TAHA Engineering Toolsby TAHA Knowledge Lab

Civil & Construction

Paint Calculator

Litres or gallons of paint for walls, using the coverage rate on your paint label.

Inputs

Calculate from
Walls shown side by side as one long strip with total length and height, containing a door and two windows that are not painted.doorwindowwindowtotal length of wallsH
All walls laid side by side: total wall length × height, minus doors and windows. That net area is painted once per coat. Diagram not to scale.

Add the lengths of all walls to be painted. For a whole room this is its perimeter.

Area one unit of paint covers in ONE coat. Different paints and surfaces vary a lot — always use the label value.

Optional, for rough or porous surfaces and losses in trays and rollers.

Result

Paint needed
6.606
Net area to paint (one coat)
33.03
Total area painted (all coats)
66.06
Gross wall area
37.8
Openings deducted
4.77

You need to cover 33.03 m² with 2 coats (66.06 m² in total). At 10 m²/L, that is about 6.606 L.

  • Paint is sold in fixed can sizes — round up to the cans available. US gallon = 3.785 L; imperial (UK) gallon = 4.546 L.

Educational estimate — check the assumptions below.

About this calculator

Estimate how much paint to buy for walls. The calculator finds the area to paint, deducts doors and windows, multiplies by the number of coats and divides by the coverage rate printed on the paint can. Coverage differs a lot between products and surfaces, so the calculator always uses the value you enter.

Formula

Walls with openings

Net area = Wall length × Height − Σ (opening width × height × number)

Paint = Net area × Coats ÷ Coverage × (1 + allowance)

Coverage is the area one litre (or gallon) covers in one coat.

Known area

Paint = Area × Coats ÷ Coverage × (1 + allowance)

Worked example

A room with walls totalling 14 m long, 2.7 m high, one door 0.9 × 2.1 m and two windows 1.2 × 1.2 m; 2 coats; coverage 10 m²/L.

  • Wall area = 14 × 2.7 = 37.80 m²; openings = 1.89 + 2.88 = 4.77 m²; net area = 33.03 m²
  • Total for 2 coats = 66.06 m²
  • Paint = 66.06 ÷ 10 = 6.61 L (1.75 US gallons)

Label in imperial units? 350 ft² per US gallon = 8.59 m²/L.

Assumptions and limitations

  • Coverage on labels is usually for smooth, primed surfaces. New plaster, rough or porous surfaces and strong colour changes use more paint.
  • Ceilings, trims and doors themselves are not included — add them as a separate area.
  • US and imperial gallons are different: 1 US gallon = 3.785 L, 1 imperial gallon = 4.546 L. Choose the one used on your label.

References

  • Exact unit definitions: US gallon = 231 in³; imperial gallon = 4.54609 L.