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How does margin work?

This may seem like an unusual question, but let's say I were to start with a $6000 capital, and I were to go long on an investment that required a $4285 (random amount) initial margin. It goes up three ticks and I sell for a gain of $75. Let's say I wanted to buy again that SAME DAY, did the previous transaction settle immediately after I closed my position? Or will I have to wait for the next trading day for me to create another position in that investment? (Is immediate settlement the purpose of most futures using a cash settlement system?)

Also, as in the same example, would it require a total of $8570 initial margin ($4285 to go long, $4285 to go short) for me to buy into a position and sell out of my position?

Once you've closed that position your margin will be available to you immediately to enter a new trade. In your example, as soon as you've sold you now have $6075 (assuming no commission) available to use as margin on your next trade.

To your second question, no you don't need extra margin to close your position. If we extend your example let's say that you are long and have $4285 of your capital tied up as margin for your trade and that you are trading 1 contract. (i.e. 1 contract requires $4285 margin.) Your trade is currently at break-even and you decide that you want to be short instead of long. You can now sell 2 contracts so that you are now net short 1 contract. Your margin will remain unchanged at $4285.
Perfect, thanks for the reply, just what I wanted to make sure of, I appreciate it.
Since you specified a very short term trade, the only thing I would add is that your day trading margin are determined by your broker. This will determine the number of contracts you would be allowed to trade.

Ok Just want to check something here. Lets say I have an initial opening deposit of $2500 and the margin is advertised as $500. Does this mean that if i trade one contract of the ES $500 will be placed aside for this trade. If I then am in a position where I am 1 point down for a $50 loss, is the margin maintained at $500 or does it become $450?
While the position is still open, that $500 is set aside and is not available as margin to add another contract to the open position. In addition, any gain or loss in your open position affects your remaining available margin. If your open position is down $50, your available margin is $1950 (= $2450 - $500), in which case the second contract would be "funded" (backed) from the remaining $1950 of available margin funds in your account.

If you close the trade and take the $50 loss on the trade, the loss is then debited from your account balance and you would then have the remaining $2450 immediately available for the next trade.