Two rooms, same walls to refresh, one trip to the paint counter. That is a very common job: the living room and a bedroom between one move-out and the next move-in, or two rooms that got scuffed in the same year. People who plan it tend to spend their energy on one question, which is whether to buy for both rooms at once or to treat them as two small jobs. I worked it through on an example pair of rooms and that question turns out to be worth one quart of paint, at most. The question people skip, one coat or two, is worth between one and three-quarter gallons and three gallons on the same walls. So this guide does the coats first and the buying second, in that order, because that is the order the numbers rank them.
I'll do everything by hand first. Then I'll show where the calculator on this site agrees, and the one place where it needs you to do the adding.
The two rooms
These are measured for this example. They are not from a source, and your rooms will differ. The point is the method, which carries over.
The living room is 12 by 14 feet with 8-foot walls. Walk the perimeter: 12 + 14 + 12 + 14 = 52 feet. Times 8 feet of height, that is 416 square feet of gross wall.
The bedroom is 10 by 11 feet, same 8-foot walls. Perimeter 10 + 11 + 10 + 11 = 42 feet, times 8 = 336 square feet.
Both rooms together: 94 feet of perimeter, 752 square feet of gross wall.
The openings, measured instead of guessed
The paint calculator here has one field for doors and windows, and it starts at 20 square feet. Its help text says "Subtract only openings you measured." I take that literally, so every opening in this example has a tape measure behind it.
Each room has one door, 3 feet by 6 feet 9 inches. That is 3 × 6.75 = 20.25 square feet, the same door as in the bathroom guide.
The living room has two windows at 3 by 4 feet, 12 square feet each, so 24. With the door, 20.25 + 24 = 44.25 square feet of opening.
The bedroom has one window at 3 by 4, so 12. With the door, 20.25 + 12 = 32.25.
Subtract, and the wall you actually paint is:
| Gross wall | Openings | Net wall | |
|---|---|---|---|
| Living room | 416 | 44.25 | 371.75 |
| Bedroom | 336 | 32.25 | 303.75 |
| Both | 752 | 76.50 | 675.50 |
675.50 square feet is the number the rest of this guide runs on. The allowance I use in every guide on this site is 10 percent, so the paint to buy per coat is 675.50 × 1.10 = 743.05 square feet of coverage, and the question is how many gallons that is.
The label decides the gallon
Coverage per gallon is printed by the maker, and the makers don't agree with each other. The three figures already on this site are Sherwin-Williams' calculator page ("A gallon of paint typically covers about 350-400 square feet"), Glidden's ("1 gallon of paint typically covers up to 400 sq. ft. on primed, smooth, non-porous surfaces"), and a Behr product page ("250-400 Sq. Ft. Coverage per Gallon"). So I run four labels: 250, 350, 400, and 450. The 450 is above every label on file here. It is in the tables as the best case only, not as a figure any of these makers promise.
Gallons for one coat = 743.05 ÷ label. Then round up to what the shop sells. I'm assuming gallons and quarts are both on the shelf, and a quart is a quarter of a gallon.
One coat or two
This is where the size of the job is actually decided.
Benjamin Moore's technical data sheet for its Regal Select line puts it plainly: "two coats are required; previously painted surfaces can be finished with 1 or 2 coats." Glidden's calculator page adds the other half: "Drastic color changes typically require a second coat."
Read together, those two sentences give you a rule you can apply to a room. If the walls are already painted and you're putting the same color, or a close one, back on, you are in the "1 or 2 coats" case, and one coat is at least on the table. If you're changing the color a lot, you are in the second-coat case. Note what neither sentence says. Neither one mentions rentals, move-outs, or saving money, and I'm not going to put those words in their mouths. A rental repaint gets one coat only when the wall and the color allow it, the same as any other repaint.
Here is what the choice costs on these two rooms, bought as one order:
| Label (sq ft/gal) | One coat | Two coats | Extra for the second coat |
|---|---|---|---|
| 250 | 2.972 gal → 3 gal | 5.944 gal → 6 gal | +3 gal |
| 350 | 2.123 gal → 2 gal + 1 qt | 4.246 gal → 4 gal + 1 qt | +2 gal |
| 400 | 1.858 gal → 2 gal | 3.715 gal → 3 gal + 3 qt | +1.75 gal |
| 450 | 1.651 gal → 1 gal + 3 qt | 3.302 gal → 3 gal + 2 qt | +1.75 gal |
The second coat is never less than 1.75 gallons here. At the low label it is 3 whole gallons. On a job of this size, that is the difference between carrying out two cans and carrying out six.
One small thing in the table is worth pointing at. At 400 and 450 the extra is the same, 1.75 gallons, even though the two-coat volumes differ by about four-tenths of a gallon. Rounding to quarter-gallon steps on both columns absorbs the difference. The shelf units smooth out some of the gap between labels, and they can do it in either direction.
So before anything else, answer this about your walls: same color back on, or a real change? Everything below is smaller than that answer.
Together or apart
Now the question people usually lead with. Should you add the two rooms up and buy once, or run each room as its own job?
I'll take two coats for this part, since it is the larger order and the one where rounding has the most room to go wrong.
Bought together, the order is the two-coat column from the table above. Bought apart, each room gets its own run with its own rounding:
- Living room: 371.75 × 2 × 1.10 ÷ label.
