A 12 by 12 room with 9 foot ceilings and 40 square feet of doors and windows needs 3 gallons for two coats. Run the next room through the same fields, then add the gallon totals before you head to the paint counter.
One room's total. Add gallons across rooms before your shopping trip.
A single-room paint job is a shopping-list problem. A whole-floor repaint is a scheduling and batching problem, and the gallon count is only the first piece of it. Run each room through the fields above with its own dimensions, door and window count, and coat total, and keep a running list of gallons per room before you commit to a store trip.
| Constant | Value used here |
|---|---|
| Coverage per gallon, per coat | 350 sq ft |
| Rounding | up to the next whole gallon |
350 square feet per gallon per coat is the standard figure most manufacturer labels list for smooth, primed drywall, including Sherwin-Williams' own paint calculator. Textured or unprimed walls will use more than this figure suggests.
Take a small home repaint: a 12 by 12 bedroom, a 12 by 11 second bedroom, and a 14 by 16 living room, all 9 foot ceilings, two coats, same color. Run each through the calculator separately and you get roughly 3, 3, and 4 gallons. Add those and the honest total is 10 gallons, not "about 3 gallons times three rooms," because square footage and door and window counts differ enough per room to matter. Buying 10 gallons in one trip, ideally boxed together in one bucket for the walls that share a sightline, keeps the tint consistent room to room.
Multiply each room's gallon count by your price per gallon and add the room totals for a full-job budget. This is also where it is worth asking the paint counter about a case discount. Many stores start a discount at 4 to 6 gallons of the same product, which a single small room will not reach on its own but a combined multi-room order usually will.
Perimeter of the 12 by 12 room is 2 x (12 + 12) = 48 feet. Multiply by the 9 foot ceiling for 432 square feet of raw wall. Subtract the 40 square feet of doors and windows entered above and you have 392 square feet of paintable wall. Two coats brings that to 784 square feet of actual coverage. Divide by 350 and you get 2.24, which rounds up to 3 gallons, leaving roughly three quarters of a gallon for touch-ups later.
Enter this room's dimensions on the live page, then use this view to print the gallon total for your supplier.
Run each room through the calculator with its own wall area, doors, windows and coat count, write down the gallons for each, then add them up. Buy the combined total from the same batch code so every room's color matches, rather than making a separate trip per room.
No. Sheen is usually chosen by room use, not by convenience: eggshell or satin for bedrooms and living rooms, semi-gloss or satin for kitchens, baths and trim where scrubbing matters. Batching rooms for a shopping trip does not mean batching them onto one sheen.
Yes, if both cans came from the same product line and sheen and were mixed to the same formula code on the lid. Pour them together and stir well before use. Do not combine leftovers across different sheens or different paint lines even if the color name matches.
Run the calculator per room, multiply each room's gallons by your price per gallon, and add the totals. A whole-house repaint often qualifies for a case discount at the paint counter that a single room does not, so it is worth pricing the full job before buying room by room.
Start with ceilings across every room first if any are getting painted, then move room by room on the walls so drop cloths and tape stay in one place at a time. Painting the largest room first also confirms your gallon estimate before you commit to buying for the smaller rooms.