You have several United Airlines Check-in options that can meet your preferences and convenience. Here are the main options:
Online Check-in: This is the most common method for most passengers. United Airlines allows online check-in via its website or mobile application up to 24 hours before the flight. You can check in ahead of time, select or upgrade your seat, and obtain your boarding pass online. You can display it on your phone or print it at home. It’s ideal for people who would not want to stand in line and directly proceed to security.
Mobile App United Airlines Check-in: United offers check-in through its mobile app for use both with Apple iOS devices as well as Androids. After you have checked in on your mobile device you will be able to download a boarding pass to your device or make changes such as changing seat assignments from there.
Voice United Airlines Check-in: United Airlines also offers voice check-in via their customer service number at +1 (844) 539-5787. You’ll receive flight details, check in, and get your boarding pass via email or text.
Self-Service Kiosks: Most major airports have self-service kiosks to check in, print your boarding pass, and drop off checked bags. Self-service kiosks save time for passengers who need to check into an airport but do not have the time to stand in a long line.
Curbside United Airlines Check-in: At United Airlines, you can check in curbside at any U.S. airport with a representative. This is possible from 4 hours up to 30 minutes before your flight. You can check in your bags and receive your boarding pass without entering the terminal with this service.
Airport Counter United Airlines Check-in: In case you require some special requests or have any problem with your booking, you can always visit the United Airlines counter at the airport. Agents here will help you with check-in, seat selection, and baggage handling.

Yes, United Airlines offers online web check-in. It is available 24 hours before your intended departure time and so allows you to check in well ahead of time so you can avoid the airport hassle. Web-checking not only saves you a lot of time but also allows you to change flight details if required, seat reservations or upgrade purchases. Once you have checked in, you can print out your boarding pass via email or have it appear on your phone; this way, bypassing the ticket counter or kiosks if you’re not carrying any luggage will be easy.

This gives the easiest of airport experiences when checking when you have to go to the United flight. The time will vary upon whether your flight is going to be domestic or international, and of course, whether you checked bags.
Domestic Flights: Done 24 hours before scheduled departure or less. If there’s no baggage to be checked at the airport, done 30 minutes before scheduled departure or less. Done up to 24 hours before scheduled departure. Can be done at self-service kiosks and also online. For carrying checked baggage, you must check-in online at least 45 minutes before flight time.
International Flights: You can check-in on international flights 24 hours before the scheduled flight, but check-in must be done 1 hour or more before the scheduled time to take the flight. Additionally, there should be another check at international points.

If you are checking your bags for your United Airlines, good to know how early you can check your bags. 
Check-in at United Airlines starts within the airport 3 hours ahead of time. Your time of check-in is likewise your baggage United Airlines Check-in time. Typically, check-in counters close 30 to 45 minutes before the time set for takeoff. For international flights, however, check-in may have an earlier time. However, in any case, it is always good to check with United Airlines itself.

You don’t need to get to your United Airlines flight exactly 2 hours before flying, but it’s highly recommended, especially if traveling internationally or if you’re bringing checked bags. You can be sure that you’ll have time enough to get through United Airlines Check-in and security, and with an extra bit of wiggle room if needed. Domestic flights often require a 1.5 hours before flight time arrival, which is sufficient if you travel without checking bags or if you check in online already.
But they may vary according to the size of the airport and when you arrive, or when there are special procedures on your flight. It’s always good to arrive much earlier, not to worry about missing your flight due to certain delays.

United Airlines has some rules regarding carry-on bags so that there is uniformity and safety for all passengers. Carry-on baggage varies according to whether it will fit in the overhead bin or under your seat.
Personal Items: This usually weighs about 15 pounds or 7 kg as far as personal items go, such as a purse, laptop bag or backpack. It has to fit under the seat in front of you.
Carry-on Bags: The maximum weight for a standard carry-on bag is 22 pounds (10 kg). Carry-on luggage should fit in the overhead bin and be within United Airlines’ size limits (22 inches long x 14 inches wide x 9 inches high).
You could be asked to check your carry-on bag at no additional charge if its weight or size exceeds those limits.

While it is technically possible to show up 1 hour ahead of your flight’s departure to check in with United Airlines, it’s not ideal especially for people carrying checked luggage. One hour before check-in may be a very challenging time, especially during hectic travel times, to queue through security lines.
For Domestic Flights: If you have checked in online and are traveling with just a carry-on, arriving 1 hour before may be enough. However, if you are checking in baggage, you should aim to arrive much earlier (ideally 90 minutes to 2 hours) so that you have enough time to check your bags, pass through security, and board your flight.
For International Flights: For international flights, it’s always recommended to arrive at least 2 hours before departure, even if you’ve checked in online. This is due to the longer lines at check-in counters, additional security checks, and the potential need for extra documentation.

The convenience and time savings are key considerations in the decision on whether to check in online or at the airport.
Online Check-in: For most travelers, online United Airlines Check-in is the best option. It’s convenient, lets you avoid lines at the ticket counter, and saves time. You can check in up to 24 hours before your flight, select or modify seats, and receive your boarding pass electronically, which makes it easy to go straight to security. If you’re traveling with only carry-on luggage, online check-in is the fastest way to check in for your flight.
Airport Check-in: In case you need to check in your baggage, make special requests, or need assistance with your flight, it might be better to check in at the airport. Although this would take more time, United Airlines’ counter agents could change seats for you if desired, handle luggage, and answer other questions.

For a domestic flight with United Airlines, it would be wise to arrive at least 2 hours prior to your flight in order to check in, go through security, and reach your gate in a relaxed manner. You should add more time to your schedule if you are flying from a larger airport or if you are traveling during peak hours.
You must arrive at 1.5 to 2 hours prior to the flight for carry-on luggage only. For checked baggage, you would have that extra time to leave your luggage, go through security and get to the gate in good time.
If you miss United Airlines check-in time: what do you do
If you fail to check-in in time for United Airlines, either online or at the airport, then you might lose your seat on that flight. United operates under tight United Airlines Check-in deadlines, hence ensuring timely departure in most instances. 
Failure to check in within the time assigned will be forwarded to customer care for possible rebooking into a later flight. It is subject to fees and charges that may apply.
You should therefore prepare ahead of time, get on schedule, and be watchful of changes in your flight so as not to miss check-in.