, sometimes. Color changes, new drywall and repairs flash through — two coats is the standard that survives hallway light.
How much paint for a ceiling?
One gallon of flat covers about 400 ft² — a 12 × 12 ceiling takes one, with a quarter of the can left for the hallway that will beg for it.
Do I need primer?
New drywall (it drinks), stain blocking, and dark-to-light changes: yes — primer covers 250-350 ft² and is a different can from paint. Same-color refreshes on sealed walls usually skip it.
How much does interior painting cost?
DIY materials run paint-and-supplies on this page’s numbers; professionally, interiors run $2.75-4.70 per ft² installed in 2026 by region and prep. The gallons never explain the invoice — prep does.
What sheen where?
Flat or matte hides walls’ sins; eggshell cleans in halls and kids’ rooms; semi-gloss lives on trim, doors and kitchens; ceilings stay flat so light forgets them. Sheen changes washability, not the gallon law.
Does dark paint need more coats?
Usually — deep colors carry less hiding power, and tinted primers earn their can. Plan two coats plus a tinted primer rather than four topcoats.
How long between coats?
Latex recoats in 2-4 hours in normal room conditions — the can’s number, not the internet’s. Cold, damp rooms stretch it; rushing the second coat rolls the first one off.
How do I estimate trim and doors?
A quart spreads about 100 ft² — enough for baseboard, casing and a door or two in an average room. Count heavy millwork in linear feet and let the quart math repeat.