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Online Calculator & Converter

Paint Calculator

Enter wall size, openings, coats and the coverage from your paint label.

ft

Sum of wall widths

ft
sq ft
sq ft
%
sq ft / gal

Spread rate stated by the paint manufacturer for one coat.

Paint needed

Enter values to see the result.

What this calculator does

It estimates how much wall paint a job needs. You supply the wall dimensions, the doors and windows you will not paint, the number of coats and, importantly, the coverage rate from your paint's label. It does not assume a coverage rate, because coverage differs between products and surfaces.

Formulas

  • Gross area = total wall length × height
  • Paintable area = gross area − (doors × door area) − (windows × window area)
  • Area to cover = paintable area × coats × (1 + allowance% ÷ 100)
  • Paint = area to cover ÷ coverage rate

1 US gallon = 3.785411784 L, and 1 sq ft = 0.09290304 m².

Worked examples

Bedroom 12 ft × 14 ft, 8 ft ceiling

  1. Wall length = 12 + 14 + 12 + 14 = 52 ft. Gross area = 52 × 8 = 416 sq ft.
  2. Openings measured as one 21 sq ft door and two 15 sq ft windows: 416 − 21 − 30 = 365 sq ft.
  3. Two coats = 730 sq ft. With a label coverage of 400 sq ft/gal: 730 ÷ 400 = 1.825 gal, so buy 2 gallons.

Metric room: walls 4 m, 3.5 m, 4 m, 3.5 m, 2.5 m high

  1. Gross = 15 × 2.5 = 37.5 m². Subtract a 1.8 m² door and two 1.5 m² windows: 32.7 m².
  2. Two coats = 65.4 m². With label coverage of 10 m²/L: 6.54 L.

Interpreting the result

The figure is the paint volume for walls only. Ceilings, trim and doors need separate estimates, often with a different product. Rough, porous or unprimed surfaces absorb more paint than the label rate suggests; an extra allowance covers this.

Common mistakes

  • Using the room's floor area instead of the wall area.
  • Forgetting to multiply by the number of coats.
  • Using a generic coverage number rather than the one on the can.
  • Subtracting openings you actually plan to paint, such as doors.

Assumptions and limitations

Walls are treated as flat rectangles of equal height. Gable ends, stairwells and sloped ceilings should be measured separately; use the area calculator for triangles and add the area to the wall length × height. Primer is not included unless you add it as a separate calculation.

Related: square footage calculator, flooring calculator, unit price calculator.

Frequently asked questions

How much area does a gallon of paint cover?

It varies by product, surface texture, porosity and application method. Use the spread rate printed on your paint's label or data sheet; this calculator does not assume a figure.

How do I measure the walls of a room?

Add the widths of all walls you will paint to get the total wall length (for a rectangular room, that is the perimeter), then measure the ceiling height. Subtract doors and windows you will not paint.

Do I need a second coat?

Often, especially when changing color or painting over a new or patched surface. The paint label usually states how many coats the manufacturer recommends.