# Dark pool

Dark pool is a private trading venue where stock orders are not displayed publicly before execution, although completed trades are reported under applicable market rules.

## What it means

A dark pool lets participants trade stocks without showing their orders in a public order book before a trade occurs. That can reduce the information revealed by a large order, but it does not make the trade invisible forever: completed trades are reported under applicable market rules. Access, matching rules and available liquidity vary by venue and broker.

A dark pool is a place an order *might* be executed, not an order type or a guarantee of a better price. A broker may route an eligible order to one; PreTrAIde does not route orders.

## How it is calculated

There is no universal formula for a dark pool. Its execution quality can be assessed using the same measures used for other venues. One is effective spread, which compares the fill with the public midpoint when the trade occurred:

`Effective spread (US$/share) = 2 × side × (fill price − reference midpoint)`

- **Fill price** is the execution price in US$ per share.
- **Reference midpoint** is the average of the publicly quoted best bid and best ask at the relevant execution time, in US$ per share.
- **Side** is +1 for a buy and −1 for a sell. This makes a higher-than-midpoint buy or a lower-than-midpoint sell a positive cost.

A zero effective spread means a fill at that midpoint, before any fees. Compare prices at consistent times; a moving quote can change the result. Effective spread measures price relative to a reference quote, not every component of trading cost.

## Worked example

Suppose a hypothetical stock has a public best bid of **US$49.98** and best ask of **US$50.02** when a 100-share buy executes in a dark pool.

1. The public midpoint is `(US$49.98 + US$50.02) ÷ 2 = US$50.00`.
2. If all 100 shares fill at US$50.00, the effective spread is `2 × 1 × (US$50.00 − US$50.00) = US$0.00 per share`.
3. If instead all 100 shares fill at US$50.01 against the same reference midpoint, the effective spread is `2 × 1 × (US$50.01 − US$50.00) = US$0.02 per share`. That is **US$2.00** across 100 shares on the effective-spread measure.

The second fill is still US$0.01 per share below the displayed US$50.02 ask in this example. Neither comparison includes broker fees, and neither tells you whether the full order would have filled at another venue.

## Why it matters before you trade

Hidden orders can limit *pre-trade* information leakage, which may matter when a large order could move a visible market. But a dark pool may have no matching counterparty at the needed size or price. An order might receive a partial fill or be routed elsewhere, depending on the broker’s instructions and routing arrangements.

Before placing an order, distinguish a potential price improvement from the chance of getting filled. Consider the displayed bid-ask spread, order size relative to daily volume, acceptable price, possible market impact and what happens to any unfilled shares. A midpoint fill can look attractive while a delayed or incomplete execution creates other costs. Venue rules and protections differ, so the word “dark” alone does not establish execution quality.

## How PreTrAIde uses it

PreTrAIde’s stated features do not include dark-pool data, venue selection or dark-pool routing. Its **pre-trade analysis** instead estimates factors relevant to execution—including bid-ask spread, participation in daily volume, market impact and liquidity—for each stock and a basket. It returns a GO, NO-GO or MODIFY verdict with reasoning and a suggested execution strategy as decision support, not a recommendation.

PreTrAIde never receives, transmits or executes an order. Users place trades themselves through their own broker; any dark-pool access and actual fill depend on that broker and the market.

## Common mistakes

- **Assuming hidden means unreported.** Orders are not publicly displayed before execution, but completed trades remain subject to reporting rules.
- **Assuming every order fills at the midpoint.** Pricing and matching depend on the venue; no match may be available.
- **Calling a smaller effective spread a guaranteed saving.** Fees, timing, partial fills and subsequent prices also affect the outcome.
- **Treating all dark pools alike.** Access, counterparties and execution rules can differ.

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Source: https://pretraide.com/glossary/dark-pool