Looking for some info on how United calculates partial refunds.

One way ticket cost $338.25, booked JAC-DEN-PHL. JAC-DEN diverted to COS then eventually made it to Denver but I misconnected. United wanted to send me through Chicago with a 7 hour overnight layover. I canceled and bought a nonstop on Southwest.

Requested a partial refund and United only gave me back $216.96. I believe refunds are based on distance but I am not sure how they are calculated. I only flew 406 miles out of the 1,964 miles on the original routing. I feel I should get roughly 80% of my money back, not the 64% that United calculated. Are they factoring in the diversion somehow? I understand how a scenic tour of Colorado added to their costs but not sure I should pay for that.

Some costs are fixed government fees, did you do the proration on just the base airfare?

This is frequently a confusing black box, so I'm kind of surprised to have derived exactly the same result as United, but hey maybe it makes sense sometimes .

One standard method is to use the ratio of fares, instead of pro rata by distance. In this case, the relevant fare would be Y on JAC-DEN and Y on DEN-PHL, and taxes and fees are separate.

The original ticket was $338.25, which would be fared as $291.21 of base fare plus $21.85 of US (a total of $313.06 of ad valorem components), plus fixed fees: 10.60ZP 5.60AY 9.00XF. ZP and XF are segment based and AY is fixed.

The fare Y (not cheapest "full Y" but the actual basis Y) on JAC-DEN is $3,022 and it's $5,911 on DEN-PHL.

So the cost apportioned and allocable to DEN-PHL is $313.06 x (5911/(5911 + 3022)) + $5.30 + $4.50 = $216.953 (guessing UA always rounds up).

I would have asked for it all back. They didn't get you where you were going and presented an unacceptable routing after the diversion. Did you see if there was anything on AA they could put you on (and then ask them to)?

I would have asked for a million dollars. We'd both have gotten a 'no.'

OP had the option of being returned to JAC, in which case it would have been a Trip in Vain and a full refund would have been allowed.

If your method worked, UA would suddenly have infinite demand for flights like XXX-SFO-MRY and XXX-SFO-SMF in the hopes that people could get free transportation to SFO because "they didn't get [me] where [I] was going."

I agree that as a general matter, that would be an issue. That's why I think it's highly relevant whether OP asked for and was rejected for an OA routing that would have avoided the overnight.

Oh I forgot about that part. Makes total sense.

Amazing math skills! Thanks.

The only evening nonstop options were Southwest (which I bought) and a redeye on Frontier. I didn't see United rebooking me on either of those. American had a nonstop the next day but that wouldn't have helped me much.

Did not ask because: 1. UA doesn't interline with Southwest or Frontier, 2. the diversion was weather related, and 3. United closed all their airport help centers (the most boneheaded move they've made in recent years through otherwise great improvement.) United did bear some responsibility, however, as it was their choice to load minimum fuel on the first flight (pilot did not sound happy about that) and it took a very long time to get fuel in COS due to ground staff being overwhelmed. I did score a $20 meal voucher for my troubles.

Side note: This was my first time on Southwest after two million miles with DL and UA. I will say the plane was new, got a nice seat with decent legroom, the staff were pleasant, wi-fi and drinks were free. (drinks were supposed to be $9 but apparently they "forget" to charge often.) Most importantly, they ran on time. This mishap completely changed my perception of Southwest.