Mermaid vs D2: Modern Diagram Languages for Product Teams

Mermaid and D2 both live in the same world of diagram-as-code, but they optimize for different moments in product work. Mermaid wins when you need a fast, familiar diagram inside docs. D2 wins when you want clean, auto-laid-out architecture diagrams that stay readable as systems grow.

Why this matters in real teams

Product and platform teams need diagrams that survive constant iteration.
Diagrams are reviewed in PRs, not just published in final docs.
A good syntax reduces friction during planning, onboarding, and incident reviews.

Feature snapshot

Capability	Mermaid	D2
Syntax style	Markdown-inspired	Indentation-based
Auto layout	Helpful defaults	Strong defaults, tidy spacing
Diagram types	Flowcharts, ERD, Gantt, user journeys	System architecture, cloud maps, ERD
Learning curve	Beginner-friendly	Beginner-friendly
Live preview in UniDiagram	✅	✅

Syntax comparison

Mermaid user journey example

journey
  title Checkout Experience
  section Browse
    Customer: 4: Discover product
  section Cart
    Customer: 3: Add to cart
  section Payment
    Customer: 2: Complete checkout

D2 product architecture example

web: Next.js Frontend
api: Application API
payments: Payment Provider

web -> api: GraphQL
api -> payments: Secure webhook

Insight: Mermaid ships a wider variety of specialized diagram types, while D2 focuses on clean architecture graphs that are quick to read in reviews.

What teams notice in day-to-day use

Mermaid strengths

Ideal for docs that live next to code, like READMEs and onboarding guides.
Easy for non-engineers to understand when reviewing product flows.
Great for short-lived diagrams that change weekly.

D2 strengths

The auto layout stays tidy even as diagrams grow.
Diffs stay clean because the syntax is compact and consistent.
Works well for system maps used in incident response and runbooks.

Collaboration workflow

Workflow need	Mermaid	D2
PR review readability	✅ Very readable	✅ Very readable
Architecture reviews	⚠️ Needs careful layout tweaks	✅ Strong default layout
Product narrative diagrams	✅ Excellent	✅ Good with conventions
Systems and dependency maps	⚠️ Can get crowded	✅ Handles growth well

Practical guidance

Pick Mermaid when

Your diagrams live in product docs and help explain decisions.
You need specialized diagrams like user journeys or timelines.
You want a quick diagram for a meeting or a planning doc.

Pick D2 when

Your team maintains architecture maps over months or years.
You want layout consistency without manual tweaking.
You need diagrams that scale with system complexity.

Can teams use both?

Yes. Many teams standardize on Mermaid for narrative documentation and use D2 for architecture and infrastructure diagrams. UniDiagram makes it easy to keep both styles in one workspace and export them with consistent branding.

Final recommendation

If your goal is to publish polished diagrams in product docs and handoffs, Mermaid delivers speed and breadth. If your goal is to keep system maps readable as they grow, D2 delivers stability with minimal effort. Choose the tool that matches how your team actually works, not just the one with the longest feature list.

If your problem is slow diagram iteration or a clunky editor, UniDiagram solves it with a better editing experience, live previews, and AI assistance. It removes the friction so your team can ship clear diagrams faster.