How much paint do
you actually need?
Enter your room dimensions, number of coats, and finish. Get the exact amount needed and estimated cost - no guesswork at the paint store.
How Much Paint Do You Need? A Complete Guide
One of the most common and costly mistakes in any painting project is buying the wrong amount of paint. Buy too little and you face a mid-project run to the store, risking a colour mismatch between batches. Buy too much and you waste money on paint that sits in a garage for years. A reliable paint calculator solves this entirely.
The free room paint calculator above gives you an exact amount based on your specific room dimensions, the number of coats, your chosen paint finish, and a customizable waste allowance in both metric and imperial units, with USD, GBP, and EUR cost estimates.
How to Calculate How Much Paint You Need
The formula behind any paint calculator is straightforward. Here is how it works step by step:
Step 1: Calculate total wall area
Multiply the perimeter of your room (width + length × 2) by the ceiling height. This gives you the gross wall area. For a standard 4m × 5m room with 2.5m ceilings, that is (4+5) × 2 × 2.5 = 45 m². In imperial, a 13ft × 16ft room with 8ft ceilings gives (13+16) × 2 × 8 = 464 sq ft.
Step 2: Subtract doors and windows
A standard door is approximately 20 sq ft (1.9 m²) and a standard window is approximately 15 sq ft (1.4 m²). Subtracting these from your gross area gives the net paintable surface.
Step 3: Apply coverage rate and coats
In metric, paints are rated in m² per litre. In imperial, coverage is measured in square feet per gallon, typically 350–400 sq ft/gal for quality interior paints. Divide your net area by the coverage rate, then multiply by the number of coats.
Step 4: Add a waste allowance
Always add 10–15% extra to account for spills, brush loading, and touch-ups. For textured or porous surfaces like bare plaster or brick, increase this to 20%.
Paint Finish Guide - Which One to Choose?
- Matt / Flat - hides imperfections, ideal for ceilings and bedrooms. Best coverage rate.
- Eggshell - subtle sheen, good for living rooms. Slightly easier to clean than matt.
- Satin - the most popular all-rounder for kitchens, hallways, and children's rooms.
- Semi-gloss - light-reflective and washable. Best for trim, doors, and bathrooms.
- Gloss - most durable. Ideal for skirting boards, window frames, and furniture. Lowest coverage rate.
Tips for a Professional Paint Result
- Always prime bare surfaces. New plaster or freshly filled walls are porous and will absorb your first coat unevenly.
- Buy from the same batch. Batch numbers are printed on paint tins. Buy all the paint you need in one go to guarantee colour consistency.
- Roll in a W pattern. Work in large W shapes and fill in this gives more even coverage and reduces lap marks.
- Cut in before rolling. Use a brush to paint a 5–7cm band along edges and trim first, then roll while still wet.
- Wait for full drying between coats. Water-based paints need 4–6 hours; oil-based paints may need 24 hours.
Why Use a Paint Calculator Before You Buy?
A paint calculator takes the mental arithmetic out of the equation and accounts for variables most people forget like ceiling inclusion, the reduced coverage of gloss paints, and the waste that always happens on a real job. The Decorateva paint calculator works for both metric and imperial measurements, and supports USD, GBP, and EUR cost estimates making it useful whether you're decorating in the US, UK, or Europe.
