For herbs on a tiny patio, the best urban planter box size is usually a rectangular container about 18 to 24 inches long, 10 to 12 inches deep, and 8 to 10 inches tall. This size holds roughly 5 to 10 gallons of potting mix, which is enough for a small, productive herb garden without taking over your patio. It is also light enough to move before watering and compact enough to fit along a wall, railing, or doorway.
If you mainly grow shallow-rooted herbs such as basil, parsley, cilantro, chives, thyme, and oregano, an 8-inch soil depth can work, but 10 inches is safer and gives the roots more room. For rosemary, sage, or any herb you plan to keep longer, choose the 12-inch depth if possible. Mint is best planted in its own smaller pot inside or beside the box because it spreads quickly and can crowd out other herbs.
A good patio planter box choice is a 24 x 12 x 10-inch model with drainage holes. If your patio is extremely tight, a 12 to 18-inch-long box that is 8 to 10 inches deep can still support two or three compact herbs. When checking planter box dimensions, avoid going much smaller than 8 inches in soil depth, because the mix dries out too fast and roots become cramped. Also avoid an oversized box if it will be hard to move or blocks your walking space.
A simple container herb garden can thrive when you place the box where herbs get at least four to six hours of sun and use a lightweight potting mix rather than garden soil. Add a saucer or drip tray to protect the patio, but empty it so roots do not sit in water. Group herbs with similar water and sun needs, and leave a few inches between plants for airflow. With the right planter size, even a tiny patio can produce fresh herbs all season.