United now appears to have a 50,000-mile limit per rolling 90 days on buying MileagePlus miles, which is why you may only see “buy 50,000 miles” instead of a larger option. A recent travel post says this restriction started in January 2026, while United’s MileagePlus products page still confirms you can buy miles through its program offers.

Why you only see 50,000

  • The purchase cap appears to be the main reason.
  • Promo pages and travel blogs note that the limit is now 50,000 miles every 90 days , even if older promotions allowed more.
  • That means the checkout page may stop you at 50,000 even when you want more.

What to check

  • Your recent purchases: If you bought miles in the last 90 days, that can trigger the cap.
  • Promo terms: Some offers cap the bonus-eligible amount separately from the normal purchase limit.
  • Account-specific limits: United can also apply offer-specific restrictions based on your MileagePlus account.

Practical takeaway

If you need more than 50,000 miles, the usual options are to wait until the 90-day window resets, buy during a different eligible period, or consider another way to top up your balance. If you want, I can also help you figure out whether buying miles is actually worth it for a specific award flight.