Why Does the Bid-Ask Spread Suddenly Widen?

A stock’s bid-ask spread suddenly widens when the best buyer and best seller move farther apart. This usually happens because fewer participants are willing to quote near the current price, uncertainty has increased, or the available market is fragmented or temporarily thin. Common triggers include breaking news, rapid price movement, premarket or after-hours trading, a pending reopening, and low-liquidity securities. A wider spread raises the cost of immediate entry and exit and can make an otherwise valid setup unacceptable.

Key takeaway: Treat an abrupt spread increase as a change in trading conditions, not as a cosmetic quote change. Recalculate the trade from the executable bid and ask. If the spread breaks the position’s cost limit, stop distance, or minimum reward-to-risk requirement, skip the trade until the quote stabilizes.

What a widening bid-ask spread actually means

The bid is the highest displayed price at which a buyer is currently willing to purchase a specified number of shares. The ask, or offer, is the lowest displayed price at which a seller is currently willing to sell. The difference is the bid-ask spread, as defined in Investor.gov’s bid and ask guidance.

The basic calculations are:

Absolute spread = ask − bid

Percentage spread = (ask − bid) ÷ midpoint × 100

where:

Midpoint = (bid + ask) ÷ 2

Suppose a stock is quoted at a $40.00 bid and a $40.02 ask. Its absolute spread is $0.02, its midpoint is $40.01, and its percentage spread is approximately 0.05%.

If the bid then falls to $39.92 while the ask rises to $40.12, the spread becomes $0.20, or approximately 0.50% of the $40.02 midpoint. The last trade might still show $40.01, but that historical print does not restore the missing buy and sell interest between $39.92 and $40.12.

For US-listed stocks during regular trading hours, the best prices submitted across exchanges contribute to the National Best Bid and Offer, or NBBO. The SEC’s staff report on equity market structure explains that each exchange has its own best bid and offer and that the best prices across exchanges collectively form the NBBO.

A sudden widening can occur when only one side moves. A buyer may cancel the $40.00 bid and replace it at $39.90 while the $40.02 ask remains unchanged. It can also occur when both sides retreat. The important fact is not which side moved first; it is that the closest displayed buyer and seller are no longer willing to trade as near to each other as before.

The displayed spread is only the top of the market. It does not show all shares available at deeper prices, and it does not guarantee that the quoted size will still be present when an order arrives. A tight spread can have very little size behind it, while a wide spread can occasionally have substantial size on both sides. Spread and depth are related, but they are not interchangeable.

Why spreads widen during news, volatility, and thin trading

A bid-ask spread is the price of immediacy visible before a trade. Participants who post bids and offers expose themselves to the risk that the market moves against them immediately after they trade. The SEC staff report notes that the spread normally compensates market makers for this risk.

When uncertainty rises, liquidity providers may cancel orders, reduce displayed size, or quote farther from the estimated fair value. The spread can therefore widen before the chart clearly shows the full move. That change is not automatically evidence of manipulation or a broker error. It often means that participants no longer agree closely enough on a safe executable price.

The most common causes are:

Breaking company or market news. Earnings, guidance, a regulatory decision, a financing announcement, a merger update, or an unexpected macroeconomic release can make the stock’s near-term value uncertain. Existing quotes may be canceled faster than new competitive quotes appear.

Fast price movement. During a sharp move, a quote near the last trade can become stale almost immediately. Liquidity providers must continuously reprice their orders or risk trading at an outdated price. Nasdaq’s analysis of spreads and market volatility found a clear relationship between higher volatility and wider spreads in its large-cap data.

Low participation or thin displayed liquidity. If few buyers and sellers are competing at the inside market, the closest bid and ask may naturally be far apart. This can affect thin small-cap stocks, newly listed securities, or otherwise quiet names even during regular hours.

Liquidity withdrawal under stress. Not every electronic liquidity provider must quote aggressively throughout the day. The NYSE’s market-making paper explains that some firms may reduce or stop providing liquidity when conditions become disadvantageous, and that liquidity can be supplied at a higher price through wider spreads or less size.

