In July 2020, our board of directors approved the 2020 Equity Incentive Plan, or 2020 Plan, under which stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares and units and other cash-based or stock-based awards may be granted to employees, consultants and directors. Shares of common stock that are issued and available for issuance under the 2020 Plan consist of authorized, but unissued or reacquired shares of common stock or any combination thereof.

A total of 3,873,885 shares of our Series 1 common stock was initially authorized and reserved for issuance under the 2020 Plan. This reserve will automatically increase on January 1, 2021, and each subsequent anniversary through and including January 1, 2031, by an amount equal to the smaller of (a) 5% of the number of shares of Series 1 and Series 2 common stock issued and outstanding on the immediately preceding December 31 and (b) an amount determined by our board of directors. On January 1, 2021 the reserve increased by 3,484,045 shares. As of December 31, 2021, a total of 7,703,241 shares of common stock remain available for future issuance under the 2020 plan.

In February 2013, we adopted the 2013 Plan under which stock options may be granted to employees, consultants and directors. Upon the completion of our IPO in August 2020, the board of directors terminated the 2013 Plan and 1,470,291 shares that were available for future issuance under the 2013 Plan were transferred and authorized for issuance under the 2020 Plan. As of August 2020, no further awards may be granted under the 2013 Plan.

We use the Black-Scholes option-pricing model to estimate the fair value of our share-based payment awards. The Black-Scholes option-pricing model requires estimates regarding the risk-free rate of return, dividend yields, expected life of the award, and expected annual volatility during the service period. The calculation of expected volatility is based on historical volatility for comparable industry peer groups over periods of time equivalent to the expected life of each stock option grant. As we do not have a significant history as a publicly traded company, we believe that comparable industry peer groups provide a reasonable measurement of volatility in order to calculate a reasonable estimate of fair value of each stock award. The expected term is calculated based on the weighted average of the remaining vesting term and the remaining contractual life of each award. We based the estimate of risk-free rate on the U.S. Treasury yield curve in effect at the time of grant or modification. We have never paid cash dividends and do not currently intend to pay cash dividends, and thus have assumed a dividend yield of zero.

Subsequent to our IPO on August 4, 2020, we utilize the quoted market price for our stock on the grant date in the fair value calculation. Prior to our IPO, we estimated the fair value of common stock at the time of grant of the option by considering a number of objective and subjective factors, including independent third-party valuations of our common stock, operating and financial performance, the lack of liquidity of capital stock, and general and industry-specific economic outlook, among other factors.

We estimate potential forfeitures of stock grants and adjust compensation cost recorded accordingly. The estimate of forfeitures will be adjusted over the requisite service period to the extent that actual forfeitures differ, or are expected to differ, from such estimates. Changes in estimated forfeitures will be recognized through a cumulative catch-up adjustment in the period of change and will also impact the amount of stock compensation expense to be recognized in future periods.

The following table summarizes the weighted-average grant date value of options and the assumptions used to develop their fair value.

	Year ended December 31,
	2021		2020		2019
Weighted-average grant date fair value of options	$	30.71	$	7.01	$	1.20
Risk-free interest rate	0.96%—1.08%		0.34%—0.84%		1.51%—2.53%
Expected volatility	54.41%—56.25%		49.64%—51.49%		46.70%—47.87%
Expected life in years	6.02—6.06 years		5.49—6.10 years		5.52—6.08 years
Dividend yield		—		—		—

A summary of the changes in common stock options issued under all of the existing stock option plans is as follows:

(in thousands, except per share amounts)	Shares			Weighted average of exercise prices		Weighted average of remaining term (years)		Aggregate intrinsic value
Options outstanding at December 31, 2019		9,327		$	2.22		8.17	$	65,294
Granted		1,384			14.85		—		—
Exercised		(2,053	)		2.25		—		—
Forfeited		(443	)		5.33		—		—
Options outstanding at December 31, 2020		8,215		$	4.30		7.65	$	491,648
Granted		263			58.36		—		—
Exercised		(2,426	)		2.46		—		—
Forfeited		(324	)		11.22		—		—
Options outstanding at December 31, 2021		5,728		$	8.77		6.95	$	168,772
Vested and expected to vest at December 31, 2021⁽¹⁾		5,490		$	6.71		6.91	$	164,384
Vested at December 31, 2021		3,629		$	3.53		6.46	$	116,440

