ConvertKit lets you add up to 100 custom fields per account. That sounds generous until you’ve been running a newsletter for two years, tagging readers by interest, purchase history, geography, onboarding date, referral source, content preferences, and engagement tier.
Then you hit the wall.
The limit isn’t documented prominently. You discover it when the “Add Field” button grays out, or when an automation fails to create a new field and your sequence quietly stops working.
What counts toward the limit
Every custom field you create—whether it’s a text field, number, or date—counts toward the 100-field cap. This includes:
- Active fields currently in use
- Archived fields you’re no longer using but haven’t deleted
- Fields created by integrations (Zapier, API calls, third-party forms)
Deleting a field frees up a slot, but only if you’re certain no automation or segment references it. ConvertKit doesn’t warn you before deletion, and there’s no “undo.”
Tags, by contrast, are unlimited. Subscribers are unlimited. It’s only custom fields—the structured data layer—that hits a ceiling.
Where operators waste field slots
Most operators I’ve audited are using 40–60 fields. The bloat comes from:
Redundant date fields. Separate fields for “trial_start,” “trial_end,” “first_purchase,” “last_purchase,” “onboarding_completed.” You can often collapse these into tags with date-based automations, or store only the field you’ll actually query.
One-off campaign tracking. A field for every lead magnet, webinar, or promo. If you’re not segmenting on it after 90 days, archive or delete it.
Text fields that should be tags. A “interests” field with comma-separated values like “SEO, WordPress, email” is harder to segment than three tags. ConvertKit’s segment builder can combine tags with AND/OR logic; custom field text matching is clunkier.
Legacy fields from old integrations. A Typeform you used once in 2024, a Zapier zap you turned off, a WordPress plugin you uninstalled. Each left fields behind.
How to design segments that scale
If you’re approaching the limit—or want to avoid it—here’s the structure that works:
Use tags for categorical data. Interests, content preferences, lead sources, engagement tiers. Tags are unlimited, combinable, and easier to audit.
Reserve custom fields for values you’ll calculate or compare. Numbers (purchase count, total spend, engagement score), dates (signup date, last click), or IDs (Stripe customer ID, external CRM reference).
Audit every 90 days. Export your field list. Flag anything unused in the last quarter. Archive first, delete after another 30 days if no automations break.
Document field purpose and owner. Keep a spreadsheet. Column A: field name. Column B: what it tracks. Column C: which automations or segments use it. Column D: date created. When you hit 80 fields, you’ll thank yourself.
What happens if you hit the cap
Automations that try to create or update a field beyond the 100th slot fail silently. The automation continues, but the field write doesn’t happen. You won’t get an error email. The subscriber moves to the next step as if nothing broke.
If you’re relying on that field for downstream segmentation—say, tagging high-intent leads based on a quiz score—you’ll lose data without noticing until you spot the gap in your reports.
ConvertKit support can’t raise the limit. It’s a hard platform cap, same across all pricing tiers.
The workaround: delete unused fields, or rethink your data model. Some operators move complex segmentation logic into an external CRM (like Brevo or a dedicated CDP) and sync only the essential fields back to ConvertKit. That adds complexity, but it scales past 100.
When to stay under 50
If you’re running a solo operation with fewer than 10,000 subscribers, aim to stay under 50 fields. It forces clarity. Every field you add should answer: “What decision does this let me make that I can’t make with tags?”
If the answer is “nothing,” use a tag.
Most operators don’t need purchase history in a custom field—they need a “purchased” tag and a “last_purchase_date” field for recency-based re-engagement. That’s two slots instead of five.
ConvertKit’s segmentation is powerful, but it rewards restraint. The ceiling exists whether you plan for it or not.
Hit a segmentation problem you can’t solve with tags? Reply and tell us what you’re trying to track—we’ll feature operator solutions in a future issue.
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