How Do I Know If a Stock Is Too Illiquid to Trade?

Take a stock quoted 4.10 bid, 4.20 ask. The spread is ten cents. On a 4.15 midpoint that’s 0.10 / 4.15, or about 2.4% of the price, gone the moment you buy at the ask and value the position at the bid.

Now a stock quoted 50.00 bid, 50.02 ask. Two cents on 50.01 is 0.02 / 50.01, about 0.04%.

Written in cents, both spreads look small. As a share of price, one is roughly sixty times the other.

The four checks

1. Spread as a percentage of price. Divide the spread by the midpoint. Cents mean nothing without the price beside them, which is argued at more length in share price is not value.

2. Size at the quote. How many shares are displayed at the bid and ask, and a few levels behind them? A Level 2 quote shows this for the venues it covers. If the displayed size at the bid is smaller than your planned position, your exit will have to reach deeper, at worse prices, unless hidden orders happen to be sitting there.

3. Volume against your order. Compare your share count with the average daily volume. Say you want 1,000 shares of a stock that averages 20,000 a day. Your order is 1,000 / 20,000, or 5% of a whole day’s trading, and getting out fast on a bad day means asking the market to absorb, in a few minutes and probably at falling prices, an amount that normally takes a good slice of the session to change hands.

4. Gaps in time and sales. Watch the tape for a few minutes. Long pauses between prints, trades in tiny sizes, and prices that jump several cents from one print to the next are all signs of a thin book.

You need more than one of these. A tight spread with almost nothing displayed can still trap you.

The sum that decides it

Liquidity only matters relative to what you plan to make or lose, so measure the spread against your risk.

In the first trade you’ve spent a third of your risk budget before the stock moves at all. That’s also assuming a normal spread on exit, which in a thin stock under pressure is the very thing you can’t count on, since the spread tends to widen exactly when you most want to sell.

The trade risk worksheet runs the same numbers with your own stop and size.

Setting your own threshold

There’s no universal cutoff. It depends on your holding period, how far your stops sit and how big your orders are.

A threshold stated as a share of risk travels well from one stock to the next. You might decide, say, that the round-trip spread must be under a tenth of planned risk and that your order must be a small fraction of average daily volume, then skip anything that fails either test. The exact figures are yours to set. Write them down, so they still hold on a day when a thin stock is moving and you want in.

Filtering out illiquid names also shrinks the watchlist, which has benefits of its own, as argued in watch fewer stocks.

When a thin stock is still worth trading

Sometimes the setup justifies the cost. If you trade one anyway, use limit orders on both entry and exit, size down so the displayed bid can absorb your whole position, and expect partial fills that leave you managing an odd-sized position. Before you exit, check the spread again, because it may have changed.