How to Use a Stock Broker

Every equity trade in the United States flows through a registered broker-dealer, yet most beginners open an account without understanding what they have actually signed up for: the account type, the order routing mechanics, the fee structure, the margin rules, or the custody and insurance protections that apply to their assets. These are not administrative details — they directly determine execution quality, available buying power, risk exposure, and what happens if the broker fails. This course removes that ignorance efficiently.

1. Cash Accounts vs Margin Accounts

The most fundamental choice when opening a brokerage account is between a cash account and a margin account. The distinction has operational consequences for every trade you place.

In a cash account, you can only buy securities using settled funds — cash that has actually cleared in your account. Under US T+1 settlement (implemented May 2024), the proceeds from a stock sale settle the next business day. Until settlement, those proceeds are not available to purchase new securities in a cash account. This limits buying-power recycling within the same trading day but imposes no interest charges, no margin calls, and no amplified loss scenarios. For investors who buy and hold, or swing traders who are not recycling capital daily, a cash account is the simpler and lower-risk structure.

In a margin account, the broker extends credit against your securities holdings, allowing you to borrow to purchase additional securities or to hold short positions. The typical equity minimum to open a margin account is approximately $2,000, though broker-specific house rules may require more. Margin accounts allow intraday buying power that may exceed settled cash, subject to broker maintenance requirements. As of June 2026, FINRA and the SEC eliminated the Pattern Day Trader designation and the $25,000 day-trading minimum equity floor. The former rule — which flagged accounts executing four or more day trades in five business days and required $25,000 equity — is no longer in effect. What remains are the general margin account rules: standard ~$2,000 equity minimum, intraday buying power governed by house rules, and maintenance margin requirements that trigger margin calls when equity falls below broker-defined thresholds.

Margin amplifies both gains and losses proportionally. A 10% decline in a security held on 2:1 margin produces a 20% loss on the equity deployed. Margin interest — typically 6–10% annually at most retail brokers, significantly lower at Interactive Brokers — accrues daily on borrowed balances and reduces net returns on leveraged positions held overnight.

2. Order Types: What You Actually Send to the Market

The order type you submit determines how your trade executes, not just what you are trading. Understanding this distinction prevents a significant category of beginner execution errors.

Critical warning on market orders: in thinly traded stocks or during fast-moving pre-market sessions, a market order can fill at a price dramatically different from the quoted price at the moment of submission. This is called slippage and can be severe in low-float, high-volatility securities. Always use limit orders in pre-market, after-hours, or any stock with average daily volume below one million shares. See the pre-market and after-hours trading guide for the specific liquidity dynamics that make market orders dangerous in extended hours.

3. Fees, Commissions, and the Hidden Cost of Execution

The proliferation of zero-commission trading beginning in 2019 eliminated explicit per-trade fees for US equities at most major retail brokers. However, “zero commission” does not mean zero cost — it means the cost has changed form.

Payment for Order Flow (PFOF): Most zero-commission brokers route retail orders to wholesale market makers (primarily Citadel Securities and Virtu Financial) in exchange for per-share payments. The market maker profits by filling customer orders at prices slightly inferior to what direct-to-exchange routing might achieve. For typical retail trade sizes in liquid large-caps, the difference is negligible — often less than $0.01 per share. For large positions in thinly traded stocks, or for high-frequency strategies, execution quality differences between PFOF and direct-access routing become material. Interactive Brokers Pro routes via SmartRouting without PFOF, charging explicit commissions ($0.005/share, $1 minimum) in exchange for superior fill quality. For most casual investors, zero-commission PFOF brokers like Robinhood or Fidelity are entirely adequate.

The bid-ask spread is the most universal and unavoidable trading cost. Every market order you submit crosses the spread: you buy at the ask and sell at the bid. In a stock with a $0.05 spread, a round-trip trade costs $0.05 per share in spread cost alone — before any commission. For a 1,000-share position, that is $50 per round-trip purely from crossing the spread. Active traders who make many trades per day accumulate substantial spread costs that must be subtracted from gross P&L to assess real-world strategy viability. Use our stock P&L calculator to include estimated spread cost alongside commission in your net P&L projections.

4. Account Protection: SIPC, FDIC, and What Is Actually Insured

The Securities Investor Protection Corporation (SIPC) protects brokerage customers if a member firm fails financially. SIPC covers up to $500,000 in securities (including up to $250,000 in uninvested cash) per customer, per brokerage. SIPC does not protect against investment losses; it protects against the broker itself becoming insolvent and misappropriating customer assets. All FINRA-registered broker-dealers are SIPC members.

Important limitation: SIPC coverage applies per brokerage firm, not per account type. Multiple accounts at the same broker (a taxable account and an IRA, for example) are aggregated toward the $500,000 limit in many circumstances, not each covered separately up to $500,000. Investors with portfolios exceeding the SIPC limit who wish to maintain full protection should consider maintaining accounts across multiple brokers. Most major brokers maintain additional excess SIPC coverage through Lloyd’s of London or similar insurers, covering assets well beyond the statutory SIPC limit.

5. Choosing a Broker for Your Trading Style

No single broker is optimal for every participant. The selection criteria depend on your trading frequency, position sizes, asset classes, platform requirements, and whether you value cost minimisation or execution quality above all.

• Fidelity — Best overall for long-term investors and retirement accounts. Superior research, fractional shares, ZERO-fee index funds, excellent cash management.
• Interactive Brokers — Best for active traders, options, international markets, and professional-grade execution. Lowest margin rates. Steep learning curve with Trader Workstation.
• Robinhood — Best for mobile-first beginners who want zero-commission equities, options, and crypto in one app. Limited research and order types vs professional platforms.
• Alpaca — Best for algorithmic and API-driven trading. Commission-free, with Python/JS SDKs, paper trading, and full programmatic control.

Once you have selected a broker, configure two settings immediately: enable Good-Till-Cancelled (GTC) as the default order duration for swing trades (rather than Day Only, which cancels unfilled orders at close), and verify your order confirmation settings so you review all orders before submission. Many costly errors result from accidental order submission before the intended parameters are set.

6. Buying Power, Margin Calls, and Intraday Rules

Buying power is the maximum dollar amount of securities you can purchase at a given moment. In a cash account, buying power equals settled cash. In a margin account, buying power equals equity plus the broker’s margin extension, subject to house rules and maintenance requirements.

A margin call is issued when your account equity falls below the broker’s maintenance margin requirement. You must deposit additional funds or liquidate positions within the specified timeframe (usually 2–5 business days). Failure to meet a margin call allows the broker to liquidate positions without your consent, at market prices, at a time chosen by the broker. This is not hypothetical risk management language — it is a contractual right brokers exercise. The solution is straightforward: never use the full margin capacity available, and position-size using our stock position size calculator so that a stop-out does not approach margin call territory.

As noted earlier, the former PDT rule (which restricted day trading for accounts under $25,000) was eliminated in June 2026. Broker house rules may still impose their own intraday margin monitoring and buying-power restrictions independent of the former FINRA rule. Always review your specific broker’s current margin policy documents; do not assume post-PDT freedom means unlimited intraday leverage.

Key Takeaways

• Stock Position Size Calculator — size every position so a stop-out cannot trigger a margin call or exceed your account risk rule.
• Stock P&L Calculator — include spread cost and commission in every net P&L projection before committing capital.