A paint order usually goes wrong in one of two places. Either the area came off a floor plan instead of off the walls, or the coverage figure was copied from the can without checking which unit it was printed in. Neither mistake shows up while you are buying. Both show up halfway through the second coat, on a Sunday, when the shop that mixed your color is shut. The tool on this page keeps geometry, coat count, waste allowance and coverage rate as four separate visible inputs, so you can see which one actually moved the answer before you carry anything to the till.
Measuring a room the way a painter bills it
- Run the perimeter at floor level. Hook the tape at an inside corner and run it along the base of each wall, recording every wall separately rather than trusting the plan. A 5 meter by 4 meter room has an 18 meter perimeter, and the tool rebuilds that from the length and width you enter. Tool: Tape measure.
- Take the height at three points. Measure floor to ceiling at both ends of the longest wall and once in the middle, then enter the largest of the three readings. On an out of level ceiling the gap between the tallest and shortest reading is real wall that still has to be covered. Tool: Folding rule or laser measure.
- Add the openings you will not paint. Measure the glazed pane and the door leaf rather than the frame, multiply height by width, and sum the results into the openings field. Leave the reveals and the architrave in the wall area. A 0.8 by 2.0 meter door leaf is 1.6 square meters and a 0.8 by 0.5 meter glazed pane is 0.4, so one of each is a deduction of 2 square meters. Tool: Tape measure and notepad.
- Read the coverage rate with its unit. Find the spread rate for one coat on a prepared surface and note whether the can prints square meters per liter or square feet per US gallon. Enter it in the same unit family as the room measurements, converting first when the two do not match. Tool: Product label.
- Count coats over the finished substrate. Count only the finish coats and leave primer out of the coat field, because primer is a separate product with its own spread rate. A color change over a dark wall, a patched repair and a chalky old surface each justify entering 2 rather than 1. Tool: Site notes.
What the coverage field is really asking for
Coverage is the spread rate for one coat on a prepared surface, and it is the only input on this page that is not a measurement you took yourself. It arrives from the product label, and labels are not printed in one unit. A can sold in metric markets states square meters per liter. A can sold in United States customary units states square feet per US gallon. The two are the same physical quantity wearing different clothes, and they are roughly forty times apart numerically, so a value pasted into the wrong field does not look obviously wrong. Using the factors in the table below, the default value of 10 square meters per liter is 407 square feet per US gallon. A label figure of 350 square feet per US gallon converts to 8.59 square meters per liter, and 400 square feet per US gallon converts to 9.82. Enter the converted number, not the printed one, whenever the label and your tape measure disagree.
Repainting a 5 by 4 meter bedroom in two coats
| Field | Value |
|---|---|
| length | 5 |
| width | 4 |
| height | 2.5 |
| openings | 2 |
| coats | 2 |
| coverage | 10 |
| waste | 10 |
The room is 5 meters by 4 meters with 2.5 meter walls, so the perimeter is 18 meters and the gross wall area is 45 square meters. The door leaf and the glazed pane entered in the openings field come to 2 square meters, leaving 43 square meters of paintable wall. Two coats put 86 square meters of coated area on the schedule, and the 10 percent waste allowance lifts that to 94.6. Dividing by a label rate of 10 square meters per liter gives 9.46 liters. Coats and paintable area move the result directly, while coverage moves it inversely. Reducing the coverage rate about 14.1 percent to 8.59 square meters per liter, equivalent to a 350 square feet per US gallon label, raises the estimate about 16.4 percent to 11.01 liters. The largest real uncertainty depends on wall measurement, surface condition, and the exact product label.
Result: 9.46 litres.
| From | To | Multiply by | Exact factor |
|---|---|---|---|
| gallon (U.S. liquid) | liter | 3.785 412 | No, rounded value |
| quart (U.S. liquid) | liter | 0.946 352 9 | No, rounded value |
| pint (U.S. liquid) | liter | 0.473 176 5 | No, rounded value |
| fluid ounce (U.S.) | milliliter | 29.573 53 | No, rounded value |
| square foot | square meter | 0.092 903 04 | Yes |
| foot | meter | 0.3048 | Yes |
| mil (0.001 in) | millimeter | 0.0254 | Yes |
Source: National Institute of Standards and Technology, NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically.
