Viral Marketing Strategy: 29 K Factor Tactics That Generated 3,528 Leads

Viral marketing for a software product means building the product so that the people using it bring in new users as they use it: by inviting colleagues, sharing what they made, or referring friends for a reward. Its strength is measured by the viral coefficient, or K-factor: the number of invites each user sends multiplied by the share of those invites that become new users. A K above 1 means every user brings in more than one new user, so growth sustains itself, and the viral cycle time, how long one round of inviting takes, decides how fast it compounds.

That is the meaning Wikipedia’s K-factor entry and First Round Review’s glossary give the term. It is different from a social media post going viral: a post can spread once and stop, while a product loop keeps running for as long as people use the product.

Key takeaways

• The K-factor is invites sent per user multiplied by the conversion rate of those invites, so five invites at a 20% conversion rate give a K of 1.0, as First Round Review’s glossary works the example (read November 2026).
• In David Skok’s 2009 viral growth model, 10 users with a K of 2 grow to 20,470 users in 20 days when one round of inviting takes two days, and to over 20 million when it takes one day.
• Dropbox’s two-sided referral reward permanently increased signups by 60%, and 35% of daily signups came from the referral program, according to Drew Houston’s April 2010 Startup Lessons Learned deck.
• Hotmail reached 750,000 users by Labor Day 1996 and 1 million within six months after adding a line inviting every email recipient to get a free account, according to Adam Penenberg’s book Viral Loop (excerpted on TechCrunch in 2009).
• In our worked example using Skok’s formula, 100 starting users with a K of 0.9 can never pass 1,000 users, while a K of 1.2 with a seven day cycle reaches about 4,850 users in 12 weeks.

How a viral loop works

Wikipedia describes viral marketing as a strategy that relies on people spreading a product to others, in the way a virus passes from one person to another. For a consumer brand that usually means content people share. For a software company the more useful version lives inside the product: a user does their normal work, and that work puts the product in front of someone who is not a user yet.

Andrew Chen describes a viral loop as everything a user does between arriving at a product and inviting the next group of new users. Every software viral loop has the same parts, and each one is something you can measure and change:

1. An existing user does something with the product that reaches another person: sends a contract to sign, a meeting invite, a video or a referral link.
2. The other person sees the product working. The invitation arrives with a reason to care, usually the thing the user wanted them to see.
3. Some of those people sign up. The share that do is the conversion rate.
4. The new users repeat step 1, and the time from one user signing up to their invitees signing up is the cycle time.

How to calculate your viral coefficient (K-factor)

The formula is short. Wikipedia writes it as k = i × c, where i is the number of invites each user sends and c is the share of invites that convert:

• Invites per user (i): how many people one new user invites, shares with or exposes the product to.
• Conversion rate (c): the share of those people who become users.
• K-factor: i multiplied by c. Five invites with a 20% conversion rate give a K of 1.0, the example in First Round’s glossary: each user replaces themselves with exactly one new user.

A K above 1 means each round of users is bigger than the last, so growth compounds without new marketing spend. A K below 1 means each round is smaller, and the loop fades out unless you keep adding users from other channels.

Some products send far more than a handful of invites. Aye Moah, co-founder and CEO of the email tool Boomerang, explained her product’s loop to a Founderpath audience with its meeting scheduler, where every meeting a user books shows the guest a Boomerang page:

If you have one Boomerang user sending out 100 meetings, they are 100 people coming across Boomerang experience, and the pitch is very simple: you just had a great meeting scheduling experience, don’t you want that for yourself?… 100 people come across, two of them convert, we have two new user[s] for free. Then those two people send out another 100 messages, you get another two users for free.

Aye Moah, Co-founder and CEO, Boomerang ($8M revenue). Watch at 8:14

In her illustration, 100 invites with a 2% conversion rate give a K of 2: a low conversion rate can still make a loop viral when each user reaches enough people. It also shows where the work is. Moah’s team treated the page a guest sees after confirming a meeting as a high leverage page, because a small lift in its conversion rate improves the loop for every user.

Why viral cycle time matters more than the K-factor

The K-factor tells you whether a loop grows. The cycle time tells you how fast. David Skok worked out the formula with his partner Stan Reiss in his 2009 post on viral marketing: customers at time t equal the starting customers multiplied by (K to the power of t divided by the cycle time, plus 1, minus 1), divided by (K minus 1). In his own example, 10 users with a K of 2 reach 20,470 users after 20 days with a two day cycle, and over 20 million with a one day cycle. Skok concluded that the most important factor in viral growth is not the viral coefficient but the cycle time.

Here is the same formula applied to a smaller, more typical case: 100 starting users, measured after 12 weeks.

• K of 0.9 (three invites at 30%): about 522 users with a 14 day cycle and about 746 with a seven day cycle. With K below 1 the total can never pass 1,000, because each round is smaller than the one before.
• K of 1.2 (three invites at 40%): about 1,292 users with a 14 day cycle and about 4,850 with a seven day cycle.

