FastAPI is a modern Python web framework built for speed with automatic API documentation and async support by default, while Flask is a lightweight, minimalist framework that has dominated since 2010 and remains simpler for small projects. FastAPI averages 3x faster request throughput in benchmarks, but Flask has 15+ years of ecosystem maturity and community libraries.
Flask vs FastAPI — Side-by-side comparison on A Versus B
Lightweight Python web framework for building web applications and APIs with minimal structure.
Modern Python web framework for building APIs with automatic documentation and type-based validation.
Request Throughput (requests/sec). Automatic API Documentation. Async/Await Support. GitHub Stars. Production Deployments (estimated). Learning Curve (hours to productivity). Data Validation Built-in
Choose FastAPI for new projects requiring high-performance REST APIs, real-time features, automatic documentation, or microservices—especially if you need async operations and modern Python (3.7+). Choose Flask for rapid prototyping, simpler applications, monolithic systems, or when you need maximum community library support and existing team expertise.
Flask: None (use SQLAlchemy separately) | FastAPI: N/A
Flask: Requires manual or third-party setup | FastAPI: N/A
Flask: No (use Flask-Login or similar) | FastAPI: N/A
Flask: Manual integration required | FastAPI: Built-in (OpenAPI 3.0)
Flask: Experimental in Flask 2.0+ | FastAPI: Native first-class support
Flask: Requires third-party library | FastAPI: Pydantic integration native
Flask: Python 2.7+ (legacy) / 3.4+ | FastAPI: Python 3.6+
Flask: ~35 MB per instance | FastAPI: N/A
Flask: No (requires Flask-SocketIO) | FastAPI: N/A
Flask: Native (NumPy, TensorFlow, Pandas) | FastAPI: N/A
Flask: Via SQLAlchemy extension | FastAPI: N/A
Flask: 40,000+ | FastAPI: ~30,000 questions
Flask: ~8,000 req/s | FastAPI: ~25,000 req/s
Flask: ~2.5M active | FastAPI: ~400K active
Flask: Manual setup required | FastAPI: Yes (OpenAPI/Swagger at /docs)
Flask: 10,000+ extensions | FastAPI: ~2,500 extensions
Flask: Yes (Werkzeug-based) | FastAPI: N/A
Average Community Response Time (GitHub Issues)
Flask: ~2,100 req/s | FastAPI: ~32,000 req/s
Flask: Synchronous (WSGI) | FastAPI: N/A
Flask: Easy (1.5/5) | FastAPI: Moderate (3.5/5)
Flask: Optional | FastAPI: Full (enforced)
Flask: N/A | FastAPI: No, requires build
Flask: N/A | FastAPI: Full native support
Flask: N/A | FastAPI: 22% (Stack Overflow 2024)
Flask: N/A | FastAPI: Native (Swagger UI + ReDoc built-in)
Flask: N/A | FastAPI: Native Python type hints with validation
Flask: N/A | FastAPI: Automatic (Swagger UI + ReDoc)
Flask: N/A | FastAPI: Requires app server (Uvicorn)
Flask: N/A | FastAPI: Yes (Pydantic native)
Flask: N/A | FastAPI: 3.5 hours (manual setup required)
Flask: N/A | FastAPI: No (requires external library)
Flask: N/A | FastAPI: Yes (default async/await)
Flask: N/A | FastAPI: ~480,000 packages (PyPI)
Flask: N/A | FastAPI: ~1.2M (PyPI: ~2.8M)
Flask: N/A | FastAPI: Manual setup required
Flask: N/A | FastAPI: Native, default behavior
Flask: N/A | FastAPI: 1,200KB (with uvicorn)
Flask: N/A | FastAPI: Native async/await with asyncio
Flask: N/A | FastAPI: High-level API framework (built on Starlette)
Flask: N/A | FastAPI: ~15,000-18,000 req/sec
Flask: N/A | FastAPI: Pydantic (integrated)
Flask: N/A | FastAPI: 500,000+ PyPI packages (Python ecosystem)
Flask: N/A | FastAPI: 6 years (released 2018)
Production Applications (market estimate)
Flask: N/A | FastAPI: Yes (Swagger UI + ReDoc automatic)
Flask: N/A | FastAPI: Yes (Pydantic built-in)
Flask vs FastAPI — Structured Comparison Data
Machine-readable comparison dataset for Flask vs FastAPI. Contains 124 attribute comparisons, 5 FAQ pairs.
Flask vs FastAPI — Attribute Category Vocabulary
Taxonomy of attribute categories used to classify comparison data for Flask vs FastAPI.
Which framework should I choose for a new project in 2026?
Use FastAPI for REST APIs, microservices, or projects prioritizing performance and modern features. Use Flask for traditional web applications, content sites, or if your team is already proficient with Flask. FastAPI has become the standard for new API projects, while Flask remains ideal for full-stack web applications and prototypes.
How much faster is FastAPI than Flask really?
FastAPI handles approximately 25,000 requests/second while Flask handles ~8,000 req/s on identical hardware—a 3x improvement. This difference compounds under concurrent load; FastAPI's native async support prevents thread blocking while Flask requires external ASGI servers. In practice, at 100+ concurrent users, Flask degrades significantly without tuning.
Partial migrations are feasible—you can run FastAPI and Flask side-by-side, gradually replacing Flask routes with FastAPI endpoints. However, full migration requires rewriting views to FastAPI's dependency injection pattern and replacing Flask extensions with FastAPI-compatible libraries. Most teams do this incrementally or rebuild performance-critical APIs in FastAPI while keeping legacy Flask code.
Flask 2.0+ added experimental async/await support, but it's not recommended for production. FastAPI's async implementation is far more mature, using Starlette's ASGI foundation. If you need true async capabilities, FastAPI is the better choice; Flask's async is a compromise for existing Flask users.
FastAPI's official documentation is excellent and modern, with interactive examples. Flask's documentation is comprehensive but assumes more developer knowledge. However, FastAPI auto-generates API documentation (Swagger UI, ReDoc) while Flask requires manual setup—a major advantage for API projects.
Consider: Request Throughput (requests/sec)
Request Throughput (requests/sec): Flask has ~8,000 req/s, while FastAPI has ~25,000 req/s.
Automatic API Documentation: Flask has Manual setup required, while FastAPI has Built-in (Swagger UI + ReDoc).
Async/Await Support: Flask has Added in Flask 2.0 (experimental), while FastAPI has Native, first-class support.
GitHub Stars: Flask has 68,000+ stars, while FastAPI has 73,000+ stars.
Consider: Production Deployments (estimated)
Production Deployments (estimated): Flask has ~2.5M active projects, while FastAPI has ~400K active projects.
Consider: Learning Curve (hours to productivity)
Learning Curve (hours to productivity): Flask has 2-4 hours for basics, while FastAPI has 4-8 hours for basics.
Data Validation Built-in: Flask has Requires third-party libraries, while FastAPI has Pydantic integration native.