Introducing

Upcoming Changes to Bid and Ask Sizes in Stocks Quotes

Oct 27, 2025

On November 3, 2025, as part of an industry-wide update driven by the U.S. Securities and Exchange Commission (SEC), we will begin publishing

and values in shares instead of round lots across all Stocks Quotes REST API, WebSocket feed, and Flat Files. This change aligns with the SEC's Market Data Infrastructure (MDI) Rules, which are designed to modernize equity market data reporting for better transparency and efficiency.

In this blog post, we'll break down why this change is happening, what exactly is changing, when it takes effect, and how it impacts you. We'll also provide clear before-and-after examples to illustrate the updates. We’ve designed this transition to minimize disruption while enhancing data usability, most users won't need to make any changes.

Why Is This Change Happening?

The SEC's amendments to Regulation NMS (National Market System), adopted in Release No. 34-101070 on September 18, 2024, respond to significant changes in U.S. equity markets since 2005. Transaction volumes have more than tripled over the past 17 years, electronic trading now dominates with latencies in microseconds, and stock prices vary widely, some exceeding $10,000 per share.

Key rationales from the SEC include:

• Modernizing Round Lot Definitions: Historically, a "round lot" has been fixed at 100 shares for most U.S. stocks. Under the new rules, round lots are tiered based on a stock's average closing price to better reflect notional value (e.g., aiming for lots worth up to $25,000). This reduces the prevalence of "odd-lot" trades (smaller than a round lot) and improves quote visibility for high-priced stocks. Tiers: 100 shares (≤$250), 40 shares ($250.01–$1,000), 10 shares ($1,000.01–$10,000), or 1 share (>$10,000).
• Enhancing Transparency: The rules accelerate the inclusion of odd-lot information in core data feeds and require bid/ask sizes to be reported in actual shares rather than round lots. This provides investors with more accurate views of available liquidity, especially for smaller orders that now make up over 50% of trades due to retail participation and algorithmic trading.

For full details, refer to the SEC Release No. 34-101070, especially pages 1 and 5–8, Section V (pages 166–214), and Section VI (pages 216–224), which explain the rationale, rule definitions, and compliance dates for round-lot and share-based size reporting.

What Is Changing?

Beginning November 3, 2025, the

and fields in Polygon.io’s Stocks Quotes REST API, WebSocket stream, and Flat Files will represent the number of shares available instead of round lots.

While the SEC’s Market Data Infrastructure (MDI) Rules only require this change for data going forward, we're going a step further by regenerating our historical flat files to ensure seamless consistency across your entire dataset. This regeneration take several weeks to complete and will begin after our API changes have been validated.

• Historical data: We’re regenerating historical Flat Files so
  bid_size
  and
  ask_size
  are expressed in shares.
• Streaming data: All new quotes published after November 3 will use share counts.
• Flat Files rollout: Updates will start with the most recent files and proceed backward in time, with new ETags applied as each file is refreshed, triggering automatic syncs.
• Other fields: No other quote attributes are affected; only size units have changed.

Any flat files dated on or after Nov 3rd, 2025 will use shares instead of round-lots, reflecting the immediate change for new data, while older files are updated during the regeneration process.

If you prefer to continue working in round lots, you can use the

field from our Ticker Overview API to convert shares back using .

Before and After Examples

Let's look at a few examples to see how this works. Historically, bid/ask sizes were in round lots (typically 100 shares).

Standard Stock (e.g., AAPL with 100-Share Round Lot)

Assume a quote for Apple Inc. (AAPL) at $150/share.

• Before (Round Lots):

  {
    "bid_price": 150.00,
    "bid_size": 5,  // 5 round lots = 500 shares
    "ask_price": 150.01,
    "ask_size": 3   // 3 round lots = 300 shares
  }
  

• After (Shares):

  {
    "bid_price": 150.00,
    "bid_size": 500,  // Direct share count
    "ask_price": 150.01,
    "ask_size": 300   // Direct share count
  }
  

Without regeneration, historical data would show sizes like "5" pre-November 3 and "500" after, potentially breaking backtests. Our update ensures everything is in shares.

High-Priced Stock with Non-100 Round Lot (e.g., BRK.A with 1-Share Round Lot)

For Berkshire Hathaway Class A (BRK.A) at ~$736,000/share (historical round lot: 1 share).

• Before (Round Lots):

  {
    "bid_price": 736000.00,
    "bid_size": 2,  // 2 round lots = 2 shares
    "ask_price": 736001.00,
    "ask_size": 1   // 1 round lot = 1 share
  }
  

• After (Shares):

  {
    "bid_price": 736000.00,
    "bid_size": 2,  // Direct share count (no change needed, as round lot was 1)
    "ask_price": 736001.00,
    "ask_size": 1   // Direct share count
  }
  

Under the new SEC tiers, BRK.A would qualify for a 1-share round lot (>$10,000), so the conversion is straightforward. For most stocks, it's a simple multiplication by 100.

Next Steps

In summary, this update to bid and ask sizes in our Stocks Quotes data feeds, effective November 3, 2025, brings greater transparency and consistency by reporting values in actual shares rather than round lots, aligning with the SEC's push for modernized market data. This change improves consistency, especially for high-priced stocks and odd-lot orders, which now make up over half of all trades.

Most customers don’t need to take action since Websocket streaming and Flat Files will update automatically. If your systems depend on round lots, you can still convert using the

field in the Ticker Overview API (for example, ).

Note on upcoming changes: The SEC’s modernization effort continues into 2026. Starting April 27, 2026 (with testing in March), the UTP SIP plans to implement additional MDI updates for odd-lot quote reporting in core market data feeds, aligning with the SEC's May 2026 compliance date for full dissemination. These enhancements will further improve transparency by including aggregated odd-lot volumes at multiple price levels per participant (up to five depths) and best odd-lot orders (BOLO). We’ll share more details and timelines as that date approaches.

For questions, contact support@polygon.io.

• Justin

  editor

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