It seems some pedals are 15mm and some are 9/16. I'd like to replace the pedals on my bike.

Here is the link to my bike. My owners manual is very generic. I can't find a 'service manual' (if anyone knows where one is I'd appreciate it) - so by looking at this will these pedals fit this bike?

The pedals:
https://www.amazon.com/gp/product/B00...cm_rdp_product

The bike:
https://www.fujibikes.com/LifeStyle/P...osstown40.aspx

Thanks for help on a basic question like this - I appreciate it

Your pedals are 9/16". French pedals are 14 mm. No pedals are 15 mm. Don't confuse wrench size with thread size.

The pedal wrench you use to put the pedals on and off is 15mm,
but that is not the tread size.

Older French stuff would be using all metric rounded down numbers, Fuji , Japan, would use fractional parts
metric equivalents come out as decimal portions of a MM 25.4=1", 22.2 =7/8" for example .
one piece cranks on cruiser bikes will have 1/2" pedals

Thanks! - this is where I got the 15mm idea

https://www.ehow.com/video_4415604_re...s-bicycle.html

At around 35 seconds into the video the mechanic mentions a 'pedal removal tool' or if you don't remove pedals that often a 15mm wrench can be used. Then I found some nice metal pedals like I wanted - and they say 9/16in so I was confused.

https://www.amazon.com/Park-Tool-PW-3.../dp/B000OZBK32

I guess I'm not interpreting something right? I noticed the tool had a 15mm side and a 9/16 side. I really didn't want to have to buy the tool as I doubt I'll swap pedals very often.

9/16" would be for the aforementioned OPC with the 1/2" thread to the crankarm.
a pedal wrench is long but fairly thin, cut from 1/8" steel, some pedals will not let you use a normal 15mm wrench they make many of them much thicker , so pedals in hand you may take them , and your bike, to the bike shop and ask for assistance,
if the 15mm open end wrench you have is too thick and as such, will not fit.

The common sizes in the US are 1/2" and 9/16". One piece cranks take 1/2" and virtually everything else is 9/16".

So why have bicycle pedals (and headsets) escaped the conversion to metric?

The fact that my front crank is 'only one ring' (I have a 7 speed freewheel) - that wouldn't make a difference would it? That wouldn't be a "one piece crank" ala the 1/2 inch I see mentioned above would it?

single ring is irrelevant. Your Fuji has this crank/bottom bracket: Crankset ProWheel composite resin/steel, 44T
Bottom bracket Sealed Cartridge Bearing ST
The crank arms both can be removed from the crank axle. Thus, it's a three piece.

With a one piece crank, the crank arms and axle are...................one piece.

Thanks, I'm learning - this forum is great - there is NOONE around to ask these question to where I'm at (at least noone I know).

You are right about this forum being great. Lots of good advice from people who work in the field.

On most 3 piece cranks the pedals are 9/16"X20TPI (threads per inch). The taps are hard to come by and expensive.

You forgot chains all have a 1" pitch. Most seatposts just use the metric size of an older standard measurement.

And fork steerers are 1", 1 1/8" or 1 1/4" diameter.

Never discount "inertia" as an answer for "why?".