Tick Size and Lot Size: Minimum Price and Trade Units

Key Takeaways

• Tick size is the minimum price increment for displayed quotes; US stocks above $1 use a $0.01 tick since 2001, with some liquid names moving to $0.005 under 2024 SEC amendments.
• Round lot definitions changed in 2024 so stocks above $250 now have lot sizes smaller than 100 shares, making NBBO quotes more reflective of real retail activity.
• Investors using tools built for the old 100-share universal round lot will misclassify orders in high-priced stocks, affecting execution quality analysis.
• Tick size directly shapes bid-ask spread economics: for very liquid names, the tick is the spread, so narrower ticks compress the cost of crossing it.

What It Is

Tick size is the minimum price increment, set under Regulation NMS Rule 612. For US stocks priced at $1 or above, the tick has been $0.01 since the 2001 decimalization moved the market off fractions. Below $1, sub-penny ticks apply. The SEC adopted amendments in 2024 that will move many liquid stocks to a $0.005 tick for quoting.

Lot size is the normal trading unit. The traditional US equity round lot is 100 shares. Anything smaller is an odd lot. A mixed lot is more than 100 shares that is not a multiple of 100 (for example, 250 shares). The 100-share convention dates to the NYSE Rule Book and is still the default display unit on the consolidated tape.

The Intuition

Tick size is a design trade-off. A smaller tick lets quotes refine more precisely, which usually tightens effective spreads. But very small ticks fragment liquidity across many price levels, which can hurt the depth at any single one. A larger tick concentrates resting orders at fewer levels, which can help execution for large trades but costs price improvement to small ones.

Lot size exists because historically only round lots had full protection on the national best bid and offer (NBBO). Odd lots printed but often did not affect the quote, which created an "odd lot world" where small orders traded at prices the tape did not advertise. SEC reforms in 2020 and 2024 tightened the round-lot definition for expensive stocks precisely because a 100-share round lot at a $3,000 share price was a poor reflection of real retail activity.

How It Works

Before 2001, US stocks quoted in fractions. Minimum increments were one-eighth ($0.125) and later one-sixteenth ($0.0625). On April 9, 2001, the exchanges completed decimalization and stocks above $1 moved to a $0.01 tick. Sub-penny quoting was banned for stocks at or above $1 in 2005 under Rule 612.

The SEC ran a Tick Size Pilot from October 2016 to October 2018 on small-cap stocks, testing a $0.05 tick on three test groups against a $0.01 control. Results were mixed and the pilot ended without a permanent change. In September 2024, the SEC adopted amendments to Regulation NMS that will allow a $0.005 tick for stocks where the time-weighted average quoted spread is at or below $0.015, estimated to cover roughly three quarters of US listed equities.

Round-lot definitions were updated in the same round of reforms. Instead of a universal 100-share lot, size now depends on price:

Share price         Round lot size
$0.00 - $250.00     100 shares
$250.01 - $1,000    40 shares
$1,000.01 - $10,000 10 shares
> $10,000           1 share

The SEC's stated goal was to bring the NBBO closer to where retail investors actually transact. A stock trading at $800 per share would historically show a 100-share NBBO, meaning the displayed quote required $80,000 of interest. Under the new tiers, a 40-share quote is protected.

Worked Example

A stock is quoted $50.10 bid, $50.12 ask. The tick is $0.01, so the minimum the ask could move down is to $50.11. A retail order to buy 80 shares at market executes at $50.12 and prints as an odd lot. The NBBO does not change because neither the buyer nor the seller adjusted a displayed 100-share quote.

Now the stock goes ex-dividend and the price rises to $280 per share. Under the updated rules, its round lot becomes 40 shares. A 60-share order is a mixed lot (one round lot plus 20 odd lot). A 30-share order is an odd lot. The NBBO is now quoted and protected for a minimum size of 40 shares, not 100.

If this same stock is later moved to a $0.005 minimum quoted increment because of tight spreads, a quote could read $280.005 bid, $280.015 ask. The half-cent increment is visible in quote feeds but trades still print to the full cent unless the broker opts in to the new grid.

Common Mistakes

1. Assuming all US stocks have a $0.01 tick. Sub-dollar stocks already trade in sub-penny increments, and the 2024 amendments put many liquid large caps on a $0.005 quoting grid. Always check the current minimum for the instrument you are modeling.

2. Believing odd lots do not affect execution. Odd lots now account for a large share of daily volume (often over 40 percent for high-priced tech names). They print on the tape and are routed like any other order, even if they historically did not update the NBBO.

3. Using the 100-share round lot for expensive stocks. For stocks above $250, the round lot is no longer 100 shares. Tools and spreadsheets built on the old assumption will misclassify orders.

4. Ignoring the link between tick and spread. In very liquid names, the tick is the spread. Cutting the tick can tighten spreads but reduce incentive to post liquidity, with the net effect varying by stock.

Frequently Asked Questions

Q: What are tick size and lot size in simple terms? Tick size is the smallest price step a stock can move. A $0.01 tick means a quote can jump from $50.00 to $50.01 but nothing in between. Lot size is the standard number of shares in a trading unit, traditionally 100 but now tiered by share price under 2024 rules.

Q: How do tick size and lot size affect investment decisions? They shape the visible cost of trading. A smaller tick means quotes can refine finer, typically tightening spreads. Lot size affects which quotes appear in the NBBO, so a stock priced at $500 with a 40-share round lot produces more accurate quotes than the old 100-share standard.

Q: What is a real-world example of tick size and lot size? A stock quoted $280.005 bid / $280.015 ask on the new $0.005 tick. The half-cent spread costs you $5 per 1,000 shares to cross. Under the old $0.01 floor, the minimum spread would have been $10 for the same order.

Q: How can investors use tick and lot knowledge effectively? Before modeling execution costs, confirm the current tick size and round-lot definition for the specific security. Tools built on pre-2024 assumptions will overstate spread costs for tight stocks and misclassify odd-lot volume in expensive names.

Q: How is tick size different from bid-ask spread? Tick size is the regulatory minimum price increment. The bid-ask spread is the actual gap between best bid and best ask at any moment. In very liquid stocks the spread equals the tick; in wider-spread names the gap between bid and ask can be many ticks.

Sources

1. U.S. Securities and Exchange Commission. "Investor Alert: Tick Size Pilot Program." https://www.sec.gov/resources-for-investors/investor-alerts-bulletins/ia_ticksize
2. U.S. Securities and Exchange Commission. "Statement on Minimum Price Increments, Access Fee Caps, Round Lots, and Odd-Lots" (Chair Gensler, September 18, 2024). https://www.sec.gov/newsroom/speeches-statements/gensler-statement-regulation-nms-091824
3. U.S. Securities and Exchange Commission. "Tick Size Pilot Program." https://www.sec.gov/data-research/tick-size-pilot-program
4. U.S. Securities and Exchange Commission. "Market Activity Report Methodology." https://www.sec.gov/securities-topics/market-structure-analytics/market-activity-report-methodology

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.