Shares vs Dollars: Should You Buy Stock by Share Count or by Dollar Amount?

“Shares or dollars” is the choice a brokerage app offers when you place an order: you either ask for a number of shares, or you ask to invest a sum of money and let the broker work out how much of a share that buys. For most brokerage orders the question used to make no sense: a share was bought whole. Fractional share programs changed that, and the right choice now depends on the amount you invest, the price of the share and the order type you need.

Buying in dollars versus buying in shares

The SEC’s investor bulletin on fractional shares sums it up: “Generally, you can place orders to buy or sell fractional shares in either dollar amounts or share amounts.” Its example: if a stock trades at $1,000, you can order 0.5 shares, or order $500 worth.

• An order in shares fixes the quantity. You know how many shares you will own; the amount spent depends on the price at execution. Brokerage firms generally offer several order types, the bulletin notes, but some do not allow certain of them for fractional shares.
• An order in dollars fixes the amount. You know how much you will spend; the quantity, often a fraction, depends on the price. The SEC bulletin notes that “some firms only allow market orders for fractional share investing.”

How much idle cash costs: a computed example

To measure the difference, we took the S&P 500 ETF and invested a fixed amount at the close of the first trading day of each month, two ways. With fractional shares, the whole amount is invested each month. With whole shares only, the investor buys as many full shares as the cash allows and carries the rest to the next month. Prices are the actual closing prices, not adjusted for dividends, and dividends are left out for both methods. Values are at the close of October 28, 2026, when the ETF was at $772.31.

With $100 a month and a share between $558 and $764 over those two years, the whole-share investor bought nothing in 21 of the 24 months and ended with $409.93 sitting in cash, 4.2% less in value than the fractional investor. With $1,000 a month, the gap fell to 0.48%. Over the full history since 1993, when the share started at $44.25, the gap was 0.47%: $100 bought two shares a month in the early years, and the cash left over was small. Even so, the whole-share investor bought nothing in 140 of the 405 months, all from mid-1998 on, once a share cost more than $100.

The rule that comes out of it: idle cash matters when the regular payment is small compared with the share price. When the payment buys several shares at a time, whole shares and dollar amounts end up close.

The whole-share investor also paid a slightly different average price: $663.36 a share on the $100 plan against $651.17 for the fractional investor, because whole shares were bought only in the months when enough cash had built up. Over the full history the order reversed, $127.53 against $128.82. The difference in average cost comes from timing, not from a better method.

What changes with fractional shares

The SEC bulletin and FINRA’s guide on fractional shares list the points to check with your broker before buying in dollars:

• Availability. Not every broker offers fractional shares, and some limit them to certain stocks and ETFs; the SEC notes that some firms only allow them for S&P 500 stocks.
• Execution. Some brokers fill fractional orders in real time; others collect them during the day and execute them in one or more large orders, which can change the price you get.
• Order types and hours. Some firms only take market orders for fractions, require a minimum amount, or limit after-hours trading.
• Dividends. Dividends are paid pro rata: the SEC’s example is 0.75 shares receiving $7.50 of a $10 dividend.
• Voting rights. You may not be able to vote the shares, depending on the broker’s program.
• Transfers. Fractions generally cannot be moved to another broker; you may have to sell them first, which FINRA notes can trigger taxes and fees.

Which one to choose

Dollar orders fit a regular investing plan; how often to invest is a separate question, covered in dollar cost averaging frequency. For the order types themselves, see order types.

Is it better to keep cash in dollars or buy stocks?

The question is sometimes meant the other way: hold cash, or buy shares at all. History gives a range, not an answer. Over the full history of the S&P 500 ETF, from January 29, 1993 to October 28, 2026, the fund returned 10.8% a year with dividends reinvested, multiplying the starting sum by 32. It also lost 55.2% from its peak of October 9, 2007 to its low of March 9, 2009, and only passed that peak again on August 16, 2012.

Cash in an account does not fall that way, but its return depends on interest rates. Money you may need within a few years is exposed to a drop like 2007 to 2009; money you can leave for decades has, in this record, been paid for the wait. More on the risk side is in bullish vs bearish markets, and other ideas are in trading concepts.