How to Measure a Room for Painting

Buy the wrong number of gallons and you are back at the store mid-job

Measuring wrong costs you twice. Run short and that is a return trip, so the new gallon might not tint quite the same. But buy too much and it sits in the garage. So every number on your receipt traces back to the math below. Get it right once.

The wall, ceiling, and trim math, step by step

1. Measure every wall’s width

   First, walk the tape along the baseboard. Then write down every wall’s width separately. Also include short ones, like a closet return. So a rectangular bedroom is four numbers. But an L-shaped room is more, and each one counts.

2. Multiply by ceiling height

   Next, measure floor to ceiling in at least two spots. Older houses can drift an inch between corners. Then multiply each wall’s width by that height. Add the results together for your gross wall area. That total comes first, before you remove doors and windows.

3. Subtract the door and window openings

   Then measure the width and height of every opening. Multiply each one out. Next, subtract the total from your gross wall area. Openings vary by house age and manufacturer, so measure what is actually built into the wall. Do not assume a stock size. What is left is your net paintable wall area.

4. Give the ceiling its own line

   A ceiling is a separate surface, so it usually gets a flat finish. Flat hides imperfections overhead better than a wall sheen. Next, measure the room’s length and width. Multiply them for ceiling square footage. Then price that paint apart from the walls.

5. Give trim and doors their own line too

   Run the tape along baseboard and casing, then add up that linear footage. Also, trim paint is bought separately, usually a quart or two in semi-gloss. So it does not come out of your wall gallon count.

6. Do the coverage math

   A gallon of paint covers about 350 to 400 square feet. That is roughly a 10-by-10 room, per Sherwin-Williams’s own paint calculator. So divide your net wall area by 350, the conservative end. Then round up to the next full gallon.

7. Double it for two coats

   Most jobs need two coats to look even. That is especially true for a real color change. So multiply your gallon count by two. Also, priming over a dark color or bare drywall is a separate budget. A gallon of primer covers about 200 to 300 square feet, per Sherwin-Williams.

8. Measure with your phone instead of a tape measure

   A LiDAR room scan captures room measurements and a 3D model of the space. It runs on a Pro iPhone. Scan the room in the SimplyWise Cost Estimator, then attach that saved scan to the estimate you build for the client.

Skip the ladder and the notebook. A LiDAR room scan on a Pro iPhone attaches to your SimplyWise Cost Estimator estimate. Free to try.

Measuring is half the job

Getting the numbers right tells you how many gallons to buy. But it does not tell you what to charge for labor and prep. It does not turn square feet into a price. Instead, that lives in our guide to estimating a paint job. A free downloadable template can help too, if you would rather start from a form.

Ready to turn those measurements into a number? Price the job from a photo in about 6 seconds, with your measurements typed into the project description. Free to try.