- Bedroom: 303.75 × 2 × 1.10 ÷ label.
| Label | Together | Living room alone | Bedroom alone | Apart, added | Cost of buying apart |
|---|---|---|---|---|---|
| 250 | 6 gal | 3.271 → 3 gal + 2 qt | 2.673 → 2 gal + 3 qt | 6 gal + 1 qt | +1 quart |
| 350 | 4 gal + 1 qt | 2.337 → 2 gal + 2 qt | 1.909 → 2 gal | 4 gal + 2 qt | +1 quart |
| 400 | 3 gal + 3 qt | 2.045 → 2 gal + 1 qt | 1.671 → 1 gal + 3 qt | 4 gal | +1 quart |
| 450 | 3 gal + 2 qt | 1.817 → 2 gal | 1.485 → 1 gal + 2 qt | 3 gal + 2 qt | same |
Rounding twice instead of once costs one quart on three of the four labels, and nothing at all at 450. That's it. You can plan these rooms either way and the paint bill barely notices.
Why it's a quart here and a carton on a floor
In the closet-and-hallway flooring guide, the same mistake, rounding twice instead of once, cost a whole carton on two of the six cartons. Here it costs a quart. The mistake is identical in both guides: you split one purchase into two and round each half up. What differs is the step size of what you're buying.
Flooring is sold in cartons of 18 to 51 square feet. Round up once too often and you can pay for up to a whole carton you didn't need. Paint, if the shop stocks quarts, is sold in steps of a quarter gallon, which at these labels is 62.5 to 112.5 square feet of coverage per step (a quarter of 250, a quarter of 450). Each round-up can waste at most one step, and a step is small. The size of the error is set by how finely the material is sold, not by the error itself.
This is also why the together-or-apart question gets bigger if your paint only comes in gallons. With gallons only, each round-up can waste up to a whole gallon instead of a quarter. I haven't built that table here because it depends on which color you pick and whether the shop has a quart of it, but the direction is clear: the coarser the can, the more the two-rooms-at-once rule is worth.
Leftovers, and why one order still helps
The quart is not the only reason to buy for both rooms at once. If the rooms share a color, a single order means you finish the second room with paint left over from the first, instead of opening a fresh can for the last few square feet. None of that shows up in a volume table. It is still a reason, and it points the same way as the quart.
What one order does not do is let you skip measuring the rooms separately. You need each room's net wall to get the total right, and you need each room's openings to subtract the right amount. Measure apart, buy together.
Doing it on the calculator
The paint calculator on this site takes one room at a time in room mode, or a total wall area in area mode. There is no two-room mode. So for the one-order run, do the adding yourself and use area mode:
- Add the gross wall areas: 416 + 336 = 752 square feet.
- Add the opening areas: 44.25 + 32.25 = 76.5 square feet.
- In area mode, enter 752 as the wall area and 76.5 as doors and windows. Pick coats, set the allowance to 10, choose "Gallons + quarts."
For the 350 label and two coats, the page prints 4 one-gallon cans + 1 quart, then 4 gal + 1 qt · 2 coats · 675.5 sq ft, and underneath the working: (752 − 76.5) sq ft × 2 coats ÷ 350 sq ft/gal × (1 + 10/100) = 4.246 gal, rounded up to the selected cans. That is the same 4.246 from the table above.
To check the "apart" column, switch to room mode and run each room on its own: 14 by 12 by 8 with 44.25 of openings, then 11 by 10 by 8 with 32.25. The living room alone at 350 comes back as 2 one-gallon cans + 2 quarts, 2 gal + 2 qt, on 371.75 square feet. Every figure in the two tables above was checked on the live calculator on September 28 and matched the page.
This is the same limit the flooring calculator has with its one rectangle, and the fix is the same: measure each space by itself, add on paper, enter the total once. The calculator also prints a reminder under every paint result that "Ceiling, primer and trim are not included unless measured separately." That applies here too. Nothing in this guide covers ceilings, trim, or primer.
What to buy, in order
- Look at the walls and the color. Same color, or close, on walls that are already painted: one coat is possible. A big change: plan on two. This decision is worth 1.75 to 3 gallons on these rooms.
- Measure each room's walls and each opening. Subtract.
- Add the rooms together, apply the allowance once, and divide by the label on the can you're actually buying.
- Round up to the shelf units. If quarts are available, buying apart costs you a quart at most. If they aren't, the gap can be bigger, so add first.
For the 675.50 square feet here: one coat is 1 gallon and 3 quarts to 3 gallons depending on the label; two coats is 3 and a half gallons to 6. Buying room by room adds a quart or nothing.
If I had to drop one of the two decisions to save time, I'd drop the together-or-apart comparison and spend the minute looking hard at the walls instead. The quart is real, but the second coat is the whole job doubling.
Is yours a same-color refresh, or a color change?
Sources
- Benjamin Moore & Co., Regal Select 550 technical data sheet: "two coats are required; previously painted surfaces can be finished with 1 or 2 coats." https://media.benjaminmoore.com/WebServices/prod/assets/stage/datasheets/TDS_0550/20180731%20550%20TDS%20US%20OKF.pdf — accessed 2026-09-07.
- Glidden (PPG), paint calculator page: "Drastic color changes typically require a second coat."; "1 gallon of paint typically covers up to 400 sq. ft. on primed, smooth, non-porous surfaces." https://www.glidden.com/paint-calculator — accessed 2026-09-09.
- Sherwin-Williams, paint calculator page: "A gallon of paint typically covers about 350-400 square feet". https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator — accessed 2026-09-09.
- Behr Paint Company, BEHR PREMIUM PLUS Interior product page: "250-400 Sq. Ft. Coverage per Gallon". https://www.behr.com/products/paint/behr-premium-plus-interior — accessed 2026-09-09.
- Wall Math paint calculator, area mode and room mode, "Area of doors & windows" help text "Subtract only openings you measured.", "Gallons + quarts" can sizes, and the line under every result "Ceiling, primer and trim are not included unless measured separately." https://www.wallmath.com/paint-calculator/ — accessed 2026-09-28.