How this room paint calculator works
For a rectangular room, the four-wall area is the room perimeter multiplied by wall height: 2 × (length + width) × height. If the ceiling is selected, the calculator adds length × width. It then subtracts the door and window areas you entered, multiplies the remaining area by the number of coats, divides by the paint's stated coverage and adds the selected allowance.
The result is the estimated volume required before rounding to retail containers. The can comparison then tests combinations of the two sizes and prices you entered. It recommends the lowest-cost combination that covers the estimate; if two combinations cost the same, it favors the one with less leftover paint. This is more useful than simply rounding everything up to one large can.
Use the coverage printed on the exact product
Paint coverage is not universal. Product type, surface porosity, texture, application method and color change can all affect the amount needed. The default 10 m² per litre is only a starting point. B&Q's wall calculator also uses 10 m² per litre, while paint manufacturers publish product-specific figures. Replace the default with the number on the tin, technical data sheet or retailer listing for the exact paint you plan to buy.
Do not mix metric and imperial coverage. In metric mode enter square metres per litre. In US customary mode enter square feet per US gallon. The unit switch converts the current measurements and container sizes so the physical room does not change.
What to include and subtract
| Part of the project | How to handle it |
|---|---|
| Four walls | Enter internal room length, width and wall height. Irregular rooms can be split into smaller rectangles and calculated separately. |
| Ceiling | Select it only if the same product, coverage and coat count apply. Run a second estimate when the ceiling uses different paint. |
| Doors and windows | Subtract openings that will not be painted. Edit the average area rather than relying on the defaults when openings are unusually large or small. |
| Trim and doors being painted | Calculate separately because enamel or trim paint normally has a different product, coverage and container size. |
| Feature walls | Calculate the wall as a separate rectangle if it uses a different color or product. |
Choose a realistic number of coats
One coat may be enough for a compatible refresh when the manufacturer allows it, but strong color changes, repairs, porous surfaces and some products may require more. A label's coverage figure may describe one coat, so multiplying by two coats approximately doubles the surface area the paint must cover. Primer is not automatically included because primer and finish paint often have different coverage and prices.
An allowance helps with roller loading, cutting in, small spills and future touch-ups, but more is not always better. Enter zero if you want the strict mathematical minimum. Use a modest allowance for a simple measured room and a larger one only when the layout, texture or application makes waste more likely. The calculator shows leftover after the suggested can purchase so you can see the trade-off.
How to compare cans without false savings
Enter the two container sizes and current prices shown by the retailer. A larger can can have a lower price per litre or gallon, yet still cost more overall if the project needs only a small amount. The recommendation minimizes the entered purchase price while still buying enough paint. It does not assume that a retailer will accept returns or that leftover mixed or tinted paint has resale value.
Before ordering, confirm that both sizes contain the same product, sheen, base and color. Check whether the listed price includes tinting and tax, and whether promotional pricing requires a membership or minimum purchase. No retailer price is built into this tool, so the result does not become outdated when local prices change.
Accuracy and safety limits
This calculator estimates quantity, not surface condition. Peeling coatings, moisture problems, repairs and unknown older paint may need assessment before sanding or preparation. Follow the product label, ventilation guidance and local rules. In the United States, the EPA's Renovation, Repair and Painting program applies specific lead-safe requirements to covered work in pre-1978 homes, child-care facilities and preschools. Other countries have their own rules.
Sources and calculation references
- B&Q wall painting calculator — example of the area, exclusions, coat-count and 10 m²/L planning method.
- Behr paint calculator — manufacturer example covering interior, exterior, stain and floor-coating projects.
- U.S. EPA Renovation, Repair and Painting Program — U.S.-specific lead-safe information for covered older properties.
Frequently asked questions
How much paint do I need for one room?
Measure the room rather than relying on a bedroom-size rule. Room perimeter, wall height, openings, coats and product coverage determine the estimate. Two rooms with the same floor area can have different wall areas.
Should I buy exactly the calculated amount?
Paint is sold in fixed container sizes, so an exact match is uncommon. Use the can-mix result as a shopping estimate, then confirm stock sizes, product coverage and return rules with the retailer before purchase.
Does the calculator include primer?
No. Run primer as a separate calculation using the primer's own coverage, coat count, can sizes and prices.
Can I use this for exterior walls?
You can calculate rectangular areas, openings, coats and coverage, but complex elevations, trim, siding profiles, access and weather exposure need project-specific judgment. Split each elevation into measured rectangles and use the exterior product's stated coverage.
This calculator is independent and currently contains no affiliate links or paid placements. It does not promise savings or a perfect quantity. Any future commercial relationship will be clearly labeled and will not change the calculation method or editorial criteria.