Getting ready to convert my tri bike to 11 speed, already made it 1x. I'm toying with gearing in my head and thinking either a 50 or 54 on the front. Currently riding the 50 but will switch to the 54 to see how I like it... I'm more of a grinder than a spinner.

Currently I'm running a 50 11-28 setup. If I threw on a 11-25 cassette for a "flat" course, would chain length be a problem?

This will be my first chain I'm putting on my bike. Local bike shop closed down so I'm doing my own wrenching these days. I put the number into a chain length calc I found but can't link yet.

and changing from a 28 to 25 is a one link difference... with same sized front ring. Just wondering if I'm going to need a chain to go with cassette changes. TIA

If 11-28 is not a problem then 11-25 will work fine as well, since it is a subset of 11-28. It will be roughly equal to using the smallest 10 cogs of your 11-28.

That makes sense and thank you for the answer.

going from 50 to 54 front will require a longer chain... reducing from 11-28 to 11-25 in rear will not. doing both changes at once MAY require a longer chain length, dependent on how tightly the original chain length was set. You may possibly need a long caged derailleur, if you maintain the 11-28 in the back, then add the 54 in the front...

If you have a good chain break & quick link tool you can try this approach: Put the whole 116 link chain on bike. Go through the gears on the stand. If chain droops & has too much slop start removing one link at a time & repeat until everything runs smooth. (Avoid having to do too much math this way).