Betterment

There’s no minimum investment at Betterment, an online investing startup, or “robo-advisor,” founded in 2010. A unique advantage of Betterment for small investors is its ability to sell fractional shares — down to one-millionth of a share — which makes it easier to spread small portfolios over several different exchange-traded funds.

But the service charges smaller accounts more than larger one. It charges a fee of 0.35 percent of assets per year for accounts of less than $10,000, if clients set up an automatic deposit of at least $100 into their account. Otherwise, Betterment charges small accounts $3 per month, which doubles the fee for a $500 account. From $10,000 to $100,000, clients pay 0.25 percent, and over $100,000 they’re charged 0.15 percent.

Charles Schwab

You need $1,000 to open a brokerage account; that requirement is waived if you set up an automatic transfer of $100 a month or open a checking account. This year, the discount broker started up its own robo-advisor, called Schwab Intelligent Portfolios, which recommends and manages a portfolio at no charge. The minimum initial investment is $5,000.

E*Trade Financial

E*Trade requires $500 to open a self-directed brokerage account, but there’s no minimum to open an individual retirement account, or IRA. If you have at least $25,000, you can sign up for E*Trade’s managed accounts, in which the discount broker manages a portfolio for you.

Fidelity Investments

Clients must have $2,500 to open a retail brokerage account and invest in the markets. For a managed account through Fidelity’s Portfolio Advisory Services, there’s a $50,000 minimum.

Financial Guard