Extended-hours trading. Premarket and after-hours sessions generally have fewer orders, higher volatility, and less price competition than the regular session. FINRA Rule 2265 specifically warns that lower liquidity and higher volatility may produce wider-than-normal spreads. It also notes that prices can differ between unlinked extended-hours systems.

A trading pause, opening, or reopening. Before continuous trading begins or resumes, the normal two-sided quote may be unstable, indicative, or unavailable. An order imbalance and uncertain clearing price can make the first executable market after the event materially different from the last quote before it.

A trader’s position size does not, by itself, explain why the displayed bid and ask were already far apart. A large incoming order can consume available shares and produce a worse average execution, but that is an order-size and depth problem. The spread should be diagnosed first as the distance between the best current prices.

Four annotated quote snapshots

The following snapshots are hypothetical. They illustrate quote conditions rather than historical trades.

Snapshot 1 — Normal regular-session quote

Time: 10:15:00 a.m. ET

Bid: $40.00 for 2,400 shares

Ask: $40.02 for 2,100 shares

Last: $40.01

Absolute spread: $0.02

Percentage spread: approximately 0.05%

The inside market is competitive and the displayed size is meaningful relative to a small retail order. A buyer demanding immediate execution starts from the $40.02 ask; a seller starts from the $40.00 bid. The quote can still change, but nothing in this snapshot alone signals abnormal deterioration.

Snapshot 2 — News-driven spread expansion

Time: 10:31:00.500 a.m. ET

Bid: $39.92 for 600 shares

Ask: $40.12 for 500 shares

Last: $40.01

Absolute spread: $0.20

Percentage spread: approximately 0.50%

The last trade barely changed, yet the executable market deteriorated tenfold from a $0.02 spread to $0.20. The quote suggests that participants have withdrawn or repriced liquidity while new information is being processed. Using the last price to calculate entry risk would conceal the actual $40.12 purchase price currently offered.

Snapshot 3 — Premarket quote

Time: 8:12:00 a.m. ET

Bid: $39.70 for 100 shares

Ask: $40.30 for 75 shares

Last: $40.05

Absolute spread: $0.60

Percentage spread: 1.50%

The stock may be liquid after 9:30 a.m. ET and still have a wide premarket spread. There are fewer displayed shares, the venues available through a particular broker may be limited, and the last trade may have occurred on a different extended-hours venue. FINRA’s extended-hours trading guidance explains that these markets can be less liquid, more volatile, and not linked in the same way as the regular-session market.

Snapshot 4 — Low-liquidity regular-session quote

Time: 1:40:00 p.m. ET

Bid: $7.35 for 200 shares

Ask: $7.50 for 100 shares

Last: $7.46

Absolute spread: $0.15

Percentage spread: approximately 2.02%

The market is open, but the best buyer and seller remain far apart and show little size. Unlike the temporary news snapshot, this may be the stock’s normal trading condition. Waiting a few seconds may not solve it. The relevant comparison is the stock’s usual spread at the same time and session—not an arbitrary one-cent standard borrowed from a different security.

These snapshots show why “wide” cannot be defined by cents alone. A $0.05 spread has a different meaning in a $5 stock than in a $500 stock, and the same percentage spread has a different impact on a scalp with a $0.10 target than on a position with a multi-dollar target.

How to measure the spread cost before entering

The quote should be translated into the trade’s actual economics.

For a marketable buy, the ask is the starting executable price. For a marketable sell, it is the bid. Investor.gov’s order-type guidance states that market orders generally execute at or near the ask for a buy and the bid for a sell, while the last-traded price is not necessarily the execution price.

Use four checks.

1. Full-spread dollar cost

If a trader buys at the ask and could immediately sell the same shares at the bid without either quote changing:

Displayed round-trip spread cost = spread × shares

With a $0.20 spread and 500 shares, that cost is:

$0.20 × 500 = $100

This is a quote-based estimate, not an execution guarantee. Price improvement, changing quotes, fees, and insufficient displayed size can change the realized result.

2. Spread burden relative to the stop

First calculate risk from the executable entry price—not from the last trade or midpoint.

