How to Measure Garden Planters

Knowing the size of a garden planter is useful for more than just comparing pots. Accurate measurements can help you choose the right saucer, estimate how much potting mix you need, determine whether a nursery pot will fit inside a decorative container, and compare planter sizes when shopping.

Measuring a planter can be straightforward when it is empty and rectangular, but round, tapered, unusually shaped, or already-planted containers require a little more care. The measurements you need also depend on what you are trying to determine.

How To Measure A Planter For A Saucer

When choosing a saucer, you usually need the outside diameter or width at the bottom of the planter. This is the portion of the pot that will actually sit inside the saucer.

If the planter is empty, turn it upside down on a level surface. Place a tape measure across the widest part of the bottom and measure from one outside edge to the opposite outside edge.

For a round planter, make sure the tape crosses the center. If you are not sure where the widest point is, move the tape slightly until you find the largest measurement.

Measuring A Planter That Is Already Full

Many planters are wider at the top and narrower at the bottom, which makes the base difficult to measure once the planter is filled.

One simple method is to place the planter on a flat surface and mark the position of both outside edges of the base using chalk or removable masking tape. Move the planter, then measure the distance between the marks.

On a deck or other surface with gaps, make sure the marks fall on solid boards so you have accurate reference points.

What Size Saucer Should You Buy?

The saucer should be slightly wider than the base of the planter so the pot sits comfortably inside without being wedged against the edges.

Adding roughly 1 to 2 inches to the measured base diameter is a useful starting point, but saucer designs vary. Check the manufacturer's interior dimensions whenever possible, especially for planters with unusually shaped bases.

How To Measure For A Cover Pot

Decorative planters are often used as cover pots or cachepots, with a nursery pot placed inside rather than planting directly into the decorative container.

For this use, measure the inside diameter or width of the decorative pot at its narrowest point, along with the usable inside height.

Keep in mind that a planter may taper inward near the bottom. Even if the opening is wide enough, a nursery pot may not slide all the way down if the interior becomes narrower.

How To Calculate How Much Potting Mix A Planter Holds

Measuring the inside of an empty planter gives the most accurate estimate of how much potting mix it can hold. Exterior measurements include the thickness of the container walls and will slightly overestimate capacity.

You also do not normally fill a planter completely to the rim. Leaving approximately an inch of space at the top makes watering easier, so the actual amount of potting mix used may be somewhat less than the calculated maximum volume.

Square Or Rectangular Planters

Measure the inside length, inside width, and usable depth.

If the measurements are in feet:

For example, a planter that measures 2 feet long × 1 foot wide × 1 foot deep has a maximum volume of:

2 × 1 × 1 = 2 cubic feet

If you measure in inches, multiply the three dimensions first and then divide by 1,728, because one cubic foot contains 1,728 cubic inches.

Round Planters

A round planter with straight sides is approximately a cylinder. To calculate its volume, you need the inside diameter and usable depth.

First, divide the diameter by 2 to find the radius.

If all measurements are in feet, the result will be in cubic feet.

Tapered Round Planters

Many decorative pots are wider at the top than at the bottom. Treating these as full cylinders using only the top diameter can noticeably overestimate how much soil they hold.

For a quick estimate, measure the inside diameter near the top and bottom and use an average diameter:

Then use that average diameter in the round-planter formula above. This is still an estimate, but it is usually more useful than calculating volume from the widest point alone.

Converting Cubic Feet To Gallons

Potting mix is often sold in cubic feet, while some planters are described by gallons. If you need to compare the two:

Remember that these calculations estimate the container's interior volume. The actual amount of potting mix needed can be lower because of the plant's root ball, the space left below the rim, planter inserts, or the shape of the container.

Planter Measurements At A Glance