# Liquidity provider

Liquidity provider is a market participant that makes shares available for others to trade, typically by posting a bid to buy, an offer to sell, or both.

## What it means

A stock trade needs someone willing to take the other side. A liquidity provider supplies that opportunity by making shares available to buy or sell, often through displayed orders. A market maker is one type of liquidity provider: it may post both a bid, the price at which it is willing to buy, and an offer, the price at which it is willing to sell. Other participants can also supply liquidity by placing orders that rest in an order book.

Providing liquidity does not mean guaranteeing a fill. Quotes can change or be canceled, and the number of shares available at a quoted price may be limited. The **bid-ask spread** is the difference between the best displayed offer and bid; **market depth** describes how many shares are available at quoted prices. Neither measure identifies every participant willing to trade.

## How it is calculated

A liquidity provider is a participant, not a numerical measure, so it has no single formula. One measure of the quoted trading conditions its orders may help create is:

`Quoted spread (US$ per share) = best offer (US$ per share) − best bid (US$ per share)`

- **Best offer:** The lowest displayed price to sell a share at that moment.
- **Best bid:** The highest displayed price to buy a share at that moment.
- **Quoted spread:** The price gap between those quotes, measured in US$ per share. Dividing the gap by the bid-ask midpoint and multiplying by 100 expresses it as a percentage.

These are market-wide quotes, not necessarily quotes from one liquidity provider. The spread alone does not show available size, commissions, market impact, or the price an order ultimately receives.

## Worked example

Suppose, hypothetically, a stock has a best bid of **US$50.00** for 100 shares and a best offer of **US$50.10** for 100 shares. Assume the displayed offer comes from a liquidity provider.

1. The quoted spread is **US$50.10 − US$50.00 = US$0.10 per share**.
2. A hypothetical 100-share buy order that immediately executes against the entire displayed offer would pay **100 × US$50.10 = US$5,010**, excluding commissions and other costs.
3. The bid-ask midpoint is **(US$50.00 + US$50.10) ÷ 2 = US$50.05**. The execution price is **US$0.05 per share** above that midpoint, or **US$5** across 100 shares.

That midpoint difference describes this assumed execution, not a guaranteed charge or the liquidity provider’s profit. If the offer changes or fewer shares remain, the order may receive a different price or fill only partly.

## Why it matters before you trade

Available liquidity affects the price and quantity at which an order might execute. A narrow quoted spread can be useful context, but it is not enough on its own: an order larger than the shares available at the best offer may reach higher-priced offers. Liquidity can also change during the trading day or around news.

A pre-trade check can compare the proposed order size with typical daily volume, consider the estimated spread and market impact, and distinguish a displayed quote from an assured execution. These checks concern execution conditions, not whether a stock’s future return will be positive.

## How PreTrAIde uses it

PreTrAIde does not identify or evaluate individual liquidity providers as a stated product feature. Its pre-trade analysis instead estimates execution conditions for each stock in a user-built basket. Its deterministic quantitative layer includes an estimated bid-ask spread, participation in daily volume, an Almgren-Chriss-style market-impact estimate, and liquidity scoring and tiering.

PreTrAIde returns a GO, NO-GO or MODIFY verdict for the basket and each line, alongside risk assessment, a model-estimated success probability, a suggested execution strategy, and plain-language reasoning. These are decision-support outputs, not recommendations or guaranteed fills. PreTrAIde does not receive, transmit, or execute orders; users trade through their own broker.

## Common mistakes

- **Treating liquidity provider and market maker as identical.** Market makers provide liquidity, but resting orders from other participants may do so too.
- **Assuming displayed shares will remain available.** Quotes and available size can change before an order arrives.
- **Treating the quoted spread as the total execution cost.** Commissions, market impact, and the actual fill price may also matter.
- **Assuming a narrow spread guarantees a full fill.** Check displayed size and the order’s size as well as the quoted prices.

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Source: https://pretraide.com/glossary/liquidity-provider