The gap begins at the junior lawyer level, and is more pronounced among seasoned attorneys at major firms, persisting even when partners possess similar levels of experience and work in the same market, according to the review by Sky Analytics Inc., a provider of software to help companies track legal spending and invoices.
For example, senior male litigators at regional firms in Los Angeles with more than 25 years of experience charged on average 8.3% more than their female equivalents—$487 an hour versus $450.
In New York, the average male partner at a 1,000-plus lawyer firm with 13 to 24 years of experience representing investment banks was billed out at about $679 an hour, nearly 25% more than the average female partner, whose rate was $544.
"It was very significant, across all categories," said Silvia Hodges Silverstein, vice president of strategic market development at Sky Analytics. Ms. Silverstein, who teaches law-firm management at Fordham and Columbia law schools, looked at three years of billing data from more than 3,000 law firms.
The findings highlight a persistent gender gap in the legal profession despite inroads made by women over the past four decades. The statistics echo results from earlier surveys that found similar discrepancies in the compensation realm.
One-third of U.S. lawyers and judges are now women—compared to roughly 4.8% in 1970, according to U.S. census data—but they remain relatively rare among the top ranks of the profession.
Women make up only 17% of so-called equity partners with ownership stakes at the 200 top-grossing U.S. law firms, according to the National Association of Women Lawyers, and they are similarly underrepresented in management roles and on powerful governing committees. By contrast, an overwhelming majority of so-called "rainmaker" lawyers credited with bringing in substantial business are men, the association's most recent survey found.
"Women are less likely than men to reach the highest levels…and when they do they are still paid somewhat less than their male peers," said Stephanie Scharf, a senior partner with Scharf Banks Marmor LLC in Chicago, and a past president of the association.
Ms. Scharf said hourly rates are often tied to a lawyer's status in the firm—whether an attorney is a department head of a particular practice, for example, or sits on the firm's executive committee—and don't necessarily reflect a person's marketability or skill level.
"To the extent women partners are not advancing into the highest levels of firms," she said, "then their billing rates will reflect that situation."
Some women attribute the broader achievement gap to the often subjective ways that law firms evaluate, reward and promote partners.
"It's all a series of backroom assessments and backroom deals," said Joan C. Williams, a professor at the University of California's Hastings College of the Law. "It's just like a petri dish for gender bias."
Firms that responded to the National Association of Women Lawyers survey said key obstacles to women's advancement include lack of business development opportunities, work-life balance issues and attrition, as women lawyers leave firms for better job opportunities elsewhere.
Some prominent women lawyers say they charge as much—and are as well compensated—as their male colleagues, but note that is not always the case for women with less power.
"If you are a woman and you control a lot of business, the firm is going to want to keep you happy," said Patricia K. Gillette, an employment lawyer at Orrick, Herrington & Sutcliffe LLP in San Francisco who is involved in a number of initiatives aimed at increasing retention of female lawyers.