The total intrinsic value of options exercised during the years ended December 31, 2021, 2020 and 2019 was $126.0 million, $72.4 million and $5.6 million, respectively. The intrinsic value was calculated as the difference between the estimated fair value of our common stock at exercise, and the exercise price of the in-the-money options. The total grant date fair value of options vested for the years ended December 31, 2021, 2020, and 2019 was $6.4 million, $11.4 million, and $9.4 million.

At December 31, 2021, 2020 and 2019, there was an estimated $11.5 million, $11.4 million and $9.4 million, respectively, of total unrecognized compensation costs related to stock options. These costs will be recognized over a weighted-average period of 2.5 years.

During the year ended December 31, 2021, we granted 1,352,746 RSUs to members of management, board members and certain other employees pursuant to the 2020 Plan, and 195,339 RSUs related to an acquisition under the 2021 Inducement Plan. The fair value of the RSU grant is determined based upon the market closing price of our common stock on the date of grant. The RSUs vest over the requisite service period of 4 years, subject to the continued employment of the employees. The following table summarizes the RSU activity, including vesting of the performance-based restricted stock units below, under the Plans for the year ending December 31, 2021:

(in thousands, except per share amounts)	Shares			Weighted average grant date fair value
Nonvested at December 31, 2019		—		$	—
Granted		1,408			24.67
Vested		—			—
Cancelled/Forfeited/Expired		—			—
Nonvested at December 31, 2020		1,408		$	24.67
Granted		1,548			57.19
Vested		(350	)		24.07
Cancelled/Forfeited/Expired		(234	)		42.97
Nonvested at December 31, 2021		2,372		$	44.10

At December 31, 2021, there was an estimated $60.9 million of total unrecognized stock-based compensation costs related to RSUs. These costs will be recognized over a weighted-average period of 3.4 years.

During the year ended December 31, 2020, we granted 1,216 PSUs to members of management pursuant to the 2013 Plan. These PSUS contained a performance clause which required us to successfully complete an IPO as well as a service condition that required continued employment. As of December 31, 2021, 810 PSUs remain unvested and outstanding. These PSUs vest on a tranche by tranche basis over the life of the service period of 1-4 years.

At December 31, 2021, there was an estimated $4.4 million of total unrecognized stock-based compensation costs related to these PSUs. These costs will be recognized over a weighted-average period of 1.7 years.

	Year ended December 31,
(in thousands)	2021		2020		2019
Cost of revenue	$	2,055	$	769	$	191
Sales and marketing		7,761		3,310		838
Research and development		5,901		2,500		666
General and administrative		9,707		4,479		1,461
Total stock-based compensation expense	$	25,424	$	11,058	$	3,156

As of December 31, 2019, we had six outstanding series of redeemable convertible preferred stock. These preferred shares were classified as temporary equity within our consolidated balance sheet as of December 31, 2019. Immediately upon closing of our IPO, the outstanding preferred stock was automatically converted into an aggregate of 29,390,733 shares of Series 1 common stock and 5,050,555 shares of Series 2 common stock. Under the terms of Series F preferred stock, dividends were required to be paid at 10 percent, which could be adjusted for the holder’s actual rate of return upon redemption. Upon completion of our IPO with an offering price of $24 per share, we met the threshold for a reduction of dividends and reduced the required dividend rate to 8 percent. We utilized a portion of the proceeds from the IPO to pay the cumulative dividends of $12.8 million to the holders of our Series F preferred stock. As of December 31, 2021 and 2020, there was no preferred stock issued or outstanding.