Getting the watering right for your urban planter boxes is crucial for plant health. Too much water can cause root rot, while too little leads to drought stress. The key is to know your planter's soil volume, which determines its water-holding capacity. Here's a simple guide to calculate it.
First, measure the interior dimensions of your box. You'll need the length, width, and depth (or height) in inches or centimeters. For a standard rectangular or square box, the formula is straightforward: Volume = Length x Width x Depth.
For example, if your planter is 24 inches long, 12 inches wide, and 10 inches deep, the calculation is: 24 x 12 x 10 = 2,880 cubic inches. To convert this to gallons (a common measure for watering), divide cubic inches by 231. So, 2,880 / 231 ≈ 12.5 gallons. This is the total volume your soil can hold.
For cylindrical planters, use the formula for the volume of a cylinder: Volume = π x (Radius x Radius) x Depth. Remember, the radius is half the diameter.
Why is this number useful? It tells you the maximum amount of water the soil can absorb before draining. A good watering rule is to provide about 10-15% of the soil's volume per session. For our 12.5-gallon box, that's roughly 1.25 to 1.9 gallons of water. Always check soil moisture an inch below the surface before watering—it should feel slightly dry. Factors like plant type, sun exposure, and season will adjust this baseline, but knowing your planter's volume gives you the perfect starting point for a thriving garden.