How do I calculate the wall area of a room?

Perimeter of the room (sum of all wall lengths) multiplied by the wall height gives the gross wall area in m². Then subtract the area of each door (approximately 1.8 m² each) and each window. For a typical 4m x 3.5m room with 2.4m ceiling height: perimeter = (4+3.5+4+3.5) = 15m, gross wall area = 15 x 2.4 = 36 m². Deduct 2 doors (3.6 m²) and 2 windows (2.4 m²) for a net of approximately 30 m².

Why does the first coat of paint always use more than the second coat?

The first coat on a bare or repainted surface penetrates into the substrate and is partially absorbed. On porous surfaces like bare plaster, render or raw timber, absorption can reduce the effective spread rate by 20-30% compared to the stated rate. The second coat goes over the sealed first coat and achieves the full stated spread rate. For new builds or bare surfaces, calculate the first coat at 80% of the stated spread rate and the second coat at 100%.

How much extra paint should I keep for touch-ups?

Keep at least one litre per colour stored in a cool, dark place for future touch-ups. Label the tin with the room name, colour code, batch number and date of purchase. Touch-up paint must come from the same batch -- even the same colour from a different batch can show as a slight variation when dry. For custom tints, keep a minimum of 1-2 litres depending on the room size.

Paint Coverage Calculator — Free Tradies Calculator

🎯

You're pricing an interior repaint on a three-bedroom house. The client wants two coats on walls and ceiling throughout, plus one coat of enamel on all the trim. Before you finalise the quote, you need the exact litre count for each paint type so the supplier order is right the first time.

Paint Coverage Calculator

Paint

Total area (m²)

Measure each wall H×W then add together

Number of coats

Coverage (m²/L)

Check tin label — typically 10–16 m²/L

Waste factor

Litres = (Area × Coats × Waste) ÷ Coverage Always check the coverage rate on the actual tin — it varies by product and surface porosity.

Reference: Dulux Paint Coverage Guide

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates the volume of paint required in litres from the surface area, number of coats, spread rate (m² per litre) and a waste factor. Supports metric (m²) and imperial (ft²) area input. Shows the litres needed and prompts to round up to the nearest tin size.

2 Formula & professional reasoning

Litres = (Area m² × Number of coats × Waste factor) / Spread rate (m²/L)
Waste factor: 1.0 = no waste | 1.10 = 10% waste (standard) | 1.15 = 15% (complex surfaces)
Spread rates: Economy interior: 10 m²/L | Standard interior: 12 m²/L | Premium/low-sheen: 14-16 m²/L | Exterior: 8-10 m²/L

Paint volume is driven by three factors: the area to be covered, the number of coats required, and the spread rate of the specific product. Spread rates vary significantly between products -- a cheap economy paint at 10 m²/L requires 20% more paint than a premium product at 12 m²/L for the same area. The waste factor (typically 10%) accounts for paint left in the roller tray, on brushes and cut-in losses at edges and corners. Always add waste before rounding up to the nearest tin, never after.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Single room -- walls and ceiling, 2 coats

Given: Area: 48 m² | Coats: 2 | Spread rate: 12 m²/L (standard interior) | Waste: 10%

Working: Net litres: (48 × 2) / 12 = 96 / 12 = 8.0 L | With 10% waste: 8.0 × 1.10 = 8.8 L

Answer: 8.8 litres needed -- order 10 L (2 × 5 L tins)

💡 Rounding to 10 L gives a 1.2 L buffer for touch-ups. Always round up to the nearest standard tin size (1 L, 4 L, 5 L, 10 L, 15 L) -- paint cannot be returned once tinted.

Standard

Large commercial repaint -- exterior walls, 2 coats

Given: Area: 320 m² | Coats: 2 | Spread rate: 8 m²/L (exterior masonry) | Waste: 10%

Working: Net litres: (320 × 2) / 8 = 640 / 8 = 80.0 L | With 10% waste: 80.0 × 1.10 = 88.0 L

Answer: 88 litres needed -- order 90 L (6 × 15 L tins)

💡 Exterior masonry paints have lower spread rates (8-10 m²/L) because the porous surface absorbs more paint than smooth plasterboard. A first coat on bare render can drop to 6-7 m²/L -- check the primer coat separately.

