Pmc Logic Paint Calculator: How It Works

The PMC Logic paint calculator works out how much paint you need for a job from three numbers: the surface area to cover, the number of coats, and the paint’s coverage rate. You enter those values, and it returns the total litres required, plus a small safety margin so you don’t run short. The result is an estimate, not a guarantee, because real-world coverage depends on the surface, the applicator, and how thickly you lay the paint on.

Formula

The core formula is simple:

Paint needed (L) = (Surface area × Number of coats) ÷ Coverage per litre

Then a 10% safety margin is added, so the full formula used here is:

Paint needed (L) = (A × C ÷ R) × 1.1

• A = surface area in square metres (m²)
• C = number of coats (unitless, usually 1, 2, or 3)
• R = coverage rate in square metres per litre (m²/L), which is printed on every paint can

The 1.1 multiplier adds a 10% buffer for waste, drips, and touch-ups. If you prefer no buffer, you can ignore the first output and use the second output (total area) divided by the rate manually.

Worked Example

Using the default values from the calculator above:

1. Surface area = 50 m²
2. Number of coats = 2
3. Coverage per litre = 10 m²/L

Step 1: Multiply area by coats: 50 × 2 = 100 m² (this is the total area to paint).

Step 2: Divide by coverage: 100 ÷ 10 = 10 L.

Step 3: Add 10% buffer: 10 × 1.1 = 11 L.

So the calculator returns 11 L as the paint needed. If you were buying 5 L tins, you’d need three tins (15 L) because you can’t buy 11 L exactly. If you were buying 2.5 L tins, you’d need five tins (12.5 L). Always round up to the nearest tin size.

How to Measure the Surface Area

The calculator only works if you give it a correct area. Here’s how to get that number for common situations:

• Walls: Measure the width and height of each wall in metres, multiply them, then add all the walls together. Subtract the area of doors (about 1.9 m² each) and windows (about 1.2 m² each) if you’re not painting them.
• Ceilings: Measure the length and width of the room, multiply them. That’s the ceiling area.
• Woodwork (trim, doors, skirting): Measure the length of each piece and multiply by its width. For a door, both faces count, so measure one side and double it.
• Irregular shapes: Break the surface into rectangles and triangles. For a triangle, multiply base × height and divide by 2.

If you’re working in feet, convert to metres first (1 ft = 0.3048 m) or use a calculator that accepts feet. Mixing units is the most common error in paint estimates.

What Coverage Rate Should You Use?

The coverage rate (R) is printed on every paint can, usually as something like “10 m²/L” or “350 ft²/gallon.” But that number is measured on a smooth, sealed surface under ideal conditions. Real-world rates are often lower:

• Fresh plaster: 6–8 m²/L because it soaks up paint.
• Rough or textured walls: 7–9 m²/L.
• Previously painted, smooth walls: 10–12 m²/L.
• Unpainted drywall: 8–10 m²/L.

If you’re unsure, use a lower rate. Running out of paint mid-wall is far more annoying than having a quarter of a litre left over. Also note that dark colours often need more coats to cover a light base, and light colours over dark bases may need 3 coats instead of 2.

Common Mistakes and Misconceptions

People often make these errors when estimating paint:

• Forgetting the second coat. Most walls need at least two coats for even colour. One coat rarely looks right unless it’s a touch-up on the same colour.
• Ignoring the primer. Primer is a separate product with its own coverage rate. If you’re priming bare surfaces, calculate that separately and add it to the total.
• Using the wrong unit. Coverage is given per litre, but some calculators ask for gallons. 1 US gallon = 3.785 L, and 1 UK gallon = 4.546 L. Mixing these up changes the result by about 20%.
• Not subtracting doors and windows. A standard door is about 1.9 m², a window about 1.2 m². If you’re painting a room with two doors and two windows, that’s roughly 6 m² you don’t need to cover – about half a litre of paint.
• Thinking the 10% buffer is optional. It’s not just for waste; it also covers the fact that your wall measurements are rarely perfect, and touch-ups after the job will need the same colour batch.

The biggest misconception is that paint calculators give an exact number. They give a minimum. Always round up to the nearest tin size, and if you’re between sizes, buy the larger one.

Limitations of the Formula

This formula assumes uniform coverage and a flat surface. It does not account for:

• Spraying: Spray application wastes 20–30% more paint than rolling.
• Very porous surfaces: Brick, concrete, or unpainted wood can absorb twice the stated rate.
• Paint quality: Cheaper paints often have lower solids content, so they cover less per litre.
• Colour change: Going from dark to light (or vice versa) may require an extra coat beyond what the formula suggests.
• Temperature and humidity: Painting in direct sun or high humidity can affect how the paint flows and how much you need.

For a typical interior wall with a roller, the formula is accurate to within 10–15%. For anything unusual, add more buffer or consult the paint manufacturer’s technical data sheet.

Frequently Asked Questions

How many square metres does 1 litre of paint cover?

Most standard interior paints cover about 10–12 m² per litre on a smooth, sealed surface. On bare plaster or rough textures, expect 6–9 m² per litre. The exact number is always printed on the tin, and it’s the number you should use in the calculator.

Do I need a primer, and does it count in the calculator?

Primer is a separate product with its own coverage rate. Use the calculator twice: once for primer (on bare surfaces) and once for the topcoat. If you’re painting over an already painted, sound wall, you can skip the primer and just use the topcoat.

Why does the calculator add 10%?

The 10% buffer covers paint lost in the roller tray, drips, brush cleaning, and the fact that your wall measurements are never perfectly exact. It also gives you a little extra for touch-ups later. If you’re on a tight budget, you can drop it, but you risk running out mid-job.

Can I use the same calculation for exterior paint?

Yes, the formula is the same, but exterior surfaces (brick, stucco, wood siding) are more porous and textured, so use a lower coverage rate. Also, exterior paint is often applied at a thicker film, so you might get only 6–8 m²/L instead of 10–12.

What if my paint tin says coverage in square feet per gallon?

Convert to metric first. 1 gallon (US) = 3.785 litres, and 1 square foot = 0.0929 m². So if the tin says 350 ft²/gallon, that’s 350 × 0.0929 ÷ 3.785 ≈ 8.6 m²/L. Use that number in the calculator.

Is it better to buy more paint than the calculator says?

Yes, always round up to the nearest tin size. If the calculator says 11 L and tins come in 5 L or 2.5 L, you’d buy 12.5 L (five 2.5 L tins). Leftover paint can be stored for touch-ups, and it’s cheaper than making a second trip to the store.

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in area | Surface area | m² | 50 | 0
in coats | Number of coats | - | 2 | 1
in rate | Coverage per litre | m²/L | 10 | 1
out paint | Paint needed | L | (area * coats / rate) * 1.1 | 1
out area_total | Total area to paint | m² | area * coats | 0