Accurately measuring a planter is a practical necessity for successful container gardening. Knowing the precise dimensions allows a gardener to purchase the correct quantity of potting mix, ensuring the plant has enough substrate for healthy root development. It is also necessary for selecting appropriately sized liners, saucer trays, or decorative cover pots that will fit the container without wasted space or awkward gaps. Taking a few careful measurements before purchasing supplies prevents the common issue of buying too much or too little soil, saving both time and money.
Essential Terminology and Tools
Before starting any measurement, it is important to understand the specific terms used in the horticultural context. The Inside Diameter or Inside Width/Length refers to the usable space across the top opening of the planter, which dictates the maximum size of the plant’s canopy or root ball. The Depth is the measurement from the inner lip down to the bottom interior surface, which determines the vertical space available for the roots. This internal depth is distinct from the overall external height of the planter, which includes the base or feet.
The planter’s Capacity is the total volume of soil it can hold, typically expressed in cubic feet, quarts, or gallons, and this is the most important number for buying potting mix. To obtain these measurements, the primary tools required are a flexible tape measure or a long ruler, and a calculator or access to an online volume tool. It is always best practice to measure the internal dimensions of an empty planter, as external measurements can significantly skew the final capacity calculation.
Measuring Standard Planter Dimensions
For containers with straight sides, such as basic cylindrical or rectangular shapes, obtaining the linear measurements is straightforward. To measure a round planter, use a tape measure to find the Inside Diameter by measuring straight across the widest point of the top opening, ensuring the tape passes directly over the center. The radius, which is used in volume calculations, is simply half of this diameter measurement.
For square or rectangular planters, measure the Inside Length and the Inside Width across the top opening, measuring from the interior edge of one wall to the interior edge of the opposite wall. After recording the width and length, measure the Depth of the container by placing the end of the tape measure on the inner rim and extending it down to the interior floor of the planter. These three figures—length, width, and depth—provide the necessary dimensions for volume calculation.
It is helpful to take all linear measurements in the same unit, such as inches, before performing any calculations. Converting inches into a larger unit like feet before calculation often simplifies the final volume conversion, especially since potting mixes are sold by volume.
Calculating Capacity for Soil and Inserts
Once the internal dimensions are accurately recorded, the next step is to calculate the total soil volume, or capacity, which is mathematically the volume of the geometric shape. For standard square or rectangular planters, the calculation is the simplest: multiply the Length by the Width by the Height (or Depth). If all three measurements were taken in feet, the result will be the capacity in Cubic Feet (L x W x H = V).
For round or cylindrical planters, the volume calculation involves the mathematical constant pi (approximately 3.14159). The formula for a cylinder’s volume is pi multiplied by the radius squared, multiplied by the depth, or pi r-squared h. Using this formula will yield the volume in the cubic unit that matches the measurements taken.
Potting mix is commonly sold in bags labeled by volume, often in quarts, gallons, or cubic feet, so conversion is necessary. To convert cubic inches to US gallons, divide the volume by 231, since one US liquid gallon is equal to 231 cubic inches. Alternatively, many online capacity calculators allow the input of linear measurements and automatically provide the resulting volume in common units like quarts or liters, simplifying the process for the gardener.
Handling Tapered and Irregular Planters
Measuring planters that taper, meaning they are wider at the top than at the bottom, requires a more complex approach because they are shaped like a frustum of a cone or pyramid. For these containers, measure the diameter or width/length at the top opening and the corresponding diameter or width/length at the interior bottom of the planter. The vertical depth measurement remains the same, taken from the inner lip to the center of the bottom interior.
The volume of a tapered round planter can be calculated using a specialized formula that incorporates the top radius (R), the bottom radius (r), and the depth (h). Most gardeners find it easier to use a dedicated online calculator that handles this geometry. For irregularly shaped containers, calculating the precise volume mathematically becomes impractical. A straightforward method involves lining the empty planter with a plastic bag and then filling it with a measured amount of water or sand, recording the total volume added to fill the container.