Cal Henderson
Co-Founder and CTO @ Slack
San Francisco Bay Area, United States
Cal Henderson is a seed investor at Slack in San Francisco Bay Area focused on Software and Information Technology. Cal Henderson is the co-founder and CTO of Slack. He is an experienced technology leader, having previously built and led the engineering team at Flickr, through its acquisition by Yahoo. As a popular speaker on engineering scalability, he authored the best-selling O’Reilly Media book “Building Scalable Websites”. He was a pioneer in the use of webAPIs, and created the basis for oAuth and oEmbed, now used by YouTube, Twitter, and many others. Cal has a Bachelor of Computer Science from the University of Central England, and was involved in London’s early online community through his work with the early digital community B3ta and his personal blog iamcal.com, which he has ran for over 15 years. Cal now resides in San Francisco.
- Total investments:
- 17
- Last investment:
- Lead investor:
- 0%
Last updated:
For AI agents: llms.txt
Industries Cal Henderson invests in
Stages Cal Henderson invests in
Countries Cal Henderson invests in
Round sizes
Round size
- Seed
- $3.5M–$13M
- Pre-Seed
- $1.4M–$4M
- Series A
- $5.5M–$11M
Investment Focus
Industry
- Software 7 (41%)
- Information Technology 4 (24%)
- Artificial Intelligence 3 (18%)
- SaaS 2 (12%)
- B2B 1 (6%)
Stage
- Seed 6 (35%)
- Series A 4 (24%)
- Pre-Seed 2 (12%)
- Series B 1 (6%)
- Funding Round 1 (6%)
Country
- United States 7 (41%)
- Finland 2 (12%)
- United Kingdom 2 (12%)
- Poland 1 (6%)
Investments
| Company | Description | Round | Links |
|---|---|---|---|
Marker
|
A writing tool for people who believe in writing | $13.4M / Seed / Jul 09, 2026 | |
Realm
|
Realm is a conversational search engine for the workplace. With Realm, employees accurately find anything their company has ever written down. Realm gives an organization perfect memory. | $4.5M / Seed / Apr 22, 2026 | |
Electric Twin
|
Electric Twin uses AI to simulate human behavior for decision-making and market analysis. | $10M / Series A / Feb 12, 2026 | |
Slashwork
|
Slashwork is reimagining professional communication through clarity, context, and human connection. | $3.5M / Seed / Feb 04, 2026 | |
Swarmia
|
Data-driven superpowers for software teams | $11.5M / Series A / Jun 18, 2025 | |
Elva
|
Elva wants to make recruiting internationally as straightforward as hiring domestically so businesses can easily add the greatest people to their teams. | $1.4M / Pre-Seed / Mar 10, 2025 | |
|
Brightband
|
Brightband develops an advanced, end-to-end Earth System AI to improve weather and climate-related decisions. | $10M / Series A / Sep 19, 2024 | |
Noded Ai
|
Noded Ai is preparing yourself for that customer meeting is tough. Noded brings them all together using your notes history and gives you exactly what you needed , when you need it. So you’re always prepared and one step ahead. | $4M / Seed / Jul 22, 2024 | |
Amplifier Security
|
Amplifier Security utilizes AI-powered technology to enable security teams to modernize their approach. The company offers security solutions for enterprises, uniting technology and human expertise to enhance an organization's security posture. | $3.3M / Pre-Seed / Apr 24, 2024 | |
Linear
|
Linear allows the user to manage software development and track bugs. Built for speed and efficiency.The founders started Linear from their own frustrations building software projects in successful high-growth companies like Airbnb, Coinbase and Uber. The founders wanted to create the tools that focus us and its team in making progress, not getin the way. Tools that are fast and efficient. Speed and performance were the main things Linear focuses on early on and keep focusing on that. | $35M / Series B / Sep 14, 2023 | |
| See all 17 investments → | |||
Co-Investors
About Slack
Slack is a cloud-based communication and collaboration platform for teams. It allows users to exchange messages, share files, and integrate with various business tools and services. Slack is commonly used across various industries, from tech startups to large enterprises.
Slack Contacts
| Name | Phone | Socials | |
|---|---|---|---|
Jason Spinell
Manager
|
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Matt Loop
Head of Asia Pacific
|
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Steve Wood
VP of Product and Developer Platform
|
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Cal Henderson
Co-Founder and CTO
|
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Eric Costello
Co-founder
|
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FAQ
What does Cal Henderson invest in?
Cal Henderson invests primarily in Software, Information Technology and Artificial Intelligence startups, most often at Seed and Series A stage. Most of the 17 investments tracked by Shizune back companies in United States. The Investment Focus section breaks down every industry, stage and country in the portfolio.
What is Cal Henderson's check size?
Cal Henderson typically joins rounds of $3.5M–$13M at Seed, $1.4M–$4M at Pre-Seed and $5.5M–$11M at Series A. These ranges are 10th–90th percentile round sizes calculated from confirmed public funding rounds, so they are a realistic guide to the rounds Cal Henderson participates in.
When did Cal Henderson last invest?
The most recent investment recorded for Cal Henderson closed in Jul 2026. Shizune tracks 17 investments in total for this profile, including round sizes, stages and portfolio companies, all sourced from confirmed public funding rounds and refreshed monthly.
Is Shizune's investor data accurate?
Yes — every stat on this profile is calculated from confirmed public funding rounds monitored across hundreds of sources since 2020. This profile was last refreshed in Aug 2026 and is updated monthly. Spotted something off? Use the “Suggest an edit” link near the top of the page.
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