As a retail investor that puts a certain monthly amount into equity, I was wondering which is the best way to proceed when purchasing stocks. By best I mean the one that would allow me to purchase the stock at a lower price. Assigning some minutes every month to the process is not a problem.

I'm used to placing limit orders between the ask/bid prices, and the order executes almost instantly. I was thinking if it could be possible to take advantage of the volatility of the stock price to set a permanent limit order with a slightly lower target price, expecting that its volatility drives it there during the trading day (or the subsequent days). Thus, purchasing the stock at a lower price.

Having the historical daily price range for a specific stock, it would be plausible to assess its volatility and set the target share price accordingly. Is my thinking flawed? Am I overlooking something? Is there other more advisable course of action for purchasing stocks?

I just want a fire-and-forget policy regarding monthly stock purchases, but I thought it could be improved, albeit slightly. Even if it would mean a small improvement, everything adds up in the long run.

Related questions

How to decide on limits when purchasing/selling stocks?

The chosen answer(@Michael Pryor) helps me deal with the guilt of not purchasing immediately. Although I wouldn't mind the order being executed a month later (as long as I'm not missing dividends), I'm well aware that as time goes by the share price would move further from the targeted price, making the execution increasingly unlikely.

Choosing the limit when making a limit order?

This one comes closer to the mark, but seems inconclusive. @DumbCoder recommends crunching numbers oneself (my approach here), but seems that the focus drifts into stock value assessment, which isn't my interest (I'm not picking stocks).


4 Answers 4


Short answer:

If you can afford the cost and risk of 100 shares of stock, then just sell a put option. If you can only afford a few shares, you can still use the information the options market is trying to give you -- see below.


A standing limit order to buy a stock is essentially a synthetic short put option position. [1] So deciding on a stock limit order price is the same as valuing an option on that stock. Options (and standing limit orders) are hard to value, and the generally accepted math for doing so -- the Black-Scholes-Merton framework -- is also generally accepted to be wrong, because of black swans.

So rather than calculate a stock buy limit price yourself, it's simpler to just sell a put at the put's own midpoint price, accepting the market's best estimate. Options market makers' whole job (and the purpose of the open market) is price discovery, so it's easier to let them fight it out over what price options should really be trading at. The result of that fight is valuable information -- use it.

Long answer using options:

Sell a 1-month ATM put option every month until you get exercised, after which time you'll own 100 shares of stock, purchased at:

option strike price - total price of all put options you sold until exercise

This will typically give you a much better cost basis (several dollars better) versus buying the stock at spot, and it offloads the valuation math onto the options market. Meanwhile you get to keep the cash from the options premiums as well.

Disclaimer: Markets do make mistakes. You will lose money when the stock drops more than the option market's own estimate.

Long answer using stock:

If you can't afford 100 shares, or for some reason still want to be in the business of creating synthetic options from pure stock limit orders, then you could maybe play around with setting your stock purchase bid price to (approximately):

bid = spot - (spot * iv * sqrt(days/365) * ndev)

spot = current mark price of stock 
iv = implied volatility of front-month ATM option (see options chain)
days = number of days you expect to wait for a fill
ndev = number of standard deviations 

See your statistics book for how to set ndev -- 1 standard deviation gives you a 30% chance of a fill, 2 gives you a 5% chance, etc.

Disclaimer: The above math probably has mistakes; do your own work. It's somewhat invalid anyway, because stock prices don't follow a normal curve, so standard deviations don't really mean a whole lot. This is where market makers earn their keep (or not).

Long answer using stock, take 2:

If you still want to create synthetic options using stock limit orders, you might be able to get the options market to do more of the math for you. Try setting your stock limit order bid equal to something like this:

stock_bid = put_strike 

Where put_strike is the strike price of a put option for the equity you're trading. Which option expiration and strike you use for put_strike depends on your desired time horizon and desired fill probability. To get probability, you can look at the delta for a given option. The relationship between option delta and equity limit order probability of fill is approximately:

probability of fill = abs(delta) * 2

Disclaimer: There may be math errors here. Again, do your own work. Also, while this method assumes option markets provide good estimates, see above disclaimer about the markets making mistakes.

  • if Black-Scholes-Merton is wrong, when is a more correct replacement going to be found? it seems like a lot of money is riding on invalid formulas, meaning a lot of money can be made with the correct formula.
    – user12515
    Dec 10, 2019 at 16:59
  • @Michael There are plenty of refinements and replacements in use, but they will always be inaccurate to some extent. A model is only a simplified approximation of reality, and no model can ever have access to all the information and processing power available to the full set of market participants.
    – stevegt
    Jan 10, 2020 at 20:49

There is no such thing as buying at the best price.

That only exists in hindsight.

If you could consistently predict the lower bound, then you would have no reason to waste your time investing.

Quit your job and bet with all leverage in.

What if the price never reaches your lower bound and the market keeps rallying?

What if today is crash day and you catch a falling knife?

I'd say the best strategy would be just buy at whatever the market price is the moment your investment money hits your account with the smallest possible commission.

  • The comission is the same for a market order and a limited order. This isn't about predicting a lower bound (which I know it's impossible), but about setting the limit order with the most appropriate target price so it gets executed within the day or soon afterwards. I wouldn't mind a market crash: there would be no disadvantage to executing the limit vs market order in that case. I've set all my orders between bid and ask prices and have been executed immediately. Your points, valid as they might be, don't answer my question. May 24, 2015 at 16:00
  • 1
    @Tachibana maybe you aren't being all that clear. What are you attempting to achieve by using a limit order? Both with your current strategy and your proposed alternative. Even now you're risking not buying the stock at all even if it usually works. Assuming stocks go up most of the time, that's money you lose. As I understand it you are investing if and only if your bet works.
    – gengren
    May 24, 2015 at 19:52
  • I suspect so. I'd still have the option to cancel. The idea is using historical data to make the probability of executing the order high enough but still slashing some cents from the purchase price. When I place the order I always ask myself how much could I push down and still get the stock within days. However, my question might be invalid in the sense that such analysis isn't quantitatively worth it when it comes to monthly purchases. May 25, 2015 at 10:56
  • Ahh, now I see where you come from. Your problem, leaving aside the tea leaves, is that your account will get money once a month. Your technical analysis will give you a value today but I suspect it will give you a different value altogether if the stock is up 8% tomorrow. If your order is filled today you have a month to feel guilty about it. If you wait till the last day you trade guilt for opportunity. To be clear, I'm not apologizing for market orders, I place orders above ask the moment I decide to get on board.
    – gengren
    May 25, 2015 at 11:31

The simplest solution to fire-and-forget is to pick something like a Target Date mutual fund made up of low-overhead index funds (within your 401k or a Roth IRA, perhaps) and set up automatic purchase to that. If you're talking about limit orders and so on, that ain't simple.

  • I might have explained my situation poorly. By fire-and-forget I mean having to assign some minutes every month to set the limit order, and then forget. An automatic purchase would be a market order (wouldn't it?) which I'd like to avoid. May 24, 2015 at 7:44

Stevegt has a great answer imo.

I get into stocks with puts (selling).

You can also buy puts at a lower strike or further out in time at the same strike to hedge against massive losses.

Back in the 90's put selling was all the rage...until the market dropped far further than many people could have imagined.

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