Davies Paint Calculator: How It Works

The Davies Paint Calculator is a free online tool that estimates how much paint you need for a room or surface. You enter the room’s dimensions, the number of doors and windows, and the number of coats; it returns the required paint volume in liters and gallons. The calculator uses standard coverage rates (typically 10–12 m² per liter per coat) and subtracts areas that won’t be painted.

Formula

The calculator uses the standard wall-paint estimation formula:

Paint (L) = (Total wall area − door area − window area) × coats / coverage rate

• Total wall area = 2 × (length × height + width × height), in m²
• Door area = 1.8 m² per door (typical 0.9 m × 2.0 m)
• Window area = 1.2 m² per window (typical 1.2 m × 1.0 m)
• Coats = number of coats you plan to apply
• Coverage rate = 11 m² per liter per coat (a common average for interior wall paint)

For US customary units, the same formula works with feet and square feet, using a coverage rate of about 400 sq ft per gallon per coat. The calculator’s default coverage rate is a middle-ground value; actual rates vary by paint brand and surface porosity.

Worked Example

Using the defaults: length = 5 m, width = 4 m, height = 2.5 m, doors = 1, windows = 1, coats = 2.

1. Total wall area = 2 × (5 × 2.5 + 4 × 2.5) = 2 × (12.5 + 10) = 45 m²
2. Subtract door area: 45 − 1.8 = 43.2 m²
3. Subtract window area: 43.2 − 1.2 = 42.0 m²
4. Multiply by coats: 42.0 × 2 = 84.0 m²
5. Divide by coverage rate: 84.0 / 11 = 7.636… L

Rounded to two decimals, the calculator shows 7.64 liters (about 2.02 US gallons). That’s the amount of paint you need for two coats on the walls, not including the ceiling or trim.

How to Use the Calculator Correctly

Enter the room’s internal dimensions in meters (or feet if you prefer). Measure from floor to ceiling, not including baseboards or crown molding. Count doors and windows that are fully inside the wall area. For a half-wall or a room with a sloped ceiling, adjust the height to the average wall height. The calculator assumes you’re painting all four walls; if you’re painting only one accent wall, use the length and height of that wall and set the width to 0.

For the number of coats: one coat is for touching up or for painting over the same color. Two coats are standard for a color change or for new drywall. Three coats may be needed for deep or dark colors over light primer. The calculator doesn’t include primer—add that separately if you plan to prime.

Common Mistakes and How to Avoid Them

• Mixing units: Don’t enter length in feet and height in meters. Convert everything to the same unit first.
• Forgetting to subtract openings: The calculator subtracts doors and windows automatically, but if you leave them at 0, you’ll overestimate by roughly 3 m² per opening.
• Ignoring surface texture: Rough plaster, brick, or unpainted drywall absorbs more paint. Add 10–20% to the result for such surfaces.
• Rounding too early: Round only at the end. If you round each intermediate step, you might end up with half a liter more than needed.
• Not accounting for waste: Paint spilled on the floor or left in the roller isn’t used. Add about 10% for waste and touch-ups.

Limitations of the Formula

The formula is a good starting point, but it doesn’t account for every variable. Coverage rates vary widely: a liter of cheap paint might cover only 8 m², while a premium paint can cover 14 m². The porosity of the surface matters—new drywall soaks up more paint than a previously painted wall. The calculator also assumes a standard door and window size; if you have sliding glass doors or floor-to-ceiling windows, adjust the counts or subtract the actual area manually.

For ceilings, use a separate calculation: length × width, then divide by coverage. For trim and baseboards, measure the linear footage and use a trim paint calculator or estimate 1 liter per 10–12 m of trim.

Comparison with Other Paint Calculators

Most online paint calculators (from major paint brands or home improvement stores) use the same core formula but differ in the default coverage rate and the assumed door/window sizes. Some ask for the total square footage directly, others ask for room dimensions. The Davies calculator is simpler: it gives a quick estimate without requiring you to know your exact square footage. However, it doesn’t let you input custom door/window sizes, which can be a drawback for non-standard openings. For a more precise estimate, measure the actual openings and subtract them from the total wall area manually.

Frequently Asked Questions

How accurate is the Davies Paint Calculator?

It’s accurate to within about 10–15% for a typical room with standard doors and windows. The main source of error is the coverage rate, which varies by paint brand and surface. For a final purchase, always round up to the nearest half liter and consider buying a small extra can for touch-ups.

Does the calculator include ceiling paint?

No, it only calculates wall paint. To estimate ceiling paint, multiply the room’s length by its width and divide by the coverage rate (about 11 m² per liter). For example, a 5 m × 4 m ceiling needs about 20 m² / 11 ≈ 1.82 liters per coat.

What if I’m painting a room with no windows?

Set the window count to 0. The calculator will then subtract only the door area. If you have no doors either (e.g., an archway), set both to 0, but keep in mind that archways and large openings reduce the paintable area more than a standard door.

Can I use this for exterior paint?

Yes, but exterior surfaces (stucco, brick, wood siding) absorb more paint and have more irregular areas. Use a lower coverage rate (about 8–10 m² per liter) and add 20% for waste. The calculator doesn’t let you change the coverage rate, so you’ll need to multiply the result by 1.2 or 1.3.

How do I convert liters to gallons?

The calculator shows US gallons (1 US gallon = 3.78541 liters). If you’re in the UK or elsewhere, note that an imperial gallon is 4.546 liters. To convert the liters result to imperial gallons, divide by 4.546.

Should I buy exactly the amount the calculator says?

No, always round up to the nearest liter or half gallon. Paint is sold in standard sizes (1 L, 2.5 L, 5 L, or 1 quart, 1 gallon, 5 gallons). If the calculator says 7.64 L, you’d buy a 5 L can and a 2.5 L can (7.5 L total) and still be slightly short—so you’d actually need 10 L to be safe. It’s better to have a little extra than to run out mid-project.

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in length | Room length | m | 5 | 0
in width | Room width | m | 4 | 0
in height | Room height | m | 2.5 | 0
in doors | Number of doors | - | 1 | 0
in windows | Number of windows | - | 1 | 0
in coats | Number of coats | - | 2 | 1
out paint_l | Paint needed | L | (2 * (length * height + width * height) - doors * 1.8 - windows * 1.2) * coats / 11 | 2
out paint_gal | Paint needed (US gal) | gal | paint_l / 3.78541 | 2
out area | Total wall area | m² | 2 * (length * height + width * height) | 2