YC Batch Codes Decoded: W24, S21, X25 & Company IDs
YC Batch Codes vs. Company IDs: Don't Mix Them Up
Two code systems confuse search traffic on YC directories:
| Code type | Format | Meaning | Example |
|---|---|---|---|
| Batch code | Letter + 2-digit year | Which cohort the company joined | W24, S21, X25, F24 |
| Company ID | Integer (often searched as ai#####) | Unique company record in a directory | 29973 → arva-ai-29973 |
W24 is not a company ID. ai29973 is not a batch. Full ID guide: how to look up a YC company by ID. This article decodes Y Combinator batch codes only.
Letter Codes: W, X, S, F + Year
| Code | Season | Typical months | URL on Guide Startups |
|---|---|---|---|
| W | Winter | ~Jan–Mar | /batches/winter-2024 style slugs |
| X | Spring | ~Apr–Jun | spring-2025, etc. |
| S | Summer | ~Jun–Aug | summer-2024, etc. |
| F | Fall | ~Oct–Dec | fall-2025, etc. |
The two digits are the batch year (usually the year the program starts or Demo Day falls). X is Spring—not "extra" or experimental. Older posts mentioning only W and S describe the two-batch era; today all four letters are standard [1].
Examples: W24, S21, X25, F24, W07
W24 — Winter 2024 batch (~251 companies in Guide Startups). Browse peers on the Winter 2024 batch page.
S21 — Summer 2021 batch (one of the largest cohorts, ~391 companies). High-growth era alumni cluster here.
X25 — Spring 2025 batch (~145 companies). Spring batches grew after YC moved to four seasons per year.
F24 — Fall 2024 batch (~93 companies in our snapshot).
W07 — Winter 2007 batch; includes alumni like Weebly. Long-tail searches like "weebly y combinator batch w07" map to this code.
Map any code → full name via glossary or what is a YC batch.
How Batch Codes Appear on Company Pages
On Guide Startups, each company profile shows a human-readable batch (e.g. "Winter 2024") that corresponds to W24. The URL uses /companies/{name-slug}-{company_id}, not the batch code.
Example: Stripe lists Summer 2009 → batch code S09. Airbnb → Winter 2009 → W09.
When researching "[startup] y combinator batch," always open the company page first; do not guess the code from founding year alone.
Batch Page URLs on Guide Startups
Batch hubs use slugified season + year:
/batches/winter-2025/batches/summer-2024/batches/spring-2025/batches/fall-2025
The index /batches lists all cohorts with company counts. Pair batch browsing with /year/2025 when you care about calendar-year graduation lists rather than season codes.
Decoding Search Queries and Long-Tail Patterns
Common search patterns and how to resolve them:
- "bluumbio y combinator summer 2021" → company name + season/year → find company, read batch field (likely S21).
- "semiotic yc" / "hlabs yc" → company or founder lookup on /companies, not batch code search.
- "ai29973" → company ID lookup, not batch—see ID guide.
- "y combinator batch codes" → this article + YC batches explained.
Conclusion
YC batch codes use W (Winter), X (Spring), S (Summer), F (Fall) plus a two-digit year. They identify cohorts, not individual companies. Company IDs like ai29973 are separate—use batch pages and company profiles together when researching the ecosystem.
Explore all batches, read how many companies per batch, and apply on apply.ycombinator.com when ready.
Frequently Asked Questions
What does W24 mean in Y Combinator?
W24 is the Winter 2024 YC batch—the cohort that started the winter program cycle and Demo Day in that year.
What is the X in X25?
X is the letter code for Spring batch. X25 is Spring 2025. S is reserved for Summer.
Is ai29973 a batch code?
No. ai29973 is a company ID pattern on Guide Startups (company_id 29973). Batch codes are W24, S21, etc.
How do I find all companies in S24?
Open /batches and select the Summer 2024 batch page, or search companies with batch Summer 2024.
Did YC always use four batch letters?
No. Historically only W and S were common; X and F were added when YC moved to four batches per year.
What batch was Stripe in?
Summer 2009 (S09). See /companies/stripe-240 on Guide Startups.
References
- 2026 Demo Day Dates – Y Combinator (four-batch calendar)