The most straightforward method is the "Buy Miles" page on United.com. Usually 3.5 cents per mile plus a 7.5% excise tax. Limit: You can buy up to 175,000 miles per calendar year.
If you need 2,000 miles to book a $4,000 Business Class seat. No Promotion
You can transfer Marriott points at a 3:1 ratio. For every 60,000 Marriott points transferred, you get 10,000 bonus miles (total 30,000 United miles). ⚠️ Is it Worth It?
Buying miles is generally a unless you have a specific high-value flight in mind. Recommendation Emergency Top-Off