What's the weight of a filled concrete planter box so we can ensure the structure below can support it?

Hey there, friend. I’m glad you’re asking this question before it’s too late. I’ve seen too many beautiful decks, balconies, and patios groan under the weight of a planter box that was heavier than the owner ever imagined. Let me break it down for you like I’m talking to a neighbor over the fence.

First, you have to think of me as two parts: the concrete container and everything I hold. A typical concrete planter box—say, a medium-sized one that’s 24 inches long, 24 inches wide, and 18 inches tall—weighs about 50 to 80 pounds when empty. That’s not bad, right? But once you fill me with potting soil (which is surprisingly dense when wet) and add plants, water, and maybe some drainage gravel, my weight can easily quadruple.

Here’s the hard math, but I’ll keep it simple. Soil weighs roughly 75 to 100 pounds per cubic foot when moist. A planter that’s 2 feet by 2 feet by 1.5 feet gives you about 6 cubic feet of volume. That’s 450 to 600 pounds of soil alone. Add the concrete weight—say, 80 pounds—plus a large plant that could weigh 50 pounds when moist, and you’re looking at a total load of around 550 to 730 pounds.

And don’t forget water! After a good rain or a thorough watering, soil can absorb up to 30% more weight. So that 600-pound load can spike to 780 pounds or more in wet conditions.

So, dear DIYer, here’s what I need from you: check the load rating of your deck, balcony, or roof structure. Most residential decks are designed for 40 to 50 pounds per square foot (psf). A filled planter box on a small area—like a 4-square-foot footprint—could exert 150 psf or more. That’s a red flag. Always use a heavy-duty base or distribute the weight with a larger support pad.

My advice? Weigh the empty planter on a bathroom scale, calculate the soil volume, and then multiply by 90 pounds per cubic foot (a safe wet-soil estimate). Add the weight of the plant and water. If the total exceeds 50% of the structure’s load capacity, please move me to a ground-level spot or reinforce the area below.

I’m a weighty friend, but I’m also a beautiful anchor for your garden. Just make sure the muscle below me is strong enough to hold me up. Stay safe, and happy planting.