(Bloomberg) -- Zenefits, a provider of human-resource toolsand services, raised $500 million in funds at a valuation of $4.5billion as more companies seek ways to streamline their employeebenefit programs.

The round, led by Fidelity Management & Research Co. andTPG, is the largest financing for a software-as-a-service businesssince 2012, the San Francisco-based company said in a statementWednesday.

Zenefits provides free online software that companies use tomanage benefits for their staff, including payroll, retirementaccounts and health insurance. Insurers pay Zenefits to offer theirservices, and the startup has signed up more than 10,000 small-andmedium-sized customers in the United States. The startup needs newfinancing to hire more staff and speed up the pace of growth,co-founder and Chief Executive Officer Parker Conradsaid.

“We’re driving this car; we’re driving it pedal-to-the-metal theentire way,” Conrad said in an interview. “If you want to growreally fast, you burn a lot of gas.”

Zenefits, which hired Yammer co-founder David Sacks as chiefoperating officer last year, said it plans to double headcount to2,000 by the end of this year. The company opened an office inArizona in November.

Zenefits last raised funds in June, attracting $66.5 million ata valuation of $500 million. Founders Fund, Khosla Ventures,Insight Venture Partners and Ashton Kutcher’s Sound Ventures alsoparticipated in the latest round, along with existing investors,including Andreessen Horowitz, Institutional Venture Partners andJared Leto.

Bloomberg LP, the parent of Bloomberg News, is an investor inAndreessen Horowitz. Zenefits is Andreessen Horowitz’s largestinvestment to date, the startup said.

As of January, Zenefits had annual recurring revenue of $20million, up from $1 million a year earlier, the company said. Thecompany forecast that it would reach $100 million in sales byJanuary 2016.

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