Paint Calculator

How to Calculate How Much Paint You Need

Calculating the right amount of paint for your room doesn't have to be guesswork. Our paint calculator uses a straightforward formula that professional painters rely on every day. If you are painting fresh plasterboard, use our drywall calculator first to make sure the wall is ready for paint.

The Paint Calculation Formula

The basic formula for calculating paint is:

Paint needed = (Wall Area × Number of Coats) ÷ Coverage Rate + 10% Waste

Where wall area is calculated as: 2 × (Room Length + Room Width) × Room Height, minus the area of doors and windows.

Step-by-Step Guide

1. Measure your room — Record the length, width, and height in meters (or feet).
2. Count openings — Note how many doors and windows are in the room.
3. Choose your coats — Most paint jobs require 2 coats for proper coverage.
4. Enter the values — Use our calculator above to get instant results.

Understanding Paint Coverage

Paint coverage varies by type and quality:

Tips for Accurate Estimates

• Textured walls absorb more paint — increase your estimate by 15-20%.
• Dark to light color changes may need 3 coats or a tinted primer.
• Always buy slightly more than calculated — leftover paint is useful for touch-ups.
• Check the can — manufacturer coverage rates are printed on every paint can.
• Prefer wallpaper instead? Use our wallpaper calculator to estimate rolls for a textured alternative to paint.

Common Room Sizes and Paint Needed

Paint Types and When to Use Each

Different rooms and surfaces benefit from different paint formulations. Choosing correctly the first time saves repaints later.

Eggshell and satin sheens hide imperfections better than gloss but stay washable, which makes them the practical default for high-traffic interior walls.

Surface Preparation Drives the Real Cost

Material costs are usually 30-40% of an interior repaint; the rest is labour, prep, and consumables. Prep effort almost always dominates the timeline:

Skipping these steps is the most common reason repaints fail within 2-3 years.

Brush, Roller, or Sprayer?

Application method affects both speed and total paint volume.

For a single room, brush + roller is almost always cheaper than spraying once you factor in masking time and overspray loss.

Common Mistakes to Avoid

• Skipping the mist coat on new plaster — Plaster sucks up paint unevenly; the first coat must be diluted 30% with water or it peels off later.
• Painting in cold rooms — Most latex paints need 10°C minimum to cure. Below that, the film stays soft for weeks and marks easily.
• Reloading the brush mid-stroke — Causes streaks. Finish the stroke, then reload.
• Over-thinning to "stretch" the paint — Reduces opacity and forces a third coat anyway.
• Ignoring the manufacturer coverage rate — Listed rates assume smooth, primed walls; rough plaster needs 15-25% more.

When to Hire a Professional Painter

Consider hiring a professional if your project involves high ceilings, specialty finishes (texture, faux, or decorative painting), or multiple rooms. A professional painter can complete the job faster and with a more consistent finish. If you are also painting the outside of your house, check our exterior paint calculator for masonry and cladding estimates. Use our free quote tool below to connect with local painters.