FastAPI is a modern Python web framework built on Starlette that prioritizes speed and automatic API documentation with async support by default, while Flask is a lightweight, minimalist framework that emphasizes simplicity and flexibility with synchronous request handling. FastAPI's built-in features result in ~3x faster request handling and automatic OpenAPI/Swagger documentation, whereas Flask requires manual setup for these capabilities.
FastAPI vs Flask — Side-by-side comparison on A Versus B
Modern high-performance Python web framework with automatic API documentation and async support.
Lightweight, minimalist Python web framework emphasizing simplicity and flexibility.
Request Performance (requests/sec). Async/Await Support. Automatic API Documentation. Learning Curve (hours to productivity). Ecosystem Maturity (years since release). Built-in Data Validation. GitHub Stars (as of 2026)
Choose FastAPI if you're building modern REST APIs that require high performance, automatic documentation, async operations, or type safety with built-in validation—ideal for microservices, real-time applications, and teams prioritizing developer experience. Choose Flask if you prioritize simplicity, have an existing codebase, need a lightweight framework for small projects, or prefer minimal dependencies and maximum flexibility in architecture decisions.
FastAPI: Yes, built-in by default | Flask: No, requires external libraries
FastAPI: Yes, OpenAPI/Swagger auto-generated | Flask: No, manual setup required
FastAPI: 15-25 hours | Flask: 5-10 hours
FastAPI: 4 years (2021) | Flask: 18 years (2010)
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) | Flask: N/A
FastAPI: Built-in (OpenAPI 3.0) | Flask: Manual integration required
FastAPI: Native first-class support | Flask: Experimental in Flask 2.0+
FastAPI: Python 3.6+ | Flask: Python 2.7+ (legacy) / 3.4+
FastAPI: Automatic (Swagger UI + ReDoc) | Flask: N/A
FastAPI: Requires app server (Uvicorn) | Flask: N/A
FastAPI: ~30,000 questions | Flask: 40,000+
FastAPI: 3.5 hours (manual setup required) | Flask: N/A
FastAPI: No (requires external library) | Flask: N/A
FastAPI: Native (async/await throughout) | 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: ~22,000 req/s | Flask: ~7,500 req/s
FastAPI: ~400K active | Flask: ~2.5M active
FastAPI: ~2,500 extensions | Flask: 10,000+ extensions
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: Yes, Pydantic models with automatic validation | Flask: No, requires add-ons
FastAPI: ~80,000 stars | Flask: ~95,000 stars
FastAPI: 500,000+ PyPI packages (Python ecosystem) | Flask: N/A
FastAPI: 6 years (released 2018) | Flask: N/A
Production Applications (market estimate)
FastAPI: ~12,000 req/s | Flask: ~4,000 req/s
FastAPI: Yes (Swagger UI + ReDoc automatic) | Flask: N/A
FastAPI: Yes (Pydantic built-in) | Flask: N/A
FastAPI: Moderate (3.5/5) | Flask: Easy (1.5/5)
FastAPI: Full (enforced) | Flask: Optional
FastAPI: No (requires FastAPI-Users, python-jose) | Flask: N/A
FastAPI: No (requires SQLAlchemy, Tortoise-ORM) | Flask: N/A
FastAPI: ~350,000 PyPI packages (FastAPI-specific: ~4,000) | Flask: N/A
FastAPI: Yes (Swagger/ReDoc) | Flask: N/A
FastAPI: ~2.8M (PyPI/month) | Flask: N/A
FastAPI: 6 (starlette, pydantic, etc.) | Flask: 1 (werkzeug)
FastAPI: Native async/await built-in | Flask: Requires Flask-APScheduler or manual async setup
FastAPI: Automatic (Swagger UI + ReDoc) | Flask: Manual (requires Flask-RESTX, Flasgger)
FastAPI: 50-100MB | Flask: ~35 MB per instance
FastAPI: ~75,000 stars | Flask: ~67,000 stars
Production Readiness Without External Server
FastAPI: Requires ASGI (Uvicorn) | Flask: N/A
FastAPI: N/A | 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: No (requires Flask-SocketIO)
FastAPI: N/A | Flask: Native (NumPy, TensorFlow, Pandas)
FastAPI: N/A | Flask: Via SQLAlchemy extension
FastAPI: N/A | Flask: Yes (Werkzeug-based)
Average Community Response Time (GitHub Issues)
FastAPI: N/A | Flask: Synchronous (WSGI)
FastAPI: N/A | Flask: ~150,000+ PyPI packages
FastAPI: N/A | Flask: Native: NumPy, Pandas, Scikit-learn, TensorFlow, PyTorch
FastAPI: N/A | Flask: No - requires WSGI server (Gunicorn, uWSGI)
FastAPI vs Flask — Structured Comparison Data
Machine-readable comparison dataset for FastAPI vs Flask. Contains 149 attribute comparisons, 5 FAQ pairs.