For a planned long entry at the ask:

Executable risk per share = ask − stop price

Then compare the spread with that risk:

Spread burden = spread ÷ executable risk per share

This ratio is a diagnostic measure, not an industry rule. It shows whether the quote gap is small relative to the room available before the trade is invalidated.

3. Spread burden relative to the target

For a long position:

Executable reward per share = target price − ask

Target burden = spread ÷ executable reward per share

If the spread represents a large share of the expected move, a small change in execution can remove the trade’s planned advantage.

4. Reward-to-risk after the quote changes

Recalculate:

Gross reward-to-risk = executable reward per share ÷ executable risk per share

Do not preserve the original ratio by pretending the entry remains at the last price. The setup must be evaluated at the price currently available.

There is no universal “good spread” in cents or percentage terms. A workable threshold depends on the security, session, order size, holding period, target, stop, and the strategy’s tested execution costs. The practical threshold is the point at which the spread causes the trade to violate a rule defined before the quote widened.

Full scenario: when a valid setup becomes a skip

This example is hypothetical and does not describe a historical trade.

At 10:15 a.m. ET, stock XYZ is quoted at $40.00 bid and $40.02 ask. A trader is considering a 500-share long position with:

1. intended entry at the $40.02 ask;
2. a structural stop at $39.82;
3. a target at $40.42;
4. a maximum displayed spread-cost budget of $25;
5. a minimum gross reward-to-risk ratio of 1.8 to 1.

At the original quote:

1. spread: $0.02;
2. executable risk: $40.02 − $39.82 = $0.20 per share;
3. executable reward: $40.42 − $40.02 = $0.40 per share;
4. gross reward-to-risk: $0.40 ÷ $0.20 = 2.0 to 1;
5. displayed round-trip spread cost: $0.02 × 500 = $10.

The quote passes the trader’s hypothetical spread budget and reward-to-risk rule.

Before the order is submitted, an unexpected company announcement appears. The bid falls to $39.92 and the ask rises to $40.12. Both sides still display at least 500 shares, so this example isolates the effect of the wider top-of-book spread rather than a multi-level fill.

At the new quote:

1. spread: $0.20;
2. executable risk: $40.12 − $39.82 = $0.30 per share;
3. executable reward: $40.42 − $40.12 = $0.30 per share;
4. gross reward-to-risk: $0.30 ÷ $0.30 = 1.0 to 1;
5. displayed round-trip spread cost: $0.20 × 500 = $100.

The chart pattern and target did not change, but the executable trade did. The spread-cost estimate is four times the $25 budget, and the reward-to-risk ratio has fallen below 1.8 to 1. The correct decision under the stated rules is to skip the entry.

Reducing the position to 100 shares would lower the dollar spread estimate to $20, but it would not restore the 2-to-1 reward-to-risk ratio. A limit order could cap the purchase price, but it could also remain unfilled while the market moves. Neither action repairs the original setup automatically.

If the spread later narrows, the trader should build a new calculation from the current bid, ask, sizes, stop, and target. A temporary return to a two-cent spread is not enough if the quote is flickering or the underlying news has changed the trade thesis.

When the usual explanation does not work

Most sudden spread increases reflect volatility, lower liquidity, or reduced competition. Several edge cases require a different diagnosis.

The quote is delayed or venue-specific. A free chart, broker widget, or extended-hours feed may not show the same market as the order ticket. Confirm whether the data is real time, whether it covers the current session, and whether it represents a consolidated regular-session quote or only one venue.

The market is locked or crossed. In a fast fragmented market, feeds can briefly show a bid equal to or above the ask because updates arrive at different times or venues. Do not treat a zero or negative calculated spread as proof that an immediate risk-free trade exists.

The NBBO is not every potentially available price. Beginning April 27, 2026, odd-lot quotations became part of required SIP core data. However, the official Nasdaq UTP SIP odd-lot FAQ states that these quotations remain unprotected and do not affect the round-lot NBBO; the SIP publishes a separate best odd-lot bid and offer. Non-displayed liquidity and possible price improvement can also produce executions inside the quoted spread. The NBBO remains a useful pre-trade benchmark, but it is not a promise of the final effective spread.

