YML vs YAML: Which File Extension Should You Use?

Both .yml and .yaml extensions work identically. Parsers treat them the same way with no functional difference.

Why Two Extensions Exist

The .yml extension is a legacy artifact from DOS and early Windows systems that enforced a three-character limit on file extensions (like .doc, .txt, .exe). Since “YAML” has four letters, it was shortened to .yml to meet this constraint.

Modern operating systems removed this restriction decades ago, allowing the full .yaml extension.

Official Standard vs Real-World Usage

Official recommendation: Use .yaml per the YAML FAQ and specifications.

Common patterns in tools:

• GitHub Actions, Docker Compose: Both work, but .yml appears frequently (docker-compose.yml)
• Kubernetes: Primarily uses .yaml for manifests
• Ansible: Community often defaults to .yml

Best Practice: Consistency

# Good: Consistent within project
config.yaml
deployment.yaml
service.yaml

# Bad: Mixed extensions
config.yml
deployment.yaml  # Avoid mixing

Recommendation

For new projects:

• Use .yaml to align with the official standard
• Stick to one extension throughout your project

For existing projects:

• Match whatever extension is already in use
• Don’t mix .yml and .yaml in the same directory

Both extensions work perfectly—consistency matters more than the choice itself.