How to use the paint calculator
- Choose metric or imperial units before entering measurements. Metric uses metres, square metres, square metres per litre and litres. Imperial uses feet, square feet, square feet per US gallon and US gallons. Changing the system restores the example so units are never silently reinterpreted.
- Measure the inside length and width of one rectangular room plus the finished wall height. Select walls, ceiling or both. The calculator unfolds the four walls as perimeter × height and calculates the ceiling as length × width.
- Add the measured combined area of doors, windows or other wall sections that will not be painted. Do not subtract openings that you plan to paint. Openings apply only to walls and cannot be larger than the gross wall area.
- Enter a whole number of coats. Then copy the coverage rate from the exact product label or current data sheet. Coverage is a per-coat area rate, so the tool multiplies the selected surface area by the coat count before dividing by coverage.
- Enter one container size that you can actually buy. Read both the exact volume and the whole-container result. The container count always rounds up so the nominal purchased volume is not below the calculated requirement.
What the paint results mean
Gross wall area is the four-wall rectangle before deductions. Net wall area removes only the opening area you supplied. Ceiling area is separate, and net paintable area is the sum of the selected wall and ceiling surfaces. Area across all coats repeats that net area once for each coat.
Exact paint volume divides coated area by the coverage rate. The tool displays both litres and US gallons for checking a quotation or label, regardless of the selected system. Exact container equivalent divides that volume by your container size; whole containers round the equivalent upward. Purchased volume therefore may be higher than exact volume because of package rounding, not because the calculator secretly added waste.
Formula: walls = 2 × (length + width) × height − openings; ceiling = length × width; paint = (selected area × coats) ÷ coverage; containers = ⌈paint ÷ container size⌉.
Worked paint examples
Metric room: walls and ceiling
A 4 m × 3 m room with 2.5 m walls has 35 m² of gross wall area and a 12 m² ceiling. Subtracting 4 m² of unpainted openings leaves 31 m² of walls and 43 m² total. Two coats create 86 m² of coated area. At 10 m²/L, the exact need is 8.6 L. With 5 L cans, that is 1.72 cans, so the purchase result is 2 cans or 10 L.
Imperial room: walls only
A 12 ft × 10 ft room with 8 ft walls has 352 ft² of gross wall area. Subtract 36 ft² of doors and windows to get 316 ft². Two coats make 632 ft². At 400 ft²/gal, the estimate is 1.58 US gal. With 1 gal cans, the result rounds up to 2 cans.
Useful planning situations
Use this estimate before comparing pack sizes, preparing a shopping note or checking a contractor quantity for one simple rectangular room. Ceiling-only mode also works for a rectangular ceiling when openings are zero.
For several rooms, irregular walls or different paints, run separate calculations and keep the measurements beside each result. Separate records make it clear which openings, coats and product rate belong to which surface.
A quoted coverage rate is nominal. Porous or textured surfaces, repairs, strong color changes, application equipment and loss in trays or rollers may change real use. Follow preparation and coverage instructions for the chosen product.
Limits and input troubleshooting
- The geometry is one rectangular room with vertical walls and a flat rectangular ceiling. Slopes, stairwells, alcoves, cabinets, trim, exterior elevations and irregular shapes are not modeled.
- There is no automatic standard door or window size. Measure and combine only the wall areas that will stay unpainted; this avoids hiding an assumption about your openings.
- The calculator does not recommend paint, primer, sheen, coats, coverage or extra allowance. Primer and topcoat with different coverage values need separate calculations.
- Use plain positive numbers without units or thousands separators. A decimal point or comma is accepted. Openings may be zero; dimensions, coverage and container size must be greater than zero.
- Results are quantity estimates, not guarantees of color coverage, finish quality, stock availability or project cost. Confirm the label, measurements and purchasable sizes before ordering.
Copying and using the estimate
The copy button creates a short summary with whole containers, exact litres and gallons, paintable area and coats. Add the product name, coverage source, container size and date to your own project note because the copied line is not a purchase order and the page does not save projects. Browser printing can preserve the displayed result, but recalculate after any measurement or product change.
Paint calculator FAQ
Does floor area tell me how much wall paint I need?
No. Floor area is length × width, while four-wall area is perimeter × height. This calculator uses the wall formula and shows ceiling area separately.
Should I subtract doors and windows?
Subtract their measured combined area only when those surfaces will not receive the same paint. Leave an opening in the total if it will be painted.
Where do I find the coverage rate?
Use the rate printed on the current product label or technical data sheet. Do not assume that two paints, substrates or application methods have the same rate.
Why is the can count higher than the exact amount?
The exact amount can include part of a container, but purchasing usually requires whole containers of the entered size. The calculator rounds that quotient up.
Can I calculate wall and ceiling paint together?
Yes only when both use the same product coverage, coat count and container size. Otherwise calculate each surface separately.
Does the result include waste or primer?
No hidden allowance is added, and primer is not included. If a separate allowance or primer is required, account for it using the relevant product instructions and a separate calculation.