Use this calculator to estimate how much paint in gallons or litres you would need to paint a room or external surface. This paint calculator supports doors and windows, number of coats and custom paint coverage efficiency. Internal painting calculation excludes the ceiling.
Calculating how much paint you will need
In order to calculate how much paint you need to paint a room, house, or an exterior surface, you need to either know the measurements by plan, or have a long measurement tape ready (10 or 20 meter / 33 or 65 foot long is advised) before you can use this paint calculator. If you are painting an interior surface, you need to know the room's width, length and height. Rooms with non-rectangular layout can be approximated to rectangles. If you need to paint a ceiling or external wall, switch to "External" in the calculator and just enter the total width and height of the wall (or width and length instead, for a ceiling).
Usually the room or exterior will have openings for windows and doors, which need to be excluded from the total surface area. Accurate measurement of the windows is important, especially if there are many of them. Our calculator only supports one set of window dimensions, so if you have a room with multiple windows of highly different dimensions, you'd need to use the average. In this calculator door dimensions are assumed to be close to construction standards (200mm by 90mm).
The other important thing to consider is the paint coverage efficiency (a.k.a. spreading rate): how many square meters or square feet you can cover with a litre or gallon of the paint. Manufacturers usually provide this information on the product label. Different paints and primers have different coverage capacity so make sure you use the right number as input in the calculator. You'd want to allow for a lower efficiency in case the surface is not smooth. More coarse surfaces require more paint to cover.
Finally, if you enter the paint price per gallon or per litre the paint calculator will output the total price of the paint required. We round up to the closest gallon or litre, but you might want to order more paint just to be on the safe side.
Basic painting tips
In case you are doing the painting yourself here are some DIY tips and painting techniques that will help you get that perfect paint job.
Mix paint once in large quantity
Due to slight differences in paint color from one can to the next and to imperfect proportions you want to avoid mixing the paint in several different containers. Instead, what you want to do is get a large, say 5-gallon (20 litres) bucket and mix all the paint for a room in it. This will ensure consistent color across all surfaces. This is one reason to use a paint calculator as making sure the amount of paint needed is calculated precisely means you will not need to mix paints a second time.
You should also consider ordering your paint pre-mixed as this will ensure top quality and even color, even if it arrives in separate smaller containers.
Clean dirty surfaces before you paint
Painting over dirty, oily surfaces like the ones often found near switches and doors will result in paint not bonding well with the wall surface making it easier to chip or peel off. To improve adhesion, it's best to threat grimy areas with a deglosser or heavy-duty cleaner for prepaint cleaning.
Roll the full height and be quick
In order to avoid ugly stripes of uneven layers of paint buildup you need to avoid rolling over paint which is partly dry. It is called "maintaining a wet edge": each stroke overlaps the previous one before it has a chance to dry, which can happen in a mere minute or two in dry and hot environments.
The way to achieve this is to roll from top to bottom, covering the full height of the wall before proceeding to your left or right. The open side of the roller has to face the part of the wall you just painted so that there is less pressure on it, making it less likely to leave stripes. The roller should be always wet enough: regular dipping into the paint should ensure that. You must not attempt to paint with a dry or nearly dry roller.
Use primer to avoid a blotchy finish
To avoid inconsistent sheen due to filler used for filling cracks and holes, apply a thick coat of primer before painting with the actual paint. It will eliminate any flashing and texture differences of the underlying surface.
Follow a painting order
If there are wooden trims on the base of the walls, paint those first and let them dry for 24-48h. Then tape the trim and any window/door frames and paint the ceiling first, then the walls. This makes sure that any paint splatters produced while working on the ceiling will later be covered by paint.
Cut the tape for razor-sharp edges
Once you've done painting (hopefully computing the required amount of paint accurately!) you do not want to just rip off the tape off trim and frames. Instead, use a sharp utility knife to cut the tape while you pull it off to get a great edge. The paint should be hard enough to slice cleanly. If you attempt this before the paint is fully dry it will be a mess while if you do not use a knife little pieces of paint will fall off during the operation, ruining the edge.
For extra-smooth trim finish sand between coats
Two or three coats of paint are usually required to cover underlying color and sheen with paint itself producing a slightly grainy texture on the trim. To avoid that, use a fine-grit sanding sponge before painting and after each coat. After each sanding use a vacuum cleaner followed by a tack cloth wiping to remove the resulting dust.
Armed with these painting tips and our online paint calculator you should be ready for a paint job to be proud of.
Cite this calculator & page
If you'd like to cite this online calculator resource and information as provided on the page, you can use the following citation:
Georgiev G.Z., "Paint Calculator", [online] Available at: https://www.gigacalculator.com/calculators/paint-calculator.php URL [Accessed Date: 05 Jul, 2020].