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Say I'm interested in buying 300 USD worth of stock A and 700 USD worth of stock B and let's say, for the sake of argument, that it's possible to buy a fractional amount of those stocks (i.e. I could buy 100.4 stocks of A and 23.54 of B). I also want the trade to be executed immediately.

A market order is not the right order for this - it's parameter is the amount of stocks I would like to buy. Those stocks are then bought at prices starting from the bottom of the 'sell' section of the order book. So it doesn't answer my requirements, as although executed immediately, I'm not guaranteed to have exactly 300 USD worth of stock A and 700 USD worth of stock B when the orders are filled.

A limit order is also not suitable - the parameters are the amount of stock to buy and the price I'm willing to buy at. If I calculate those parameters carefully, it might still place my order in the order book and it won't be executed immediately.

What is the correct order type in this case? And if it doesn't exist, why?

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Major stock exchanges like the New York Stock Exchange do not allow the purchase and sale of partial shares. Some brokerage firms are willing to buy whole shares of stock and provide fractional shares to investors. These are firms that cater to small investors and they are likely to offer unsophisticated trading platforms.

Complex option orders (Spreads, Straddles, Strangles, Iron Condors, etc.) can be placed as Combo orders where all legs are included. These are industry wide.

Some brokers offer Stock-Stock orders where one can customize the number of stocks (long, short or both), and the the number shares. An example might be Buy 53 ABC, buy 41 RST and sell 102 XYZ.

There are also Basket Orders where one can can include multiple order types. Here's a link to an example:

https://www.interactivebrokers.com/en/index.php?f=579

  • I wonder if OP is willing to be flexible and only buy full shares up to the point his money runs out? Even if that's the case, I'm still not sure there's a buying option for this on most trading platforms, is there? In other words: "buy whole shares using my pool of $300 until the money remaining is less than the share price." Does an order type like that exist? A standard market or limit order requires one to specify the number of shares desired, not the amount of money you want to spend. In practice the share price isn't likely to move so fast that it matters, but hypothetically... – jaypops96 Jan 16 at 15:23
  • I have never heard of dollar based orders. It's B/S number of shares with price. If it exists, it's a unicorn. Yes it's TLDR but in the Basket Order link I posted, about 1/2 way down there's an Excel spreadsheet. One could set up a formula that takes the desired ratio of stock A to B and determines the maximum number of whole shares of each that can be bought with $300 that satisfies the ratio. AFAIC, this is way too much effort for $300. Add a few zeros, different story. – Bob Baerker Jan 16 at 15:59

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