FastAPI vs Flask — Attribute Category Vocabulary
Taxonomy of attribute categories used to classify comparison data for FastAPI vs Flask.
Which framework is faster, FastAPI or Flask?
FastAPI is approximately 3x faster than Flask. Benchmark testing shows FastAPI handles ~12,000 requests/second while Flask handles ~4,000 requests/second on comparable hardware. This performance advantage comes from FastAPI's async-first design built on Starlette, whereas Flask handles requests synchronously by default. For most small projects, this difference is negligible, but for high-traffic APIs or real-time systems, FastAPI's throughput advantage is significant.
Flask 2.0+ (released 2021) added native async/await support for view functions, but it remains limited. Flask's async implementation doesn't provide the same concurrency benefits as FastAPI because Flask's ecosystem (middleware, extensions) is designed synchronously. To use async effectively in Flask, you typically need to use external libraries like gevent or implement async manually. FastAPI's async support is first-class and deeply integrated throughout the framework, making it the better choice for async-heavy applications.
Do I need to manually document my API with FastAPI?
No. FastAPI automatically generates interactive OpenAPI (Swagger UI) and ReDoc documentation from your code's type hints and docstrings. Simply define your endpoints with Pydantic models and type annotations, and documentation is instantly available at `/docs` (Swagger) and `/redoc` (ReDoc). Flask requires manual documentation setup using tools like Flasgger, Connexion, or manually writing OpenAPI/Swagger YAML files. This saves significant development time with FastAPI.
Yes, Flask remains highly relevant and production-ready, especially for small-to-medium projects, prototypes, and teams that value simplicity. Flask's mature ecosystem (18+ years), extensive community resources, and proven track record make it ideal for educational projects and systems where flexibility matters more than raw performance. However, for new API-heavy projects, teams increasingly prefer FastAPI's built-in features and modern async support. Many organizations continue maintaining Flask applications successfully.
Which framework should a beginner choose?
Flask is better for beginners due to its minimal learning curve (5-10 hours to first API) and straightforward syntax. FastAPI's type hints, async concepts, and automatic validation—while powerful—require more foundational Python knowledge. Beginners typically learn Flask faster and can build functional applications immediately. Once comfortable with Python and async concepts, graduating to FastAPI is straightforward. Starting with Flask is a reasonable path for long-term learning.
Consider: Request Performance (requests/sec)
Request Performance (requests/sec): FastAPI has ~12,000 req/s, while Flask has ~4,000 req/s.
Async/Await Support: FastAPI has Native, built-in by default, while Flask has Limited, requires external libraries.
Automatic API Documentation: FastAPI has OpenAPI/Swagger auto-generated, while Flask has Manual setup required.
Consider: Learning Curve (hours to productivity)
Learning Curve (hours to productivity): FastAPI has 15-25 hours, while Flask has 5-10 hours.
Consider: Ecosystem Maturity (years since release)
Ecosystem Maturity (years since release): FastAPI has 4 years (2021), while Flask has 18 years (2010).
Built-in Data Validation: FastAPI has Pydantic models, automatic validation, while Flask has None, requires add-ons.
GitHub Stars (as of 2026): FastAPI has ~75,000 stars, while Flask has ~67,000 stars.