I have an old mountain bike which has a triple crank. Being that the chainrings need to be replaced, I am considering a 1 X setup. Measuring from the middle chainring, the chainline is 55mm. How would I calculate the needed chainline for a 1 X setup?
Thanks

There's nothing to calculate. The correct chainline for a frame doesn't change, and it should be intuitive that a single chainring would be the same distance as the middle chainring on a triple. Changing the crankset often requires a different bottom bracket (spindle length) to achieve the correct chainline for the bike.

However, are you sure you're measuring correctly? A typical "old mountain bike" has a chainline of 47.5-50mm. What bicycle are we talking about, exactly? Has the bottom bracket and/or crankset ever been replaced? It's possible the wrong BB was previously installed, which resulted in an incorrect chainline for that bike. Here's a good reference.

All About Bicycle Chainline

Giant ATX 770
Bottom Bracket 73 X 118
Replaced once with identical size
I was thinking of replacing it with Hollowtech ll bottom bracket.

Is there any reason to believe that the correct chainline for that bike isn't the industry-standard 47.5-50mm, and that somehow, somewhere a mistake was made? Is the bike stock, otherwise? Are you the one that installed the last BB?

If you can find specs for that bike that say otherwise, then of course go with the published specs. But I'd be really surprised if the correct chainline for that bike isn't the standard mountain triple which is 47.5-50mm. Especially a Giant, from the early 90s.

Thanks, I will take another measurement.

​​​​​​https://www.sheldonbrown.com/chainline.html

I stand corrected. I measured again and it was 50mm. Will I be able to change over to a Hollowtech ll bottom bracket?

Think so - which 1x crank are you planning to use?

On paper, you want the center ring to line up perfectly with the center cog of your cassette. (presuming an odd number of cogs, else "split the difference" if an even number)
However, some of us match it to our specific needs.
For example, I tend to use my 3X9 on the middle ring and outer cogs mostly, so I use a slightly longer spindle to better match those cogs.

If you're going to replace with external cup BB and modern 2-piece cranks then the BB length is irrelevant. Assuming it's a double crank and you want to run a single ring, most people mount the 1x chainring on the inside of the spider.

Ideally, "by the book," for a 1x, you want the front chainline to be about 2 mm larger (further outwards) compared to your rear chainline.
If you run it with a "normal" cassette (i.e. not the ones with over 40T on the largest sprocket), you could go without the 2mm offset (how far inwards you can move the front chainring also also depends on your chainstay length).
How to measure the front and rear chainline

As far as I know, Shimano 1x cranks are designed to provide the wider chainline when used with Shimano Hollowtech II bottom brackets (and, if your frame has British, or "at least" Italian thread, you should be able to source a Shimano Hollowtech II BB).

P.S.
I see going from 3x to a 1x as a step back (1x drivetrain pros and cons).

Relja

Thanks for your informative reply. I am thinking of the options. Everything from finding another triple crankset to purchasing a new bicycle. I like the frame because it is steel.

Another option is to just replace the worn chainrings.

Replacing 2 chainrings would probably cost me more than an entire crank.

No, not even close. Figure out what sizes you need and price them out. Much cheaper (and easier) to replace chainrings. Plus if you buy a new crankset you'll probably need a new bottom bracket too.

46 teeth, 36 teeth x 110mm

Very common sizes. You'll find that a set of replacement chainrings is much cheaper than an entire crankset, and even more so if you have to replace the BB too.