Paint Calculator

1. Calculating how much paint you need
2. Paint calculation example
3. Basic painting tips

    Calculating how much paint you need

In order to calculate how many gallons or litres of 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 tool 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 how much paint do I need 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. 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 output will include the total price of the paint required. The tool rounds up to the closest gallon or litre, but you might want to order more material just to be on the safe side.

    Paint calculation example

One needs to paint two rooms top to bottom, excluding the ceiling and needs to know how many gallons of paint to purchase. Both rooms have two windows each 3 feet wide by 4 feet high. Each room also has a door with standard dimensions (19.4 square feet or 1.8 square meters). The dimensions of the first room are 12 feet by 15 feet, and the second is 12 feet by 18 feet. Both are 8 feet high. What amount of paint is needed to cover all walls with two coats of paint?

First, calculate the total internal surface area of the rooms: (8 x 12) x 2 + (8 x 15) x 2 = 192 + 240 = 432 square feet for the first one; (8 x 12) x 2 + (8 x 18) x 2 = 192 + 288 = 480 square feet for the second room. In total: 912 square feet.

Second, calculate the total surface area of the windows: 4 x (3 x 4) = 48 sq ft. and then add the two doors: 48 + 2 x 19.4 = 86.8 sq ft.

Third, subtract the surface area of the windows and doors from the total initially calculated to get how the paint needs to cover: 912 - 86.8 = 825.2 square feet.

Finally, if the paint is known to cover 108 ft² per gallon, and given that two coats are needed, divide the square footage by the paint coverage, then multiply by the number of coats to get: (825.2 / 108) x 2 = 7.64 x 2 = 15.2 gallons needed for the two rooms. Since usually you can purchase paint in whole gallons only, and also since one should always buy a bit more to be on the safe side, 16 gallons of paint need to be bought for this interior painting project.

    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 more reason to use a calculator as making sure the amount needed is calculated precisely means you won't run out and need to mix second batch.

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 work 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 work on the ceiling first, then on the walls. This makes sure that any 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 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 it is fully dry, a mess is guaraneteed, 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 tips and our online paint calculator you should be ready for a paint job to be proud of.