Chapter 02Paint · Problem-solving guide
Ran out of paint mid-wall? How to match the second can
Stop at a natural break, like a corner or the edge of a window or door, not in the middle of a wall. Take the old can to the store so they can read the brand, product, color and sheen off the label. Sherwin-Williams says paint and tinting work within tolerances, so cans bought at different times may not match exactly. Mix what's left of the old paint into the new before you go on. Next time, buy the full amount at once and pour it all into one bucket.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026; Benjamin Moore: storing leftover paint, checked October 10, 2026; This Old House, how to roll-paint a wall (boxing your paint), checked October 17, 2026.
- Stop at a corner, window or door, never mid-wall1 corner Sherwin-Williams painting FAQs, October 17, 2026
- Bucket for mixing every can of one color together5 gallon This Old House, how to roll-paint a wall (boxing your paint), October 17, 2026
- Short in the worked example with 2 gallons, not 2 and a quart44 sq ft Arithmetic from the figures on this page
Why the new can may not match
Sherwin-Williams explains it on its painting FAQ: paint, tint and the tinting machines all work within set tolerances, so cans bought at different times may not match exactly. This Old House goes further. Even two cans bought together can show a slight difference when you switch from one to the other on a wall.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026; This Old House, how to roll-paint a wall (boxing your paint), checked October 17, 2026.
The difference is usually small. It shows most where old and new meet on one flat wall in raking light. So the fix is to make sure they never meet in the middle of a wall.
What to do now, in order
Stop at a natural break
Finish to a corner, or the edge of a window or door, and stop there. Sherwin-Williams gives the same advice for running short mid-project.
Read the old can
Brand, product line, color name and number, sheen. Take a photo of the label and the lid, or take the can itself.
Buy the same product and sheen
Same line, same finish. A different sheen in the same color looks different on the wall.
Mix old into new
Pour what's left of the old can into the new paint and stir before you go on. Sherwin-Williams says to mix the remainder of the original with the new product.
Finish whole walls
Start the new mix at the corner where you stopped and paint the rest of each wall from it.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026; Benjamin Moore: storing leftover paint, checked October 10, 2026.
Not sure the old paint is still good?
Benjamin Moore says most opened paint lasts about two years, and paint that smells sour or turns chunky or jelly-like after stirring is past use. Don't mix that into the new can.
Source: Benjamin Moore: storing leftover paint, checked October 10, 2026.
How it ran short in the first place
Take the paint calculator's worked bedroom: 12 by 14 ft with 8 ft walls, one door and two windows. That is 372 sq ft of wall, or 744 sq ft for two coats. Sherwin-Williams' calculator assumes about 350 sq ft per gallon for walls, and Benjamin Moore lists 400 to 450 for one of its lines. At 350 that is 2.13 gallons, so the calculator says buy 2 gallons and 1 quart.
Source: Sherwin-Williams paint calculator, checked October 10, 2026; Benjamin Moore Regal Select (matte) product page, checked October 5, 2026.
Round down to 2 gallons and they cover 700 sq ft at 350. The last 44 sq ft of the second coat has no paint, and that is about half of one short wall. If your paint covers nearer 450, 2 gallons would have done it. You can't know which until you're on the last wall, which is why the calculator rounds up.
Source: Sherwin-Williams paint calculator, checked October 10, 2026; Benjamin Moore Regal Select (matte) product page, checked October 5, 2026.
Sherwin-Williams also says the per-gallon figure is theoretical, because surface texture and paint left on rollers and brushes aren't in it. Rough or porous walls, a big color change and a thick roller nap all push real coverage down.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026.
Let the tool do the arithmeticPaint coverage calculatorPut in the room and the coats. The calculator gives a range from the low and high coverage figures and rounds the buy up to the next quart, so the second coat doesn't run out on the last wall.Example: Buy 2 gallons and 1 quart (12 x 14 ft room, 2 coats)Open the toolNext time: buy once, mix once
This Old House calls it boxing: pour all the cans of one color into a 5-gallon bucket, stir it well, and use that mixed paint for the whole project. Buy the full amount from the calculator in one trip so every can comes from the same visit.
