Paint Calculator
Estimate paint quantity needed to cover any room.
About the Paint Calculator
The Paint Calculator estimates the amount of paint needed to cover the walls (and optionally the ceiling) of a rectangular room. Enter the room dimensions, the number of coats, the paint coverage, and the number of doors and windows to subtract — the tool returns the total paintable area and the liters of paint to buy.
The standard coverage figure is 10 square meters per liter of paint, which is the typical rating for interior wall paint applied at standard thickness on a non-porous surface. Porous surfaces like new drywall or bare plaster absorb more paint and reduce coverage to 6-8 m²/L; previously-painted surfaces in good condition can reach 12-14 m²/L. Always check the manufacturer’s coverage figure on the paint can.
Most paint jobs need two coats: a primer-equivalent first coat to seal the surface, and a finish coat for the final color. Dark colors over light backgrounds may need three coats; bright colors like red or yellow often need a tinted primer plus two finish coats. The calculator’s default of 2 coats matches the manufacturer recommendation for most interior wall paints.
The standard door subtraction is 1.8 square meters (a 0.8m x 2.1m interior door); a standard window subtraction is 1.4 m² (1.2m x 1.2m). These are conservative defaults — if your doors or windows are significantly larger, override the calculations by subtracting their actual area from the wall area first.
How It Works
The wall area is computed as the perimeter times the wall height: 2 * (length + width) * height. This is the gross wall area before subtracting openings.
If ceiling painting is enabled, the ceiling area is length * width — the floor area of the room.
Door and window areas are subtracted from the gross wall area. Each door is assumed to be 1.8 m² (0.8m x 2.1m, standard interior door). Each window is assumed to be 1.4 m² (1.2m x 1.2m). For non-standard sizes, calculate the actual area and adjust the door/window counts accordingly.
The net paintable area is max(0, wall + ceiling - doors - windows). The max(0, ...) prevents a negative area if the user enters more doors and windows than the wall area would support.
The total area to paint is net area * coats, since each coat covers the same surface. The liters needed is ceil(total_area / coverage) — rounded up to the nearest liter, because paint is sold in whole units and you cannot buy 0.7 liters.
The gallon conversion uses 1 L = 0.264172 US gallons. For Imperial gallons, multiply liters by 0.219969 instead. The square-foot conversion uses 1 m² = 10.7639 ft².
Edge cases: zero dimensions are rejected. A coat count above 10 is rejected (likely a typo). A coverage figure below 1 is rejected. The ceiling-paint option is a checkbox; if unchecked, only walls are painted.
Worked Examples
Default: 5x4m room, 2.4m height, 2 coats, 10 m²/L coverage, ceiling painted, 1 door, 2 windows. Wall area: 2 * (5+4) * 2.4 = 43.2 m². Ceiling: 20 m². Doors: 1.8 m². Windows: 2.8 m². Net: 43.2 + 20 - 1.8 - 2.8 = 58.6 m². Total: 117.2 m². Paint: ceil(117.2 / 10) = 12 L. A typical small bedroom or office.
3x3m bathroom, 2.5m height, 3 coats (dark color), 8 m²/L coverage (porous tile), ceiling excluded, 1 door, 1 window. Wall: 45 m². Doors: 1.8. Windows: 1.4. Net: 41.8. Total: 125.4. Paint: ceil(125.4 / 8) = 16 L.
8x6m living room, 2.7m height, 2 coats, 12 m²/L coverage (previously painted), ceiling painted, 2 doors, 4 windows. Wall: 2 * 14 * 2.7 = 75.6 m². Ceiling: 48. Doors: 3.6. Windows: 5.6. Net: 114.4. Total: 228.8. Paint: ceil(228.8 / 12) = 20 L.
4x4m bedroom, 2.4m height, 1 coat (touch-up only), 10 m²/L, ceiling excluded, 1 door, 1 window. Wall: 38.4. Doors: 1.8. Windows: 1.4. Net: 35.2. Total: 35.2. Paint: ceil(35.2 / 10) = 4 L.
When to Use This Tool
- Estimating paint needed for a room refresh before a trip to the hardware store.
- Comparing the cost of repainting vs. hiring a professional (labor is typically 70-80% of a pro job).
- Planning a multi-room repaint by running the calculator per room.
- Checking that the paint quantity quoted by a contractor matches the area-based estimate.
- Deciding between 1-gallon and 5-gallon containers based on total liters needed.
- Estimating paint for an exterior wall (use outdoor coverage figures, typically lower).
- Calculating primer and finish coat quantities separately for new drywall.
Limitations & Disclaimer
The calculator assumes a rectangular room with flat walls and uniform ceiling height. Rooms with sloped ceilings, bay windows, alcoves, or non-rectangular floor plans require manual area calculation. The door (1.8 m²) and window (1.4 m²) defaults are standard interior sizes; oversized openings need manual adjustment. Coverage varies by paint type, surface porosity, and application method — the 10 m²/L default is a manufacturer’s typical rating, not a guarantee. Trim, baseboards, doors, and window frames are not included; calculate them separately with enamel paint coverage figures. See our disclaimer for full terms.
Frequently Asked Questions
How accurate is the 10 m²/L coverage figure?
It is the manufacturer’s typical rating for interior wall paint applied at standard thickness on a non-porous, previously-painted surface. New drywall or bare plaster absorbs more paint and reduces coverage to 6-8 m²/L. Rough surfaces like stucco or brick reduce coverage further. Always check the coverage printed on the specific paint can you plan to buy — it varies by brand and product line.
How many coats do I really need?
For repainting over a similar color in good condition: 1 coat may suffice. For repainting over a different color: 2 coats is standard. For dark colors like red, navy, or black over white: 3 coats plus a tinted primer is typical. For new drywall: 1 coat of primer plus 2 finish coats. For glossy finishes over matte: 2 coats to even out the sheen.
Should I subtract doors and windows?
Yes — subtracting openings prevents buying too much paint. The standard door is 1.8 m² (0.8m x 2.1m); the standard window is 1.4 m² (1.2m x 1.2m). For oversized doors (sliding glass doors, French doors), use 3-4 m² each. For non-standard windows, measure and enter the actual area as the door count (since the calculator treats each ‘door’ as 1.8 m²).
Do I need primer?
For new drywall, bare plaster, or a significant color change (e.g. dark red over white), primer is essential. For repainting over a similar color in good condition, you can often skip primer. The calculator does not model primer separately; if you need primer, run the calculator twice — once with primer coverage (typically 8-10 m²/L) and once with finish paint coverage.
How much extra should I buy for touch-ups?
Buy 10-15% extra above the calculated amount, or round up to the next can size. Paint color can vary slightly between batches, so buying all your paint at once (in the same batch) is essential for color consistency. Leftover paint keeps 5-10 years if sealed properly, so excess is not wasted — it is touch-up insurance.
What about painting trim and baseboards?
Trim, baseboards, doors, and window frames use a different paint (typically gloss or semi-gloss enamel) and have a different coverage rate (typically 12-14 m²/L). Calculate trim area separately: perimeter of the room times 0.1m (typical baseboard height) plus door and window frame perimeters times 0.05m. Run the calculator a second time for trim.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team