By RenoWorks Hampshire · Published 28/10/2026
Measure surfaces rather than floor area
For each rectangular wall, multiply its length by its height and add the results. A room's floor area does not tell you its wall area, especially with high ceilings or an irregular shape. Measure awkward sections separately and sketch the room so no wall is counted twice. Work in metres throughout. Record ceilings and woodwork separately because they may use different products and coverage rates.
Account for openings consistently
Measure substantial windows and doors if you intend to deduct them, then subtract their area from the appropriate wall total. Small openings and detailed reveals can make exact deductions less useful than they appear, so state your assumptions. Include any reveals that will receive paint. Do not deduct a door from the wall calculation and then forget to estimate the separate coating for the door and its frame.
Use the actual product's coverage
Read the coverage and coat guidance for the exact finish you will buy. Litres required equals paintable area multiplied by coats, divided by square metres covered per litre per coat. For example, an assumed 36 square metres, two coats and an assumed 12 square metres per litre gives six litres. These are worked-example inputs, not a claim about any particular paint or room.
Allow for the real background
Published coverage is a guide, not a guarantee for every surface. Texture, suction, repairs, application method and a strong colour change can affect consumption. Primer and sealer need their own calculation. Use the manufacturer's advice for an unfamiliar background and make any contingency visible rather than silently inflating every measurement. Compare available tin sizes only after calculating a sensible total; the largest tin is not automatically the least wasteful purchase.
Keep a useful decorating record
Label your room schedule with colour, finish, product and quantity, and retain batch details. If several tins will be used on one uninterrupted surface, ask the supplier about managing colour consistency. Store any remaining paint only as the manufacturer directs, securely away from children and frost where relevant. Check your local authority's disposal instructions for unusable paint; do not pour leftover coatings or cleaning residues down a drain.
Related questions
Does the calculator include primer?
Only if a tool explicitly calculates that product separately. Keep primer, wall finish, ceiling paint and trim paint as distinct quantities, each with its own coverage and coat assumptions.
Should I always add a fixed percentage?
Use a reasoned allowance for the surface and application rather than a universal number. Check product advice, note the allowance and round to suitable pack sizes after calculating the requirement.
Further reading
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