Source: This Old House, how to roll-paint a wall (boxing your paint), checked October 17, 2026.
- Buy the calculator's rounded-up amount, not the low end of the range.
- Box the cans in one bucket and stir before you start.
- Before you close up, set aside a little of the mix for touch-ups. Sherwin-Williams says to label it with where it was used.
- Write the brand, color, sheen and room on the lid, as Benjamin Moore suggests, so the next can is easy to match.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026; Benjamin Moore: storing leftover paint, checked October 10, 2026.
Checklist to print
Fill this in before you go back to the store:
- Where you stopped (corner, window or door edge)
- Brand and product line from the old can
- Color name and number
- Sheen (flat, matte, eggshell, satin, semi-gloss)
- Photo of the label and the lid
- Wall area left to paint and coats left
- New can bought: same product and sheen
- Old paint mixed into the new before going on
- Touch-up jar set aside and labeled with the room
Tips and tricks
- Tip 01
Leave a wet edge at the corner
Finish the coat right into the corner so the next start has a clean line to work from, and the meeting point sits where light changes anyway.
- Tip 02
Check in daylight
Look at the wall where old and new meet the next morning, in the light from the window. That is when a small difference shows.
- Tip 03
Ask for the formula
Ask the store to read the tint formula off the old label or their records, and to tint the new can to it. Bring the can so they can see what they're matching.
- Tip 04
Keep the receipt with the record
A paint record with the brand, color, sheen and date makes the next can a two-minute errand.
Source: Benjamin Moore: storing leftover paint, checked October 10, 2026.
What we would do
Stop at a corner and mix old into new
Finish to the nearest corner, window or door and stop. Take the label details to the store, buy the same product and sheen, and stir what's left of the old paint into the new before you go on. Next time, buy the calculator's rounded-up amount and box it in one bucket.
Questions
Why doesn't my new can of paint match the old one?
Sherwin-Williams says paint, tint and tinting machines all work within tolerances, so cans bought at different times may not match exactly. A different sheen, or old paint that has aged on the wall, can also show.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026.
What is boxing paint?
Pouring all the cans of one color into a 5-gallon bucket and stirring, then painting the whole job from that bucket. This Old House says it averages out small differences between cans.
Source: This Old House, how to roll-paint a wall (boxing your paint), checked October 17, 2026.
Can I mix two cans of the same color?
Yes, if it's the same product and sheen. Sherwin-Williams' advice for running short is to mix the rest of the original paint with the new product before you continue.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026.
Where should I stop if I'm about to run out?
At a natural break: a corner, or the edge of a window or door. Never in the middle of a flat wall.
Source: Sherwin-Williams painting FAQs, checked October 17, 2026.
How do I avoid running out next time?
Measure the room, put it through the paint calculator and buy the rounded-up amount in one trip. Then box the cans before you start.
The tool and more guides
Chapter 02Paint coverage calculatorWork out how many gallons of paint a room needs from its size, doors, windows and coats, using coverage figures published by paint makers.Open the tool
Problem-solving guideHow much paint do I really need?Work out gallons from wall area, coats and the coverage on the label, and see why a can runs out before the wall is done.Read the guide
Know-howOne coat or two? Reading the coverage line on a paint canThe coverage on a can is square feet per gallon for one coat. Two coats double the paint. When one coat can do, and what a second costs in a bedroom.Read the guide- Chapter 02All paint toolsPaint quantity tools built on coverage figures published by paint makers, plus a printable record of colors, codes and leftover cans.Open the chapter
Sources
- Painting FAQs, Sherwin-Williams. Checked October 17, 2026.
- 6 Tips For Storing Leftover Paint & Shelf Life, Benjamin Moore. Checked October 10, 2026.
- How To Roll-Paint a Wall Like a Pro, This Old House. Checked October 17, 2026.
- Paint Calculator, Sherwin-Williams. Checked October 10, 2026.
- Regal Select Interior Paint - Matte, Benjamin Moore. Checked October 5, 2026.
Every outside number on this page names its source and the date we checked it. See them all on the sources page. Spotted an error? Tell us.