Paint Calculator
About Paint Calculation
The Paint Calculator estimates the amount of wall paint needed in litres from the area to be painted, the number of coats and the surface type. It is useful for planning the painting of individual walls, rooms and other indoor spaces. It can also estimate the amount of primer required for one coat over the treated area.
Guidelines and recommendations
How the wall area is calculated
Rectangular room. The total area of the four walls is calculated from the room length A, width B and height H. All dimensions are entered in metres, and the resulting area is expressed in m2.
S = 2 × (A + B) × H
Individual wall. The area is the wall length L multiplied by its height H. The same method can be used for an irregularly shaped room if the combined length of its walls is determined first.
S = L × H
Known area. If the wall area has already been measured, that value is used directly in m2. No additional geometry calculation is performed.
How windows and doors are deducted
Area of openings. For each opening type, the width is multiplied by the height to obtain the area of one opening, then multiplied by the number of identical openings. Window and door areas are added together. If the combined opening area is already known, it is used directly.
Sopen = nw × bw × hw + nd × bd × hd
Here n is the number of openings, b is their width and h is their height. The indices w and d refer to windows and doors respectively.
Net paintable area. When openings are deducted, their combined area is subtracted from the total wall area. If the openings exceed the wall area, the calculated paintable area is set to 0 m2. If openings are not deducted, the full wall area is used.
Spaint = max(0, S - Sopen)
How paint coverage is converted into consumption
Specific consumption. Paint coverage can be entered in m2/l or ml/m2. The first unit indicates how much area one litre covers in one coat. The second indicates how many millilitres are needed to cover one square metre in one coat.
Unit conversion. The volume calculation uses consumption in ml/m2. If the entered value is in m2/l, the inverse conversion is applied:
q = 1000 / R
Here R is coverage in m2/l and q is consumption in ml/m2. For example, 8 m2/l corresponds to 125 ml/m2. For calculations, consumption is rounded to a whole ml/m2.
Rounding when switching units. When converting to ml/m2, the value is rounded to the nearest 10 ml/m2. When converting back, it is rounded to the nearest 0.1 m2/l. Switching units repeatedly can therefore slightly change the entered value and the result.
Practical guidelines. Interior paints often provide coverage of about 6-10 m2/l per coat, approximately 100-167 ml/m2. Actual consumption depends on the paint formulation, hiding power and application conditions. Where available, use the manufacturer's stated consumption for the particular product.
Surface type coefficients
Consumption adjustment. Once the base paint consumption has been determined, a coefficient is applied to account for surface characteristics. The calculator uses these values:
- Smooth, prepared surface -
1.00. - Plaster or concrete -
1.10. - Textured surface with relief -
1.20. - Coarse-textured surface -
1.35. - Porous surface, such as aerated concrete -
1.50.
For example, a coefficient of 1.20 increases estimated paint consumption by 20%, while 1.50 increases it by 50%, compared with a smooth prepared surface. These coefficients are approximate assumptions used by the calculator, not values prescribed by European standards.
Calculating paint volume for one or more coats
One coat. The net paintable area Spaint is multiplied by the specific consumption q and the surface coefficient k. The volume in millilitres is divided by 1000 to obtain litres.
V1 = Spaint × q × k / 1000
Multiple coats. Total paint volume is calculated by multiplying the volume for one coat by the number of coats N. The range is from 1 to 3 coats. Each coat is assumed to have the same specific consumption and surface coefficient.
Vtotal = V1 × N
For example, an area of 40 m2, consumption of 150 ml/m2 and a surface coefficient of 1.20 require 7.2 l for one coat and 14.4 l for two coats. Final volumes are displayed to the nearest 0.01 l.
How primer consumption is calculated
Primer consumption. Primer units are converted using the same principle as paint. Consumption can be entered in m2/l or ml/m2. Internally, it is converted to ml/m2 and rounded to a whole number.
Primer volume. One coat is calculated over the net paintable area. The area is multiplied by the specific primer consumption, and the result is converted into litres.
Vprimer = Spaint × qprimer / 1000
Here qprimer is the specific primer consumption in ml/m2. The surface type coefficients used for paint are not applied to primer. Primer consumption does not change the calculated amount of paint.
Calculation assumptions and precision
Calculation assumptions. All coats are assumed to use the same amount of paint. Differences between the first and subsequent coats are not calculated separately. The texture and absorbency of the substrate are represented by a single surface coefficient. No additional waste allowance or separate application losses are added.
Result precision. Dimensions are used in metres, areas are displayed to 0.01 m2, and volumes to 0.01 l. Intermediate areas are not rounded before consumption is calculated. The result may therefore differ slightly from a manual calculation based on rounded areas.
Related European standards
EN 13300:2022 "Paints and varnishes. Paints and varnishes for interior walls and ceilings. Classification". This standard establishes a system for classifying coatings by their properties. It helps compare paint performance but does not specify a universal consumption rate per square metre.
EN ISO 6504-3:2019 "Paints and varnishes. Determination of hiding power. Part 3. Determination of hiding power of paints for masonry, concrete and interior use". This standard describes methods for determining coating hiding power. Hiding power affects the ability to cover a substrate at a given paint consumption.
EN ISO 11998:2026 "Paints and varnishes. Determination of wet-scrub resistance of coatings". This standard describes testing the resistance of a coating to repeated wet scrubbing. The property is used to assess paint performance and is not a paint consumption coefficient.
These standards concern the properties and testing of paint coatings. The surface coefficients and quantity formulas in the calculator are used for preliminary consumption estimates and are not calculation requirements imposed by these standards.
FAQs
Why can the same wall area require different amounts of paint?
Paint consumption depends on substrate absorbency and texture. For example, the same area is assigned a coefficient of 1.00 for a smooth prepared wall and 1.50 for a porous surface. With all other inputs equal, estimated consumption for the porous surface is therefore 50% higher.
Which paint consumption value should I use?
Use the value supplied by the manufacturer of the specific paint whenever possible. If the packaging states coverage in m2/l, you can enter it directly without converting it manually into ml/m2. Actual consumption may differ from the stated figure because of surface condition and application technique.
Why is the amount of paint for two coats exactly twice that for one?
The calculator assumes identical consumption for each coat, so paint volume is directly proportional to the number of coats. In practice, the first coat can consume more material, especially on absorbent substrates. This difference is not calculated separately, while substrate properties are represented by the overall surface coefficient.
How important is it to subtract windows and doors from the wall area?
Subtracting openings reduces the paintable area and therefore the calculated paint volume. In rooms with large windows or several doors, the difference can be significant. Smaller openings usually have less effect, but deducting them gives a more accurate application area.
Why isn't primer consumption increased for porous surfaces?
Unlike paint, primer is calculated directly from the specified consumption without an extra substrate coefficient. The entered value is assumed to be appropriate for the selected primer and treated material. For very absorbent surfaces, use the manufacturer's consumption guidance for the relevant substrate.