Paint Coverage Calculator
Enter room size, ceiling height, openings and number of coats to get total area, litres needed and how many cans to buy.
Quick answer
With the default inputs: Room length 5 · Room width 4 · Ceiling height 2.8 etc.Paintable area 66.4 m²
Edit the inputs below to recalculate.Inputs
Result
Paintable area
66.4m²
| Wall area | 46.4m² |
| Ceiling area | 20.0m² |
| Primer (1 coat) | 6.6L |
| Top coat (2) | 13.3L |
| Total paint | 19.9L |
| 5 L cans | 4cans |
| 18 L cans | 2cans |
Coverage depends on substrate absorption and application method; follow the product datasheet.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Wall area | A_wall = 2 × (L + W) × H − openings |
| Ceiling | A_ceiling = L × W |
| Total area | A = A_wall + A_ceiling |
| Primer | V_primer = A ÷ coverage |
| Top coat | V_top = A × coats ÷ coverage |
| Cans | N = ceil(total volume ÷ can size) |
A 5×4 m room at 2.8 m with 4 m² of openings: 46.4 m² of wall plus 20 m² ceiling = 66.4 m². That is 6.6 L primer and 13.3 L of top coat, about 20 L total, so two 18 L cans.
How to work it out
- Measure the net perimeter, deducting fixed cabinetry that backs onto the wall.
- Use the clear height from finished floor to ceiling soffit, not the structural storey height.
- Wall area = net perimeter × clear height − opening area. Ceiling is a separate length × width, never multiplied by height.
- Multiply by the number of coats and divide by the spreading rate. Take 85% of the published rate: thinning, roller retention and substrate absorption all take a cut.
- Round up to whole containers. Base coat and top coats are costed separately — one primer plus two finish coats is standard.
Where it goes wrong
- Using floor area × 3 — only roughly valid for 2.8 m ceilings in regular residential rooms; on double-height spaces it is off by more than 30%.
- Taking the label spreading rate at face value — it is lab data on an ideal substrate; a porous render or skim absorbs half of it on the first coat.
- Skipping the primer — the primer seals the substrate, resists alkali and provides key. Without it the finish effloresces and flakes, and the rework costs far more than the primer.
- Costing dark or saturated colours at the same coverage — they usually need three coats and cannot be thinned, so allow about 40% more.
Material and cost
Contract emulsion 15-40 CNY per litre, branded interior paint 80-150, imported 200+. Labour 12-18 CNY/m² per coat; full preparation including skim and sanding brings it to 35-60 CNY/m². Figures cover paint only, not skim or labour.
On site
- Check the moisture content of the skim before painting; above 10% it will blister within months. A heater can accelerate drying but allow at least 24 hours.
- Paint from different batches differs in shade. Keep one room to a single batch and leave half a container sealed for touch-ups.
- Do not force ventilation for the first week — the film is still curing and strong draughts crack it. Below 5 °C, do not apply.
Basis
- GB 50210-2018 Code for acceptance of building decoration, Chapter 10 (painting)
- JGJ/T 29-2015 Specification for construction and acceptance of architectural coating
- GB 18582-2020 Limit of harmful substances in architectural wall coatings
FAQ
Why not just multiply floor area by 3?
Floor area × 3 is a rough rule of thumb. The real ratio depends on ceiling height and room shape: about 2.8-3.2 for a squarish room at 2.8 m, over 3.8 at 3.3 m, and more than 5 for a long corridor. Measuring the actual dimensions is accurate to within about 5%.
Is one primer plus two top coats necessary?
For new or patched walls, yes. Primer seals the substrate and evens out absorption, which saves top coat and prevents patchy colour. On sound existing walls a combined primer-and-topcoat product works. Going from dark to light needs an extra coat or a white primer first.
Why does the calculated volume come up short?
Three reasons: highly absorbent substrates such as new plaster or unsealed drywall use 20-30% more; dark colours cover about 20% less area per litre; and roller application uses less paint but often needs an extra coat. Add 10% on top of the result rather than ordering exactly.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Paint Coverage Calculator. 2026-10-25. https://jianzhujisuan.com/en/paint-coverage
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.