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Paint calculator

Brand sites will gladly estimate gallons if you stay inside their product family. This page is the independent version: wall area from length, width, and ceiling height, minus doors and windows, times coats, divided by coverage, with a waste trim so you are not one quart short on the last wall. Default coverage is a conservative 350 sq ft per gallon per coat on smooth, primed drywall. Texture, dark-to-light color changes, and cheap paint eat coverage. The label on the can you buy beats our default — change the field.

ft
ft
ft
count
count
sq ft/gal
Label on the can wins
10%
0510152025

+10% so you don't come up short

ORDER ESTIMATEPAINT
Gallons to buy3 gal
Paintable area334 sq ft
Coverage used350 sq ft/gal
Coats2
Waste10%
How it calculates
walls = 2×(L+W)×H − doors×20 − windows×15; gallons = ceil(area × coats ÷ coverage × (1 + 10% waste)).

Estimate only — not a contractor bid. Confirm bag yields and local codes before you pour or order.

When to use

One or more rectangular rooms; comparing one-coat marketing claims against a two-coat plan; adding ceiling paint as a separate line.

When not to

Exterior body + trim + shutters as one blob — break elevations into separate runs. Automotive paint — out of scope. Cabinet spraying with conversion varnish — different world.

Assumptions

Door ≈ 20 sq ft, window ≈ 15 sq ft defaults (editable). Default 2 coats. Coverage default 350 sq ft/gal. Primer optional at ~250 sq ft/gal. Waste default 10%.

Worked examples

Example 1

12×12 room, 8 ft ceilings, 1 door, 2 windows, 2 coats, 350 sq ft/gal, 10% waste
→ Walls ≈ 334 sq ft; ≈ 3 gallons (ceil)

Example 2

Same room, add ceiling, 1 coat ceiling only at 350
→ +144 sq ft → extra gallon line item

Ceilings often get flat paint; walls eggshell — do not assume one SKU.

How it calculates

Measure the room, subtract openings, multiply by coats, divide by coverage, round up gallons.

  1. Enter length, width, and ceiling height.
  2. Count doors and windows; optionally include the ceiling.
  3. Set coats, coverage, waste, and optional primer.
  4. Buy the gallon count on the ticket — same batch when you can.
wall_sqft = 2 × (L + W) × H
paintable = wall_sqft − doors×door_sqft − windows×window_sqft (+ ceiling L×W)
gallons = ceil(paintable × coats × (1 + waste) / coverage)

Common traps

  1. Accent walls — calculate the accent separately if the color differs.
  2. “One coat” paint still wants a second coat when you bury a dark red.
  3. Wet film vs boastful labels. If the can says 400 but you are rolling heavy texture, plan 300.
  4. Sheen changes mid-wall show every roller stop — buy enough to finish a plane from one batch when possible.

Next tool

Coverage-first / leftover math → paint coverage calculator.

Question wording → how much paint.

FAQ

Often about 2–3 gallons for walls at two coats, depending on openings and coverage — run the numbers rather than memorizing a meme.

Related

Estimate only — not a contractor bid or engineered takeoff. Confirm bag yields and local codes before you pour or order.