An updated regulatory filing with the U.S. Securities and Exchange Commission shows the company plans to sell 22.4 million shares and its existing shareholders expect to sell 7.5 million shares.

Fitbit, however, won't receive capital from the sale of the stock held by its existing investors, the filing said. The San Francisco company publicly announced its plan to go public in last month, after which wearables rival jawbone launched a legal action against Fitbit accusing it of "systematically plundering" its employees, trade secrets and intellectual property.

Fitbit Surge - helped drive a surge in registered users and an optimistic IPO share price. Image: Fitbit

According to the updated filing, Fitbit appears to be a healthy company. As of March 31, the company has sold more than 20.8 million devices since it was founded in 2007. The company more than doubled the number of devices it sold in the first three months of 2015 compared to the year-ago period. The company sold 3.9 million devices through March 31, compared to 1.6 million devices in the first three months of 2014.

For the three months ending March 31, Fitbit's revenue totaled $336.8 million. That's up from $108.8 million compared to the year-ago period. Profit during that time soared, increasing to $48 million from $8.9 million.

Meanwhile, Fitbit appears to be quickly growing its number of paid active users, with 2.8 million people signing up for services in the first three months of 2015. As of March 31, the company had 9.5 million active users compared to the 6.7 million active users it had at the end of 2014.

Fitbit sells a range of wearable devices that track a person's physical activity and offer analytics on this data in mobile apps and an online dashboard. Other Fitbit products include a Wi-Fi-equipped scale and a tracker

The wearable device market is becoming increasingly competitive, with Fitbit's devices facing off against smartwatches from tech heavyweights like Apple and Samsung as well as fitness trackers from companies like Garmin and Jawbone.

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