I know that the movement of a security's stock price is based on the
volume of supply and demand, but how can I calculate how many shares
need to be purchased in order to move a security stock price up $1?

Let's say Yahoo is $37 per share today, how can I find out how many
shares need to be purchased in order to move yahoo stock price to $38.

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Clarification of Question by natural00-ga on 04 Nov 2004 22:36 PST

Hello Markj,

Thank you for your comments.  I know that it's kind of like a demand
and supply relationship, but is there any formula we can use for
calculation?  Let's say if for Yahoo, currently there is 1000 shares
(ask size) offering at this moment, and the current price is $37, does
that mean if I bought all 1000 share at this moment, then the price
will move up?  If yes, how much the price will move up (1 cents, 5
cents, 50 cents)?  I just don't understand the relationship of ask
size, bid size, and the movement of stock price.

It depends on how many Yahoo shares are being offered for sale at the
time you are offering to buy shares.   The larger the number of shares
being offered for sale, the larger the number of shares you would have
to offer to buy to cause an increase of $1 in the agreed sale price. 
Because of the highly dynamic nature of the stock market, this is an
oversimplification, but that's the principle that applies.

as markj said, this was a great oversimplification.

there is probably not a good way to make that determination for a
liquid stock like yahoo.  because of the possibility for short
selling, and the presence of market makers, you would be hard pressed
to be able to determine anything on the fly.

The information you need is the market depth of the stock, also called
Level 2: it shows the best bid and offer of every market participant
who is publicly posting a quote. You'll find some details and even
print screens at http://daytrading.about.com/cs/nasdaqlevelii/a/level_ii.htm

One remark: Some orders are hidden, they don't appear into the Level
2. To make it simple, you will need to buy more shares to move the
price by $1 that the volume displayed in Level 2. The more volume
you'll have in the market depth, the more 'liquid' the instrument will
be.

For example, right now (11/05 at 6AM ET), VODAFONE listed on the
London Stock Exchange is worth 142.5 BP. If you want to move the price
to 143.5, you'll need to buy at least 26,000,000 shares, worth more
than 3,7 billion pounds!