Halving the cycle time nearly quadrupled the result at K 1.2, while the same change added only about 220 users at K 0.9. Nathan Latka, Founderpath’s founder and CEO, made the same point to a December 2024 webinar audience: when you choose between a higher coefficient and a shorter cycle, choose time, because one person invited fast beats 10 people invited slowly.

In practice that means asking for the invite on day one, inside the first task a user completes, rather than in a reminder email a week later. A loop that depends on a user’s next project, next hire or next event will always be slow, however good its conversion rate.

The main types of viral loops, with examples

Most SaaS products that grow virally use one of five loops. The table shows how each one spreads, a verified example and its source.

Loop type	How it spreads	Example	Source
Collaboration	Using the product needs a second person	A contract in PandaDoc needs another person to sign it, so one user turns into two	Nathan Latka, December 2024 webinar
Shared output	What a user makes or sends is seen by people who are not users	A Loom video’s viewers watch it, get curious and sign up	Shahed Khan, Loom, 2019 interview
Powered by badge	The product’s name rides on everything users publish	Hotmail’s line at the bottom of every email, inviting recipients to get a free account	Penenberg, Viral Loop, via TechCrunch 2009
Incentivized referral	Users invite others for a reward, often on both sides	Dropbox’s referral program rewarded both the referrer and the new user	Drew Houston, April 2010 deck
Word of mouth	Users recommend the product without being asked or paid	Loom grew with no paid marketing while users talked about it with peers	Shahed Khan, Loom, 2019 interview

Collaboration and shared output loops

These are the loops people mean when they call a product inherently viral. An e-signature tool cannot be used alone, and a scheduling tool always reaches the person being scheduled. Loom’s loop was its videos: co-founder Shahed Khan told Nathan Latka in 2019 that Loom had about 1.1 million registered users across about 21,000 companies and had not paid for marketing. He described the loop this way:

If you send someone a Loom video, they’ll watch it, they’re curious, and they’ll sign up. There are a couple of growth tactics that we have implemented such as once a user is done watching a video, we try and prompt them to sign up to either reply to a Loom video with another Loom video… we use those different product hooks and triggers to kind of continue the growth loop.

Shahed Khan, Co-founder, Loom. Watch at 2:52

Watch Shahed Khan’s full interview on Loom’s viral growth before its first paywall, with transcript.

Powered by badges

A powered by badge puts your name on the forms, emails and pages your customers publish. Hotmail is the classic case: in Penenberg’s account, investor Tim Draper pushed for a line at the bottom of every message, the founders shipped it without the “P.S. I love you” he had proposed, and the service reached 750,000 users by Labor Day 1996 and 1 million within six months (TechCrunch excerpt). Our guide to powered by virality covers where to place a badge and how to measure it.

Incentivized referrals

When sharing is not part of normal use, a reward can create the loop. Dropbox is the best documented example. Drew Houston’s April 2010 Startup Lessons Learned deck says a referral program with a two-sided incentive, inspired by PayPal’s $5 signup bonus, permanently increased signups by 60%. In the 30 days to April 2010, users sent 2.8 million direct referral invites; 35% of daily signups came from the referral program and another 20% from shared folders and other viral features, as Dropbox grew from 100,000 registered users in September 2008 to 4 million in January 2010.

Smaller companies run the same play. Loom experimented with a referral reward of $5 in Loom credits for every person a user referred, which Khan said encouraged people to invite coworkers when Loom had about 100,000 to 200,000 users (watch at 3:25). A paid commission to customers or partners is the affiliate version of this loop, covered in our affiliate marketing guide.

Word of mouth

Word of mouth is the loop you cannot fully engineer, but you can feed it: a product people want to talk about, and places where they talk. Our guide to building a community around your product covers the second part.

How to choose a viral loop for your product

Start from how your product is already used, not from the loop you admire. Three questions narrow it down:

• Does using the product need another person? If yes, build the collaboration loop first: make the invited person’s first experience excellent and give them a reason to sign up from inside it.
• Do other people see what your users make? If yes, put your name on the output, with a link that lets a viewer start their own.
• Would users refer others for a reward? If the product is valuable enough that users recommend it, a two-sided reward speeds that up.
• If none apply, rely on word of mouth and on an offer good enough that people pass it on, and use paid or outbound channels to keep growing while K stays below 1.

What if your product isn’t inherently viral?

Most B2B products are used by one person or one team, and nobody outside sees the work. Latka opened the virality section of the same December 2024 webinar with exactly that objection: founders hear about a document signing company growing through virality and conclude that their product cannot. His answer was to manufacture virality with economic incentives that get new or free users to send invites faster, send more of them and raise the acceptance rate: a powered by link, a free plan and an affiliate program.