Convert the label figure before entering it in the coverage field. A rate of 350 square feet per US gallon is 8.59 square meters per liter, and the default 10 square meters per liter is 407 square feet per US gallon.
Buying paint in cans, not in liters
The calculator returns a continuous quantity and a shop sells discrete containers, so the last step of a paint estimate is always rounding. The worked example below asks for 9.46 liters. If the range you are buying comes in 5 liter cans, that is two cans and 10 liters on the shelf, leaving 0.54 liters of headroom that is genuinely useful for touch up later. If the same range only comes in 2.5 liter cans, it is four cans, and the shortfall from buying three is not something a careful roller technique will recover. In United States customary units the arithmetic shifts again, because one US gallon is 3.785412 liters and a five gallon pail holds 18.93 liters. There is a second reason to settle the whole quantity in one order. Tinted color is mixed in batches, and two cans mixed on different days can differ enough to show as a band across a long wall in raking light, which is why painters box multiple cans into one bucket before starting.
Federal VOC ceilings that shape the product choice
The federal rule at 40 CFR part 59 subpart D caps the volatile organic compound content of architectural coatings sold in the United States, and the ceiling depends on what the coating is. Flat coatings are capped at 250 grams per liter and nonflat coatings at 380. Primers and undercoaters sit at 350, floor coatings at 400, quick dry enamels at 450, varnishes at 450, and waterproofing sealers and treatments at 600. The limits are expressed as grams of VOC per liter of coating thinned to the manufacturer maximum recommendation, excluding the volume of any water, exempt compounds, or colorant added to tint bases. Two consequences reach the estimate on this page. The federal limit is a ceiling, and state or air district rules can set a lower one, so a product line can be reformulated for one market and not another. A reformulated version does not have to spread at the same rate as the one you used last year, which is exactly why the coverage figure has to be read off the can in front of you.
Common mistakes
- Entering the floor area of the room instead of the wall area.
- A 5 by 4 meter room has 20 square meters of floor but 45 square meters of gross wall, so the floor figure orders under half the paint the walls need. Enter length and width as the room footprint and let the tool build the perimeter, then check that the gross area it implies is larger than the floor area.
- Pasting a coverage rate printed in square feet per gallon straight into a field expecting square meters per liter.
- The two units differ by a factor of about 40, so a label rate of 400 becomes an implied 400 square meters per liter and the estimate collapses to a fraction of a can. Multiply the square feet per gallon figure by 0.092 903 04, divide by 3.785 412, and enter the result, which puts 400 square feet per gallon at 9.82 square meters per liter.
- Applying the waste allowance twice, once in the tool and again when rounding up to whole cans.
- The 10 percent allowance already added 8.6 square meters of coated area to the worked example, and rounding 9.46 liters up to 10 adds a second cushion on top of it. Treat the allowance as the working margin and the rounding as the packaging margin, and record both figures so a later change of can size can be re-costed without guessing.
- Counting a primer or sealer coat as one of the two finish coats.
- Primers are a separate category with their own spread rate and a separate federal VOC ceiling of 350 grams per liter, so their volume does not come out of the finish paint total. Run the calculation twice, once with the finish coverage rate and the finish coat count, and once with the primer rate at one coat, then order the two products separately.
- Deducting openings that will still need cutting in around the frame.
- A door leaf and a glazed sash are not painted with wall paint, but the reveal, the architrave and the band of wall the roller cannot reach still absorb material and time. Check the openings figure against the pane and leaf areas before you accept it. The same door and window measured at the frame instead, at 0.9 by 2.05 and 1.0 by 0.7 meters, deduct 2.55 square meters rather than 2, which drops about 0.12 liters off the worked example and leaves nothing for the slower brush work at every edge.
