The casino withholds the taxes, you only walk out with what’s left over.
When I sold a house many years ago, the tax instructions said to figure my taxes on the gains using the capital gains rate and using the normal income rates … and then use the method that has the least amount of taxes owed. In my case, capital gains was 28% and normal income was still in the 17% bracket … so I reported the gains as normal income.
If I get audited, how could I prove how much money I lost at casinos?
If you’re gambling enough for it to matter on your taxes, you should be keeping a record of your buy-in and cash-out for each session. The casino will also have records of your buy-in and cash-out linked to your comp account, if you have one.
But in truth it’s very, very difficult to track this sort of thing. The IRS is basically relying on people self-reporting this type of thing.
If I buy a lotto ticket and win millions, I pay taxes on it. Can I use losing lotto tickets to offset the winnings?
Yes. In fact, there was recently a fraud scheme based around this.
That’s the exact reason I have box full of losing lotto tickets.
From the link provided in post #2 of this thread:
If I walk into a casino and put $10,000 on 27 black and lose, I’m not going to have a receipt or statement of it. Am I supposed to ask the casino for one before I lose?
You normally get a receipt at the teller when you buy chips and when you cash in chips.
Your best opportunity to learn is to go to an actual casino and see how it works firsthand.
Only if you want the tax deduction. 
Professional gamblers are encouraged to also keep a log of their activity to supplement the limited receipts that casinos provide.
Of course, casinos are becoming increasingly sophisticated with their membership cards. It’s possible they can give you a printout showing every bet, payout, buy in and cash out you made in the last five years. This can make things easier on everyone, but the amount of information available really depends on the individual casino. It’s not mandated by federal law that they keep track of anything for you.
Caveat #1: You can only deduct gambling losses on Schedule A. If you take the standard deduction, then you cannot take gambling losses.
Caveat #2: Casinos have to report winnings if they exceed $1200. But a lack of a paper trail does not mean that you don’t have to report your winnings! Of course, if your winnings are small potatoes, the IRS is unlikely to take notice, but you are still supposed to.
Well 27 is typically red so yes, you’d lose that bet! 
Pretty much every casino these days has a “player’s card.” You can request a record of wins/losses if you get one of those (and also get comps).
A receipt is not automatic in my experience, at least with small transactions. Also, I think most people buy chips at the table?
I can’t get a comp card because I (for the most part ), play blackjack and count cards. When you count cards, you want to keep a low profile, the last thing I want is for them to know my name.
If I win big in a casino but lose in another casino, the casino I won in would have no record of it.
The IRS will accept a personal log of gambling wins / losses. It is the way professional gamblers do it. I did it for many years. It must be filled out immediately after each gambling session, not made up at the end of the week, month, or year. As long as it looks legit they will accept it. You cannot file as a gambler and show losses year after year after year.
It is extremely rare for blackjack players to buy chips at the cashier cage – almost all buy chips at the table.
The $1200 report applies only to slot machine payouts and certain side bets like those on Caribbean Stud where you win a relatively large amount for a small wager. No reports are made to the IRS on table games wins or losses.
The casino is required to report cash transactions of $10,000 or more in a 24 period but that Currency Transaction Report is unrelated to wins or losses and applies only to players buying chips. If you exchange $10,000 or more cash for chips in a 24 period the casino must file a CTR and you must identify yourself or they must refuse to allow you to play until you do provide ID. This is not related to win/loss, only to cash purchases of chips. It is the same report that would be filed if you bought a car for cash or deposited over $10k cash at your bank. If you are a previously identified customer they may not demand ID – they will simply file the report using the information you previously provided.
Refusing to give a name in a casino where you regularly play BJ will attract extra attention since most players want to be rated so they get comps. Having the player’s card makes you blend in, not stand out.
And, indeed, 27 is red.
Come to England and do your gambling here - well, if you are a winner that is. All of your winnings are yours to trouser without anyone from HMRC breathing down your neck. If you like roulette, there is another small advantage in that we only have one zero on the wheel.
As it happens I am off to collect my winnings on the Ashes series. Good odds as we were supposed to be the underdogs. Shame I only put a tenner on them. Still, at 6/4 I get £110 back.
I withdraw cash from my savings account and use my debit card for all my non-gambling expenses (food, beer, hotel etc). Afterwards I deposit the remaining cash back into my savings account. This gives me (and the IRS) a clear idea of my losses if I should ever happen to need this to offset my winnings, which I’m told is unlikely the way I split jacks everytime.
We built a model to demonstrate whether card counting worked or not and the results were entirely underwhelming. We made the decision to walk or double at the halfway point of a six deck shoe. One problem was so many hands not played at the end in a winning situation and that in a winning situation, well, everybody wins whether they count or not. The only significant advantage we found was walking away when the shoe was “staked” against the player.
It seems too much work for not enough gain.
And not by accident I assure you. The casinos neutered that threat 40 years ago. The ever-unrealistic gamblers are still learning. It’s taking longer than even the most optimistic casino had hoped. Ka-Ching.
LSLGuy
Prior long-time Las Vegas resident.