The offer itself can also do the spreading. Adam Robinson, founder and CEO of the website visitor identification tool RB2B, told a Founderpath webinar audience in August 2024:

I just don’t think people in SaaS emphasize how important your core offer is… you can create an offer that is so compelling that it actually has virality on its own without any other engineering virality tricks.

Adam Robinson, Founder and CEO, RB2B. Watch at 38:02

His example was RB2B’s free plan, pitched on his LinkedIn banner as identifying anonymous website visitors and pushing their LinkedIn profiles to Slack for free (RB2B names him founder and CEO). A free offer that is easy to explain gets passed on even without an invite button.

How to measure your K-factor and cycle time

Measure the loop by cohort, not across all users at once, so old users who stopped inviting do not drag the number down and a launch spike does not inflate it:

1. Pick a cohort: the users who signed up in one week.
2. Count their invites: invites, shares or referral links sent by that cohort during one cycle. Divide by the cohort size to get i.
3. Count conversions: how many of those invited people signed up. Divide by the invites to get c, and multiply i by c to get K.
4. Time the loop: the median number of days from a cohort member signing up to their first invitee signing up is your cycle time.
5. Repeat each week and watch the trend, not one reading.

Count only invitees who stay. First Round Review’s glossary warns that a good virality score does not guarantee sustainability: a product can acquire a burst of users who then churn, and, as one founder told First Round, a K-factor is easy to spin so it looks good in a pitch. A loop that brings in users who leave a week later is not compounding anything.

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Frequently asked questions

What is viral marketing for a SaaS product?

Viral marketing for a SaaS product means designing the product so its users bring in new users as they use it, by inviting colleagues, sharing what they made or referring others for a reward. It is measured with the viral coefficient, or K-factor, and the viral cycle time.

How do you calculate the K-factor?

Multiply the average number of invites each user sends by the share of those invites that convert into new users. Five invites with a 20% conversion rate give a K-factor of 1.0, meaning each user brings in one new user.

What is a good K-factor?

A K-factor above 1 means each user brings in more than one new user, so growth sustains itself. Below 1 the loop fades unless you add users from other channels, but it still multiplies them: with a K of 0.5, every 100 users you acquire bring in about 100 more over time.

What is viral cycle time?

Viral cycle time is how long one round of the loop takes, from a user signing up to the people they invite signing up. In David Skok’s 2009 model, halving the cycle time from two days to one turned 20,470 users after 20 days into over 20 million, which is why he called cycle time more important than the K-factor.

What are examples of viral marketing in SaaS?

Dropbox’s two-sided referral reward permanently increased signups by 60% (Drew Houston, April 2010). Hotmail added a free email invitation to every message and reached 1 million users within six months. Loom grew because people who received a Loom video watched it and signed up, and e-signature tools spread because every contract needs a second signer.

What is the difference between a viral loop and word of mouth?

A viral loop is built into the product: using it, sharing its output or claiming a referral reward exposes new people to it. Word of mouth happens when users recommend the product on their own. Both bring in users without paid marketing, but only a loop can be measured and tuned step by step.

Can a product that isn’t inherently viral still grow virally?

Yes, partly. Products without a built-in reason to invite others can add one: a powered by badge on what users publish, a free plan, a two-sided referral reward or an affiliate commission. An offer good enough that people pass it on can also spread without an invite button.

Sources and method

Reviewed November 3, 2026. Definitions follow Wikipedia’s K-factor and viral marketing entries and First Round Review’s K-factor glossary, among the pages Google’s AI Overview cited for these searches on that date. The growth formula and its example are from David Skok’s 2009 forEntrepreneurs post; the 12 week worked example is our own calculation with that formula, assuming each new user invites during one cycle after joining. Dropbox figures are from Drew Houston’s April 2010 deck as captured by the Internet Archive in January 2013; Hotmail figures are from the TechCrunch excerpt of Adam Penenberg’s Viral Loop. Founder quotes come from talks published on Founderpath, linked to the moment they are said, and are lightly cleaned transcripts with fillers removed; the results they describe are self-reported.

The earlier version of this post listed 29 examples, most with revenue figures taken from interview titles. We removed the ones we could not check against a primary source, including Tally, ClickFunnels, Weebly, Turtl, SuperLemon, JivoChat, Arum, Yello, Loomly, Smartsheet, Surfer SEO and the B2C example Tribute. We corrected the Loom example (the interview describes a $5 credit referral reward, not a feature unlocked by invites, and gives no 1.8x or 2.6x figures) and dropped a linked calculator that no longer loads. We also removed the old worked example, which put 100 users with a K of 0.9 at 724 or 3,194 users after 30 days: with K below 1 the total can never pass 1,000, and Skok’s formula gives about 282 and 427. Removing an example does not mean it was false, only that we could not verify it.