score:3
Usually domestic flights don't have allowance at all on most of the airlines, at least those mentioned.
You have to pay additional charge to check in your bags. Up to 18kg per bag would definitely be fine, you would probably be OK with 21kg regardless of what the sites say as well. United state on the site you quoted 50lb, which is almost 23kg.
You're a paying customer, you're not getting this service for free (unless its an international connection, then its your international allowance anyway).
Upvote:4
Typically, airlines in the USA use a piece-based baggage allowance policy. Each piece can usually be up to 50 pounds (23 kg). However, this policy may vary by ailrine, and in many cases, a fee is required for checking bags. On some of the ultra-low-cost airlines, you may even be charged for your carry-on baggage/hand luggage.
Kayak, FareCompare, and Airfare Watchdog maintain baggage fee and/or allowance tables. These sites link to airline websites for more details.