Calculator
How much will painting my room actually cost?
Measure the room, take off the doors and windows, and see how much paint to buy. Add the prices you expect to pay for cans, primer, and supplies. The math stays in your browser. Nothing here is a store price or a product link.
Result
Add the room size to start.
Length, width, and height are enough for a rectangular room. Doors, the ceiling, primer, and supply prices can wait.
- Still needed: length
- Still needed: width
- Still needed: height
A room you can check by hand
Take a 12 ft by 10 ft room with 8 ft walls. Leave the ceiling out. Subtract one 21 sq ft door and one 15 sq ft window. Paint two coats at 400 sq ft per gallon, and add a 10% waste allowance.
Walls
2 × (12 + 10) × 8 = 352 sq ft.
After openings
352 − 21 − 15 = 316 sq ft. That is the paintable wall area.
Before waste
316 × 2 ÷ 400 = 1.58 gallons.
After 10% waste
1.58 × 1.10 = 1.738 gallons.
Cans
One-gallon containers round 1.738 up to 2. Quarts and larger pails use the same quantity and a different count.
The button on the calculator fills those measurements. It does not fill a price. A cost appears only after you type one.
Questions
How is the wall area calculated?
For a rectangular room, the walls are the perimeter times the height: 2 × (length + width) × height. A 12 ft by 10 ft room with 8 ft walls is 2 × (12 + 10) × 8 = 352 sq ft before any doors or windows.
Why subtract doors and windows?
You do not paint the opening. Each door or window is removed once from the wall area, and the coats are applied to what remains. A 21 sq ft door and a 15 sq ft window take the 352 sq ft example down to 316 sq ft.
How does the waste allowance work?
It is added on top of the amount the coverage rate implies. Ten percent means multiply by 1.10. In the example, 1.58 gallons becomes 1.738 gallons. It is not a store fee, and it is not applied a second time at the register.
Why buy more paint than the exact amount?
Cans are sold whole. 1.738 gallons in one-gallon containers rounds up to 2 containers, with 0.262 gallons left in the last can. Choose a quart, a gallon, a 5 gallon pail, or a litre size and the count changes. Type a size if yours is different.
Should the ceiling be included?
Only if you are painting it. The ceiling is length × width, and it uses the same coats, coverage, and waste allowance as the walls. It is left out of the 316 sq ft example. Openings are not subtracted from the ceiling.
What if the walls are different sizes?
Open Advanced options and measure each wall. The calculator adds width × height for every wall, then subtracts doors and windows. Those walls replace the rectangular-room formula. They are not added on top of it.
Can I use metres, and can I price it in Canadian or US dollars?
Yes. Use feet, square feet, and gallons, or metres, square metres, and litres. CAD and USD only label the prices you type. The calculator does not convert currencies or look up an exchange rate. Switching units converts measurements you have already entered.
Where do the prices come from?
From you. A blank price counts as zero so the quantity can still be calculated. This site does not check what a can costs in Canada or the United States, and it does not list products for sale.
What about primer and supplies?
Primer is off until you turn it on. It uses the same paintable area, with its own coats, coverage, container, and price, and the same waste allowance. Brushes and rollers, painter's tape, trays and drop cloths, and any other preparation cost are amounts you enter. They appear on the shopping list so you can check off tools you already own.
Does a lower total mean I should paint it myself?
No. This page is materials and supplies. It does not price your time or a painter's quote. If you have a quote, open the DIY vs. hire calculator and put this project's total in as the material cost.
What the formula leaves out
Rectangular room, unless you measure walls
Wall area is 2 × (length + width) × height. Individual walls replace that formula. They are not added on top of it.
Openings come off once
Each door and window is subtracted once from the wall area. Coats multiply what remains. Openings are not subtracted from the ceiling.
The ceiling shares the paint
If you include it, length × width is added and painted with the same coats, coverage, and waste allowance as the walls.
Waste is added on top
Paint after waste is the coverage figure multiplied by (1 + the waste percentage). Ten percent multiplies by 1.10. It is not a second, hidden deduction.
Cans round up
You cannot buy a fraction of a container. The count is the paint after waste divided by the container size you enter, rounded up. Leftover in the last container is shown.
Prices are yours
A blank price counts as zero. The site does not look up store prices, taxes, or what is in stock in Canada or the United States. CAD and USD are labels only.
Primer is the same area
When primer is on, it uses the paintable area with its own coats, coverage, container size, and price. It is not added to the wall area.
Labour, tax, and a painter's quote are not in this total unless you type them into a price. The DIY vs. hire calculator is the place to put a quote next to your time. The method page keeps the formulas, and the disclaimer still applies.