FastAPI is a modern async-first framework built for high-performance APIs with automatic documentation, while Flask is a lightweight, synchronous framework prioritizing simplicity and flexibility. FastAPI handles ~3x more requests per second in benchmarks, but Flask has broader ecosystem maturity and community resources.
Flask vs FastAPI — Side-by-side comparison on A Versus B
Lightweight synchronous Python web framework focused on simplicity and flexibility.
Modern Python web framework for building APIs with automatic documentation and async support
Request Handling Speed. Async/Await Support. Auto-Generated Documentation. Data Validation. Learning Curve. Community Size. Production Maturity
Choose Flask if you're building simple REST APIs, prefer minimal dependencies, need extensive third-party ecosystem support, or value a gentle learning curve for junior developers. Choose FastAPI if you're building modern, high-performance APIs requiring async operations, automatic API documentation, or type-safe request validation out-of-the-box.
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: ~95,000 stars | FastAPI: ~80,000 stars
Flask: No (manual or extension-based) | FastAPI: Yes (Pydantic integration)
Flask: Python 2.7+ (legacy) / 3.4+ | FastAPI: Python 3.6+
Flask: ~35 MB per instance | FastAPI: 50-100MB
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: ~2.5M active | FastAPI: ~400K active
Flask: Manual setup required | FastAPI: Yes (OpenAPI 3.0)
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: ~7,500 req/s | FastAPI: ~22,000 req/s
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: N/A
Flask: 1 (werkzeug) | FastAPI: 6 (starlette, pydantic, etc.)
Flask: Requires Flask-APScheduler or manual async setup | FastAPI: Native async/await built-in
Flask: Manual (requires Flask-RESTX, Flasgger) | FastAPI: Automatic (Swagger UI + ReDoc)
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)
Flask: N/A | FastAPI: Automatic (Swagger UI + ReDoc)
Flask: N/A | FastAPI: Requires app server (Uvicorn)
Flask: N/A | 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: 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: N/A | 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)
Production Readiness Without External Server
Flask: N/A | FastAPI: Requires ASGI (Uvicorn)
Flask vs FastAPI — Structured Comparison Data
Machine-readable comparison dataset for Flask vs FastAPI. Contains 146 attribute comparisons, 5 FAQ pairs.
Flask vs FastAPI — Attribute Category Vocabulary
Taxonomy of attribute categories used to classify comparison data for Flask vs FastAPI.
Should I use Flask or FastAPI for a new project in 2026?
For simple CRUD APIs or monolithic applications, Flask remains excellent. For microservices, real-time features, or APIs requiring high concurrency, FastAPI is the better choice. FastAPI's growth rate (2,000+ stars/month) suggests it's becoming the modern standard for new Python APIs, but Flask's maturity makes it reliable for production at any scale.
Is FastAPI significantly faster than Flask?
Yes. FastAPI consistently handles ~3x more requests per second (22,000 vs 7,500 req/s) in benchmarks due to native async support and Starlette's optimized ASGI server. However, for applications with <100 concurrent users, this difference is imperceptible. The performance gap matters for high-traffic APIs and real-time applications.
Flask 2.0+ has experimental async view support, but it's not first-class. You'd still need Celery for background tasks or Flask-APScheduler for scheduling. FastAPI's async is native and recommended for all new projects. If your project heavily relies on async patterns, FastAPI is the better fit.
Flask has more third-party tutorials due to age and popularity. FastAPI generates interactive API documentation automatically (Swagger UI + ReDoc), eliminating manual docs work. For API documentation, FastAPI wins. For general learning resources, Flask has more available due to its 15-year head start.
What's the migration path from Flask to FastAPI?
There's no direct upgrade path. Migration requires rewriting your application, though logic often transfers cleanly. Many teams run both frameworks simultaneously in a microservices architecture. The effort is ~30-50% rewrite effort, making migration viable for performance-critical applications but not necessary for stable, low-traffic APIs.
Request Handling Speed: Flask has ~7,500 req/s, while FastAPI has ~22,000 req/s.
Async/Await Support: Flask has Manual with extensions, while FastAPI has Native built-in.
Auto-Generated Documentation: Flask has Manual setup required, while FastAPI has Automatic (Swagger UI + ReDoc).
Data Validation: Flask has Manual or extension-based, while FastAPI has Built-in with Pydantic.
Learning Curve: Flask has Minimal (50-100 lines for basic app), while FastAPI has Moderate (100-150 lines).
Community Size: Flask has ~95,000 GitHub stars, while FastAPI has ~80,000 GitHub stars.
Production Maturity: Flask has 15+ years (since 2010), while FastAPI has 5+ years (since 2018).