From September 1 through September 30, converting hotel points to UA miles earns a 30% bonus. You need to register first through the link below before transferring points for it to qualify. The maximum bonus is 25,000 extra miles. In practice, aside from transferring Marriott points, the other options are generally poor deals.

The Marriott-to-UA transfer ratio is 3:1.1. On top of that, Marriott’s standard transfer bonus gives you 5,000 extra miles for every 20,000 points transferred. Under this promotion, the sweet spot is transferring 180,000 Marriott points, which gets you: 66,000 miles (Marriott transfer plus the UA uplift) + 15,000 miles (the 5,000 bonus for each 20,000 transferred). Those 81,000 miles then receive the 30% bonus, so the final total is 81,000*1.3 = 105,300 UA miles. The 30% bonus portion is exactly 24,000, so it does not exceed the cap. Overall, 180,000 Marriott points for 105,300 UA miles is a pretty solid deal. If you value Marriott points at about 0.8 c/p, then you are effectively buying UA miles at a cost of 1.44 c/p.

Also worth noting: Marriott is currently offering a 50% bonus on purchased points. Buying 150,000 points costs $1,250, which comes out to 0.83 c/p. Based on the math above, that works out to buying UA miles for about 1.5 c/p. That can be quite a good deal if you plan to redeem UA miles for business- or first-class award tickets.