In the past, I have been teaching designers that the proper usage of the forward slash was that if the slash is separating things without spaces, then there are zero spaces on each side of the slash. Likewise, if there are spaces in the things separated by the slash then there should be a space on each side of the slash. This rule is true for words!! I researched this over and over, and kept finding this rule, so I applied it to the numbers and conversions in patterns. I told designers it was either 131 yds / 120 m OR 131yds/120m.

THAT’S NOT TRUE for numbers.

I sent in a specific question through the Grammarly website to ask if my advice to all of you for the last few years was correct or not. Good “Pawel” got back to me within the DAY!

Bottom line: I was wrong.

Pawel says, and I will quote at length:

Unfortunately, I am not a tech editor specialist; however, after checking multiple sources, I found out that:
 
The correct formatting for measurement conversions with a slash is:
 
c) 131 yds/120 m
 
Spaces around units: Each unit (e.g., yds, m) should have a space after the number — 131 yds, 120 m.
Slash usage: Do not put spaces around the slash when it represents “per” or “to” in conversions — yds/120 m, not yds / 120 m.
This follows standard style guides such as the Chicago Manual of Style, SI unit conventions, and APA guidelines, which all recommend:
Use a space between the numeral and the unit, but no space around a solidus (slash) when used to indicate a ratio or conversion.
 
Please do not take this information for certain. I would contact a senior colleague or peer in order to confirm this information.

With all the guides that he quotes (Chicago, SI, APA), I’m willing to take this for the rule.

Therefore, moving forward, the rule is: Use a space between the number and the unit, but NO SPACE on each side of the slash.

What to do now? Where do you go from here if all your previous patterns are now wrong? Take it easy on yourself, this is my problem and we are going to work moving forward. I will do my best to go back and correct the instructions I have given designers. I think designers can start using the appropriate spacing on patters moving forward. If you want to go back and correct old patterns, that’s entirely up to you. But, I certainly wouldn’t put pressure on any designer to resubmit the updated patterns. The fundamental information in the patterns remain the same.

I am so very, very sorry for enforcing this wrong rule for so long.