Nearly 30 percent of California public companies will have to add women to their board of directors by year end to comply with a 2018 state law aimed at boosting gender diversity in the boardroom.

The Women on Boards survey released last week by San Diego-based Board Governance Research looked at the board makeup of 642 publicly traded companies headquartered in California. It found 184 firms will need to find a woman director this year under the law.

Most are small publicly traded companies with a market capitalization of $300 million or less, said Annalisa Barrett, author of the study and chief executive of Board Governance Research.

"Almost 200 this year are going to have to add women to their boards, and certainly by the 2021 deadline, there is going to be significant change in the boardrooms of California companies," she said.

In October, former Gov. Jerry Brown signed a law that requires publicly traded corporations headquartered in California to include at least one woman on their boards of directors by the end of this year.

By the end of 2021, companies with five-member boards must have at least two female directors; corporations with six or more directors would need at least three women. Companies that fail to comply will face six-figure financial penalties.

Based on the board makeup today, California public companies will have to fill 1,060 board seats with female directors by 2021, according to survey.

The goal of the law is to close the gender gap in the corporate world. It is expected to face legal challenges — though no lawsuits have been filed to date.

Companies still have time to comply, so no fines have been issued to challenge in court, said Dan Eaton, an attorney with Seltzer, Caplan, McMahon & Vitek in San Diego.

And public companies are likely reluctant to file a lawsuit that could be seen as opposing gender diversity, he said.

California boardrooms are predominantly male. About 85 percent of nearly 5,000 board seats of public companies headquartered in the state are held by men, according to the study.

The larger the company, the more likely it is to have women board members. Of the state's companies that are both on the Fortune 1000 and Russell 3000 indexes, 97 percent have at least one female director. The Fortune 1000 and Russell 3000 are indexes of the largest publicly traded companies in the U.S. based on market capitalization.

Today, 30 percent of Fortune 1000 companies in California already comply with 2021 requirements of the new law regarding female directors.

For smaller California public companies, only 8 percent currently meet the law's 2021 benchmark.

"The larger companies are much further along," said Barrett. "They have always had women on the board and have been responsive to shareholders calling for diversity on the board. It is the smaller companies that have yet to act."

Diversity and independence on corporate boards is being driven in part by large institutional shareholders, said Barrett. Some studies have shown that companies with higher levels of gender diversity have stronger financial performance, she said.