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If you are a MileagePlus member or have United Credit card
Once you've accumulated a bunch of United miles you may want to redeem them for air travel. However, you should first of all understand how many United miles you will need for an award ticket.
Domestic travel awards start from 12,500 miles one-way in economy class. So, you will need at least 25k miles for a round-trip ticket. If you want to get an award ticket to Europe, that'll cost you 30,000 miles one-way in economy class, and the round-trip award travel will cost 60k miles. Once you are sure you have enough miles, you can log in to your MileagePlus account to book an award flight. Note that when you use miles to book a flight, you will need to pay some fees and taxes that are not covered with United miles.
Air travel is not the only way you can use United miles. There are other travel redemption options like hotel awards, card awards, cruise awards. If you have a United credit card you can use miles to reimburse the card's annual membership fee and for airfare statement credit.
If you are not a member of the United loyalty program but have a miles rewards card
Things are much easier for those who have a regular travel rewards credit card that allows earning miles but not tied to any airline and United in particular. A good example of such card can be the Discover it® Miles card. You just use such a card to buy an airline ticket and then apply your miles to that purchase. That's it, no additional fees or taxes. The redemption ratio is usually 100 miles = $1.
With travel rewards credit cards, you will always know how many miles you'll need when you know the cost of the airline ticket. And if you are short of miles, that is not a problem, in most cases you will be able to redeem miles for a part of the airfare.