Paint Coverage Calculator
Turn room size and coats into paint cans
Paint coverage, coats and cans FAQ
How many square feet does a gallon of paint cover?
Most interior wall paints cover roughly 250–400 sq ft per gallon for one coat, depending on colour, sheen and surface texture. Always use the coverage printed on your can and type that number into the calculator so the estimate matches the brand you are buying.
Do I really need two coats?
Two coats are standard for good colour evenness and durability, especially when you change colour or paint over patched walls. Some high-hiding products may cover in one coat on light colours, but many manufacturers still recommend two. If in doubt, leave coats at 2 and treat any extra paint as future touch-up stock.
Should I subtract doors and windows?
Subtracting door and window area makes the estimate tighter. Leaving them in gives you a little extra paint for cutting in, touch-ups and future repairs. For most small rooms you can skip the subtraction; for larger jobs with many windows and doors it is worth entering a total openings area.
Does this include ceilings, trim or primer?
No. This calculator is tuned for walls only. Ceilings, trim, doors and primer are often different products with their own coverage rates. You can still use the same maths: measure the area you want to paint, multiply by coats, then divide by the coverage on that specific can.
What if my room is not a perfect rectangle?
Many rooms are L-shaped or have small jogs. A simple trick is to break them into a couple of rectangles, run the calculator for each, and add the paint totals together. For very complex spaces, using a tape measure and sketch on paper plus this tool will still get you close enough to avoid multiple store trips.
How much extra paint should I keep?
Many people like to keep 10–20% extra beyond what the maths suggests for future touch-ups. Because this calculator rounds up and you might not subtract every opening, you will usually end up with at least a small spare amount if you follow the estimate.
How to use this paint coverage calculator
The idea is to keep it simple: measure the room once, check the coverage on your can, and turn that into a total number of gallons or litres for all coats.
1. Pick US / Imperial or metric
Choose whether you measure in feet or metres. The labels on the boxes update so you always type length, width, height and opening area in a single system.
2. Measure the room
Measure room length and width wall-to-wall, then measure the wall height from floor to ceiling. The calculator assumes four walls at the same height and uses the perimeter to get total wall area:
- Perimeter = 2 × (length + width)
- Wall area = perimeter × height
3. Decide whether to subtract doors and windows
If you want a tighter estimate, add up the area of doors and windows you won’t paint and enter that as “Doors & windows area”. If you prefer a small safety margin, leave the box blank and the tool will treat every wall as a full rectangle.
4. Enter coverage and coats
Look at the fine print on your paint can to find the coverage rate (for example “350 sq ft per gallon” or “10 m²/L per coat”). Type that into the coverage box and set the number of coats you plan to roll. Two coats is a good default for most walls.
5. Read paint needed in gallons and litres
When you tap Estimate paint needed, the summary shows:
- Total wall area before and after subtracting openings.
- Net area per coat and across all coats.
- Total paint for every coat in gallons and litres, both exact and rounded up.
- A short note reminding you that real coverage can vary with surface and application.
The layout stays short so you can screenshot it or read it off your phone while you are in the paint aisle.
6. Copy the summary into your notes or quote
Use the Copy summary button to paste all the numbers into a notes app, email or quote request. That way you and your painter or supplier are always looking at the same figures when you talk about how many cans to buy.
How the paint coverage maths works
The calculator uses the same wall-area maths paint brands and DIY guides use: find the wall area, subtract openings, multiply by coats, then apply the coverage from your can.
1. Wall area from room size
With a rectangular room:
Perimeter = 2 × (length + width)
Wall area = perimeter × wall height
In US mode, this gives area in square feet (ft²); in metric mode it gives square metres (m²).
2. Subtracting doors and windows
If you enter an openings area, the tool subtracts it from the wall area:
Net area = wall area − openings area
If the openings box is blank, net area is the full wall area, giving you a built-in safety margin.
3. Coats and total coverage
For C coats:
Total coverage area = net area × C
4. Applying the coverage rate
Let R be the coverage rate from your can:
- US: R is in sq ft per gallon (ft²/gal).
- Metric: R is in m² per litre (m²/L).
Then:
Paint needed (gallons or litres) = total coverage area ÷ R
The calculator shows both the exact number and a rounded-up figure so you know how many full cans to buy.
5. Converting between sq ft, m², gallons and litres
To keep things easy to compare, the tool also converts between units:
- 1 ft² ≈ 0.092903 m²
- 1 m² ≈ 10.7639 ft²
- 1 US gallon ≈ 3.785 litres
That way you can measure in metres, but still think in gallons, or the other way around, depending on how your local paint store sells product.
References and further reading on paint coverage
These resources follow the same basic area and coverage ideas used in this calculator:
- Sherwin-Williams — Paint calculator — room-size based estimates and coverage guidance.
- Behr — Paint & stain calculator — examples of wall area, coats and coverage per product.
- Benjamin Moore — Paint calculator — uses wall dimensions and assumes two coats for quick estimates.
- Lowe’s — Paint calculator — explains typical 350–400 sq ft per gallon coverage ranges.
- Behr — How much paint to buy — real-world notes on coverage, coats and surface conditions.
Use this calculator as a fast planner, then confirm final paint quantities with the coverage printed on your specific cans and any advice from your paint supplier.