Figure out how much paint to buy for a room
Enter room length, width, and wall height, subtract door and window area, choose coats and a surface add-on, then see gallons (or liters) and how many cans to buy. Optional ceiling area uses length × width so one pass covers walls and ceiling together.
Results
Wall area: 312 sq ft
Ceiling area: 120 sq ft
Effective area (coats + surface): 950.4 sq ft
Paint needed: 2.715 gal
Cans to buy: 3
Formula
Walls = 2×(L+W)×H − openings; Ceiling = L×W; Effective area = net area × coats × (1 + surface); Paint = effective area ÷ coverage; Cans = ceil(paint ÷ can size)
Worked steps (check these)
- Wall area = 2×(12+10)×8 − 40 = 312 sq ft
- Ceiling area = 12×10 = 120 sq ft
- Area = 312 + 120 = 432 sq ft
- Effective area (coats + surface) = 432 × 2 × (1+0.1) = 950.4 sq ft
- Paint needed = 950.4 ÷ 350 = 2.715 gal
- Cans to buy = ceil(2.715 ÷ 1) = 3
Estimates only — confirm with product labels and a licensed pro. Not a substitute for structural or roofing advice.
The hardest part of paint shopping is not the color chip — it is knowing whether one gallon will finish the room or leave a half wall stranded. Under-buying means a second trip and a can from a different batch that may not match. Over-buying leaves opened gallons that dry out before the next project.
This page uses the same classroom formula many paint estimators start from: wrap the four walls, subtract openings, optionally add the ceiling, multiply by coats and a surface factor, then divide by the can’s stated coverage. The result rounds up to whole cans so you leave with a shopping number, not a decimal you have to interpret at the shelf.
Always skim the label for coverage on your surface type. Fresh drywall, popcorn ceilings, and deep reds drink more paint than a satin repaint over a similar color. When in doubt, buy the extra can the tool suggests rather than stretching a thin second coat.
How to estimate paint
- Choose imperial or metric units
- Enter length, width, wall height, and door/window area
- Set coats, surface add-on %, coverage, and can size; toggle ceiling if needed
- Read paint needed and cans to buy in the results
Why use this paint calculator
- Sees walls and optional ceiling in one pass.
- Coats and surface % sit in the form — no mental multipliers.
- Coverage and can size are editable so store brands still fit.
- Shows both theoretical gallons and cans to buy.
- Works in feet/gallons or meters/liters.
Good for
- Bedroom or office repaint before guests arrive
- Open-plan living room where openings are large doors and windows
- Ceiling refresh in the same color as the walls
- Comparing one-coat marketing claims against a two-coat plan
- Tenant turnovers where you need a quick material list
Common paint mistakes to avoid
- Forgetting doors and windows — large openings can shave a full gallon off a small room.
- Using the can’s “up to” coverage on rough or porous walls; drop coverage or raise surface %.
- Mixing sheens or batches mid-wall; buy enough of one batch for the whole surface.
- Skipping primer on new drywall or stain bleed-through, then wondering why two coats look thin.
- Measuring in mixed units (feet walls, meter openings) — pick one system and stick to it.
Worked paint examples
| Input | Output | Note |
|---|---|---|
| Room 12×10×8 ft, openings 40 sq ft, 2 coats, 10% surface, include ceiling, 350 sq ft/gal, 1 gal cans | Walls 312 sq ft + ceiling 120 = 432; effective 950.4; ~2.715 gal → 3 cans | walls=2*(L+W)*H−openings; paint=(area×coats×(1+surface))/coverage |
| Same room, walls only (no ceiling), 2 coats, 0% surface, 350 sq ft/gal | Walls 312 sq ft; effective 624; ~1.783 gal → 2 cans | Skipping the ceiling drops the buy quantity by about one gallon can |
| Metric: 3.6×3×2.7 m, openings 3.5 m², 2 coats, 10% surface, ceiling, 10 m²/L, 5 L cans | Use metric mode; divide effective area by coverage, then ceil by can size | Match coverage to the can label in m² per liter |
Paint calculator FAQ
- How do you calculate wall area?
- Walls = 2 × (length + width) × height, minus the total area of doors and windows. That matches a simple rectangular room takeoff.
- Should I include the ceiling?
- Check Include ceiling when you are painting it in the same color or product. Ceiling area is length × width. For a different ceiling paint, run the tool twice or leave the ceiling off and estimate separately.
- What is surface add-on %?
- Extra paint for textured walls, new drywall, or a big color change. Ten percent is a common starting point; smooth repaints in a similar color can use less.
- How many coats should I enter?
- Most interior jobs need two coats. Deep color changes or covering stains may need a primer coat counted separately or a third finish coat — follow the product label.
- Coverage is not 350 sq ft per gallon — what then?
- Replace the coverage field with the number on your can. Thicker paints and primers often cover less area per gallon.
- Planning tile or flooring in the same room?
- Use Tile calculator or Flooring calculator with the same length and width so floor materials match the paint takeoff.
- Does this work for exterior siding?
- The geometry is similar, but exteriors add trim, soffits, and weather exposure. Use the numbers as a start, then confirm with the exterior product’s coverage chart.