Old paint, pre-1978 homes and the point to stop
Repainting stops being a straightforward material calculation as soon as the surface underneath might contain lead. In the United States the Renovation, Repair and Painting rule covers paid work that disturbs painted surfaces in homes, child care facilities and preschools built before 1978. Work that disturbs six square feet or less of interior painted surface per room, or 20 square feet or less of exterior painted surface, is treated as minor repair and maintenance rather than a renovation. Converted with the exact factor in the table, six square feet is 0.557 square meters and 20 square feet is 1.858 square meters, which is a patch about 750 mm by 750 mm inside and about 1.36 meters square outside. Those areas are smaller than most people picture. Window replacement and demolition of painted surfaces stay covered whatever the area. Sanding back a whole wall in a house of that age is not a quantity problem, it is a certified contractor problem, and the paint volume is the least important number in the job.
Frequently asked questions
Does this estimate include the ceiling?
No. The formula builds wall area from the perimeter and the height, so the ceiling is not in it. For the worked example the ceiling is a separate 5 by 4 meter surface, which is 20 square meters, and it usually takes a different product in a different sheen. Run the numbers a second time with the ceiling area in place of the wall area, or use the wall area tool when the room shape is more complicated than four straight walls.
How do I convert a coverage rate printed in square feet per gallon?
Multiply the printed figure by 0.092 903 04 to turn square feet into square meters, then divide by 3.785 412 to turn US gallons into liters. A label rate of 350 square feet per US gallon becomes 8.59 square meters per liter, and 400 becomes 9.82. Both factors come from the conversion table on this page, and the square foot factor is an exact value rather than a rounded one.
Is a 10 percent waste allowance enough for a repaint?
It depends on what you are asking the allowance to absorb. In the worked example 10 percent adds 8.6 square meters of coated area, which is 0.86 liters, and that comfortably covers roller loading, tray residue and cutting in on a plain rectangular room. Heavy texture, a deep color change over a pale wall, or a lot of cut lines around joinery all push real consumption higher, so raise the figure rather than pretending the geometry changed.
Why does the answer swing so hard when I edit the coverage field?
Coverage is the divisor in the formula, so it acts on the whole coated area at once while the other inputs act on part of it. Holding the worked example steady at 94.6 square meters of coated area, a coverage rate of 10 gives 9.46 liters and a rate of 5 gives 18.92 liters. Halving the spread rate doubles the order. That is why the label figure deserves more checking than any measurement you took with a tape.
When should I stop calculating and call a certified contractor?
In the United States, when the building predates 1978 and the work will disturb more than six square feet of interior painted surface in a room, or more than 20 square feet outside. Those are 0.557 and 1.858 square meters, which is a small patch. Window replacement and demolition of painted surfaces are covered whatever the area. At that point the paint quantity stops being the governing number and certified work practices take over.
Related calculators
Sources
- NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically National Institute of Standards and Technology. Supplies every factor in the conversion table, including square foot to square meter as an exact 0.092 903 04 and gallon to liter as 3.785 412. Accessed 2026-09-06.
- 40 CFR Part 59 Subpart D, Table 1: VOC Content Limits for Architectural Coatings U.S. Electronic Code of Federal Regulations. Source of the VOC ceilings quoted on this page: 250 grams per liter for flat, 380 for nonflat, 350 for primers and undercoaters, and 600 for waterproofing sealers. Accessed 2026-09-06.
- Frequent question on the six square foot interior and 20 square foot exterior thresholds under the RRP Rule United States Environmental Protection Agency. Confirms the six square foot interior and 20 square foot exterior thresholds, and that window replacement and demolition of painted surfaces remain covered. Accessed 2026-09-06.
- Paint Calculators PaintCare. Backs the instruction to take accurate measurements and to check the coverage rate of the paint being bought instead of assuming a spread rate. Accessed 2026-09-06.
- Renovation, Repair and Painting Program United States Environmental Protection Agency. Source of the scope sentence on this page, that the rule reaches anyone paid to perform work disturbing painted surfaces in homes, child care facilities and preschools built before 1978. Accessed 2026-09-07.
This is a planning result, not a supplier quote; sellers use different pack, bag, board, roll, and bundle conventions, so confirm the exact product label and selling unit before you order.