Advanced

Multi-surface house repaint -- three paint types

Given: Main walls 140 m² at 12 m²/L | Ceiling 45 m² at 14 m²/L | Trim 22 m² at 10 m²/L | All 2 coats | 10% waste

Working:

Walls: (140 × 2 × 1.10) / 12 = 308 / 12 = 25.7 L | Ceiling: (45 × 2 × 1.10) / 14 = 99 / 14 = 7.1 L | Trim: (22 × 2 × 1.10) / 10 = 48.4 / 10 = 4.8 L

Answer: Walls: 26 L | Ceiling: 8 L | Trim: 5 L | Total: 39 L across 3 product lines

💡 Calculate each paint product separately -- different spread rates and sheen levels mean you cannot mix them in one calculation. Order walls and ceiling paint first; trim enamel is typically a separate product with a lower spread rate.

4 Sanity check

Spread rate reference by surface type

Smooth plasterboard interior: 12-16 m²/L | Textured interior: 10-12 m²/L | Exterior render/masonry: 8-10 m²/L | Bare timber: 8-10 m²/L | Trim enamel: 10-12 m²/L

Always check the product data sheet -- spread rates vary by brand and product tier.

Always add waste before rounding to tin size

Apply the waste factor (10%) to the net litres first, THEN round up to the nearest tin

Rounding before adding waste leaves you short on large jobs.

Standard tin sizes

1 L, 4 L, 5 L, 10 L, 15 L (most brands) | Some products: 2 L, 8 L, 20 L

Confirm available sizes with your supplier before ordering -- not all sizes are stocked for every tint.

Imperial conversion

1 m² = 10.764 ft² | 1 US gallon = 3.785 L | US spread rates in sq ft/gallon: divide by 41.6 to get m²/L

5 Common errors

Error	Cause	Consequence	Fix
Using the spread rate from the tin without checking the actual surface porosity	Applying the manufacturer's stated spread rate to a porous or textured surface	Significantly underestimating paint quantity -- running short mid-job	The stated spread rate is for a smooth, non-porous surface in ideal conditions. Reduce by 15-25% for textured, porous or previously unpainted surfaces. A first coat on bare plaster or render will always use more paint than the second coat.
Not deducting doors and windows from gross wall area	Calculating paint from the gross wall area without subtracting openings	Overordering paint by 5-15% on rooms with multiple doors and windows	Subtract door area (approximately 1.8 m² each) and window area from gross wall area before calculating. For rooms with many openings, the saving is significant.
Ordering tinted paint without confirming the tint is the same batch across all tins	Not specifying a single tint batch when ordering multiple tins	Subtle colour variation across tins -- visible as streaks or bands on finished walls	For tinted colours, order all tins for a room in one order and ask the supplier to tint from the same base batch. For very large areas, request the supplier mix all tins together and stir before decanting.
Calculating paint by room area without separating walls from ceiling	Treating ceiling and wall area as one calculation	Using wall paint (low-sheen) on the ceiling, or ceiling paint (flat) on walls -- wrong product applied	Always calculate ceiling area and wall area separately. Ceiling paint is typically a flat white applied at a higher spread rate; wall paint is low-sheen or semi-gloss. Keep them as separate line items in the estimate.

6 Reference & regulatory links

🇦🇺 Australia

🇺🇸 United States

7 Professional workflow

Common tools used alongside this one:

📋

Job Quote Calculator

Job Quote Calculator uses paint volume and cost per litre to build the materials cost line in the quote

→

Input	Result
Smooth interior, 12 m²/L, 2 coats	Area / 6 L per m² covered
Exterior masonry, 8 m²/L, 2 coats	Area / 4 L per m² covered
10% waste	Multiply net litres by 1.10 then round up