Fix Next.js "Port 3000 is Already in Use"

The error

Running npm run dev or next dev shows:

- error Port 3000 is already in use.
Use `next dev --port <port>` to use a different port.

Next.js 13+ auto-detects port conflicts and shows this message. Older versions may show the raw EADDRINUSE error.

Fix 1: portrm (instant)

$ ptrm fix 3000
  Port 3000 in use
  node (PID 67890) - next-server
  Killed PID 67890. Port 3000 is free.

$ npm run dev  # starts on port 3000

Fix 2: Kill manually

# macOS
$ lsof -ti :3000 | xargs kill -9

# Linux
$ fuser -k 3000/tcp

# Windows
> netstat -ano | findstr :3000
> taskkill /PID <PID> /F

Fix 3: Use a different port

Next.js supports port selection via CLI flag or environment variable:

# CLI flag
$ next dev --port 3001
$ next dev -p 3001

# Environment variable
$ PORT=3001 npm run dev

# .env.local
PORT=3001

Fix 4: Update package.json

// package.json
{
  "scripts": {
    "dev": "next dev -p 3001",
    "predev": "ptrm fix 3001"
  }
}

Common causes

Next.js with Turbopack

When running next dev --turbo, Turbopack uses the same port as the dev server. If a Turbopack process crashes, it may leave the port occupied:

$ ptrm 3000
  Port 3000
  PID    73201
  Name   next-server (turbopack)

$ ptrm fix 3000  # kills the zombie turbopack process

Running multiple Next.js projects

If you work on multiple Next.js projects simultaneously, assign a unique port to each:

# Project A: .env.local
PORT=3000

# Project B: .env.local
PORT=3001

# Project C: .env.local
PORT=3002