How to Estimate the Quantity of Paint to Buy for a House

Ready to switch up the look of your home by painting it? That must be exciting! You want to shop for paints, brushes and other materials you may need. But exactly how many gallons of paint will do? It can be a bit of a headache when the paint bucket runs empty halfway through the project. It's even worse if you must do some paint mixing to arrive at the preferred colour.

Photo by Daniel Chekalov on Unsplash

You can make the project more fun while keeping it within budget by determining the quantity of the materials you need right from the start. It helps you save time and cost as you can buy your materials in bulk and avoid more trips to the store. There are free online tools that can help you estimate the quantity of paint you need. But if you'd like to DIY, this article will show you how to measure the volume of paint your project needs.

Calculate the Quantity of Paint Needed for a Room in Six Steps

What you need depends on the surface area of the wall that you want to cover. To estimate the volume of paint that you need for a room,

1. Measure the length of the room by adding together the length of each wall.
2. Measure the height of the room — from the floor to the ceiling.
3. Multiply your answers from steps 1 and 2 to find the total wall area.
4. Get the surface area of doors and windows.
5. Subtract the surface areas of doors and windows from that of the room since you may not want to paint them.
6. Divide this result by the paint coverage (350 ft² per gallon) to arrive at the number of gallons to purchase.

The result you get from step four is the area of the room that you need to paint.

How much paint do I need per square foot?

Generally, you can expect 1 gallon (3.79 litres) of paint to cover about 350 ft². But you can check the label on the paint bucket you want to purchase to see its coverage or spread rate. To calculate the exact quantity of paint your project needs, divide the surface area of the wall you want to paint by 350ft (or the spread rate on the label).

Working Example

Let us use a sample room of 12ft x 15ft. It is 12ft tall and has a door (8ft x 4ft) and two windows (4ft x 4ft).

Step 1: Measure the length of the room by adding together the length of each wall. 

                12 + 15 + 12 + 15 = 54ft 

Step 2: Measure the height of the room — from the floor to the ceiling 

                Height = 12ft

Step 3: Multiply your answers from steps 1 and 2 to find the total wall area

                54ft × 12ft = 684ft² 

Step 4: Measure the surface area of the doors and windows

                Door — 8ft x 4ft = 32ft²

                Windows — 4ft x 4ft = 16ft x 2 windows = 32ft²

Step 5: Subtract the surface areas of doors and windows from that of the room

                684 ft² - 32 ft²- 32 ft² = 620 ft² 

Step 6: Divide this result by the paint coverage (350 ft² per gallon) to arrive at the number of gallons to purchase

                620 ft² ÷ 350 ft² = 1.8

That is approximately two (2) gallons or 7.56 litres of paint. In Nigeria, this translates to approximately two (2) small buckets of paint.

How Much Paint Does The Ceiling Need?

Step1: Multiply the length of the ceiling by its width to get its area

                12 ft × 15 ft = 180 ft² 

Step 2: Divide the area by 350 or the square feet coverage on the label of the paint bucket

                 180ft ÷ 350ft = 0.514 gallons or 2 litres of paint.

From the calculation above, you need 0.514 gallon or 2 litres of paint for a single coat. However, ceilings typically require at least two coats of paint. Meaning you should get at least 4 litres to paint the ceiling.

Additional Information

Note that some factors can make you use more than a gallon per 350 ft². You may need to apply more than one coat depending on 

• the type and colour of the paint, 
• the specific nature of the paint colour, 
• the nature of the surface you are coating and 
• the method of application.

For example, an unpainted or drywall will absorb a lot of paint. You will use more paint when applying more than one coat. When painting walls that are unfinished, heavily patched or previously painted in a darker colour, you should expect to use more paint.

For your building projects, learn how to calculate the number of blocks you need from your floor plan and what you can build on a half plot of land. Also, see building design ideas and their cost estimate.