The stock is halted or entering an auction. During a halt, opening auction, closing auction, or reopening process, a continuous two-sided quote may not be the correct benchmark. FINRA explains that a trading halt can occur for important news or a significant order imbalance. Wait until the applicable auction or continuous session establishes an executable market before applying a normal spread test.

The security is not an ordinary US-listed common stock. OTC securities, foreign listings, ETFs with illiquid underlying holdings, and other products can have additional pricing and liquidity mechanics. Verify the product and venue before assuming a stock-spread framework explains the quote.

The displayed spread looks inconsistent with executions. A trade can print inside the spread through price improvement, or a stale last trade can sit outside a new quote. Investor.gov’s execution guide notes that quotes apply to specific share quantities and can change while an order is routed. Compare precise timestamps, bid and ask data, sizes, and individual fills before concluding that the broker mishandled an order.

The spread decision framework

Use this sequence whenever a stock’s spread suddenly widens.

1. Confirm the symbol, session, and market-data status.
2. Record the bid, ask, bid size, ask size, last trade, and exact timestamp.
3. Calculate the absolute and percentage spread.
4. Compare the quote with the stock’s recent baseline during the same session—not only with yesterday’s close or another security.
5. Check for earnings, filings, economic releases, trading pauses, order imbalances, or other time-sensitive events.
6. Recalculate entry from the ask for a buy or the bid for a sell.
7. Recalculate the stop distance, target distance, reward-to-risk ratio, and dollar spread cost.
8. Compare the planned order with the displayed size, while recognizing that deeper liquidity is a separate execution question.
9. Use a limit price only if the trade can tolerate a partial fill or no fill. Do not chase the ask merely to preserve the original idea.
10. Skip the trade if any pre-defined cost or risk rule fails, if the quote source cannot be verified, or if the spread is changing too quickly to calculate reliably.

Hi2morrow methodology: We classify the spread as acceptable, conditional, or disqualifying. “Acceptable” means the current executable quote passes every pre-trade cost and risk limit. “Conditional” means the trade remains valid only with a smaller size, a defined limit price, or a narrower quote. “Disqualifying” means the spread alone breaks the dollar-cost budget, minimum reward-to-risk ratio, or reliable-exit assumption. A visually attractive chart does not override a disqualifying quote.

Professional analysis — Khasan Kadyrov: The most useful information in a sudden spread expansion is not the extra number of cents; it is the withdrawal of agreement between buyers and sellers. From a risk-management perspective, that is a new market state. The trader should not ask how to force the original entry through it. The correct question is whether the setup still works at the prices that are actually executable.

A bid-ask spread widens when the best buyer and seller retreat from each other because liquidity, competition, or confidence in the current price has deteriorated. The appropriate response is to verify the quote, identify the cause, and rebuild the trade calculation from the bid and ask. If the spread turns a valid setup into an over-budget or under-rewarded trade, waiting or skipping is part of execution discipline—not a missed opportunity.

Khasan Kadyrov is a hi2morrow analyst and an economist with five years of experience in the US stock market.

Editorial note: Substantively updated on August 4, 2026. The article was checked against current Investor.gov and FINRA guidance, FINRA Rule 2265, the SEC’s equity-market-structure and after-hours materials, and official Nasdaq and NYSE market-structure research. Market-data coverage, order handling, and extended-hours access vary by broker and venue and should be verified before trading.

Educational material only. Not investment advice.

Sources

1. Investor.gov: Bid Price/Ask Price
2. Investor.gov: Types of Orders
3. Investor.gov: Executing an Order
4. SEC: Staff Report on Equity and Options Market Structure Conditions in Early 2021
5. SEC: Investor Bulletin—After-Hours Trading
6. FINRA Rule 2265: Extended Hours Trading Risk Disclosure
7. FINRA: Extended-Hours Trading—Know the Risks
8. FINRA: Trading Halts, Delays and Suspensions
9. Nasdaq: Have Spreads Changed Over Time?
10. NYSE: Market Makers in Financial Markets
11. Nasdaq UTP SIP: Odd Lot Quotes FAQ

Author: Alexander Styopin trader with 24 years of trading experience and an economic analyst at hi2morrow

Originally published: June 18, 2026

Substantively updated: August 4, 2026