Concrete Planter Calculator
Calculate concrete mix amount for DIY concrete planters from inner and outer mold dimensions. Enter values for instant results with step-by-step formulas.
Formula
Concrete Volume = Outer Volume - Inner Volume = (L x W x H) - ((L - 2t) x (W - 2t) x (H - b))
Where L, W, H are the outer length, width, and height of the planter, t is the wall thickness, and b is the bottom thickness. The inner mold dimensions are derived by subtracting twice the wall thickness from length and width, and the bottom thickness from the height.
Worked Examples
Example 1: Standard Rectangular Planter
Problem: Calculate concrete needed for a planter with outer dimensions 18 x 12 x 10 inches, wall thickness 1.5 inches, and bottom thickness 1.5 inches.
Solution: Outer volume = 18 x 12 x 10 = 2,160 cubic inches\nInner dimensions = (18 - 3) x (12 - 3) x (10 - 1.5) = 15 x 9 x 8.5 = 1,147.5 cubic inches\nConcrete volume = 2,160 - 1,147.5 = 1,012.5 cubic inches = 0.586 cubic feet\nWeight = 0.586 x 133 = 77.9 lbs\nBags needed: 2 x 60 lb bags or 1 x 80 lb bag
Result: Concrete needed: 0.586 cu ft (1,012.5 cu in) | Weight: 77.9 lbs | One 80 lb bag
Example 2: Set of Four Small Planters
Problem: Calculate concrete for 4 small planters, each 8 x 8 x 6 inches with 1 inch walls and 1 inch bottom.
Solution: Per planter outer volume = 8 x 8 x 6 = 384 cubic inches\nInner dimensions = 6 x 6 x 5 = 180 cubic inches\nConcrete per planter = 384 - 180 = 204 cubic inches = 0.118 cubic feet\nTotal for 4 = 0.472 cubic feet\nTotal weight = 0.472 x 133 = 62.8 lbs\nBags needed: 2 x 60 lb bags or 1 x 80 lb bag
Result: Total concrete: 0.472 cu ft | Total weight: 62.8 lbs | One 80 lb bag covers all four
Frequently Asked Questions
What type of concrete mix is best for making planters?
For DIY concrete planters, a standard Portland cement-based mix works well for larger planters, while a finer concrete countertop mix is ideal for smaller and more detailed planters. Quikrete or Sakrete commercial mixes are readily available and produce good results. For a smoother finish, use a sand-based mix without large aggregate. Many crafters prefer mixing their own blend using one part Portland cement, one part perlite or vermiculite, and one part peat moss to create a lighter-weight hypertufa planter. Adding fiber reinforcement or using a fiber-reinforced mix helps prevent cracking during curing. Avoid rapid-set concrete for planters because you need working time to mold the shape properly.
How thick should the walls of a concrete planter be?
Wall thickness for concrete planters typically ranges from 1 to 2 inches depending on the overall size of the planter. Smaller planters under 12 inches can use walls as thin as 1 inch, while larger planters over 24 inches should have walls at least 1.5 to 2 inches thick to provide structural integrity. The bottom should generally be the same thickness as the walls or slightly thicker to support the weight of soil and water. Thicker walls increase the weight significantly but improve durability and frost resistance. If you live in a climate with freeze-thaw cycles, consider walls at least 1.5 inches thick to prevent cracking from ice expansion within the concrete pores.
How do I prevent my concrete planter from cracking?
Preventing cracks in concrete planters requires attention during both mixing and curing stages. First, do not add too much water to the mix because excess water creates weak spots and shrinkage cracks as it evaporates. The ideal consistency should be like thick oatmeal or cookie dough. Second, cure the planter slowly by covering it with plastic sheeting and misting it with water daily for at least five to seven days. Rapid drying is the primary cause of cracking in concrete projects. Third, adding polypropylene fiber reinforcement to the mix significantly reduces cracking. Fourth, avoid making walls too thin for the planter size. Finally, consider adding a concrete bonding agent to the mix for improved flexibility and crack resistance.
Do concrete planters need drainage holes and how do I add them?
Yes, drainage holes are essential for concrete planters to prevent root rot and waterlogging that kills plants. Most planters need at least one drainage hole per 6 inches of planter width. You can create drainage holes by inserting greased dowels or PVC pipe sections into the bottom of the mold before pouring concrete. Position the dowels vertically through the bottom form and remove them after the concrete has set but before it fully cures, typically after 24 to 48 hours. Alternatively, you can drill holes using a masonry bit after the concrete has fully cured, though this risks cracking. Each drainage hole should be at least half an inch in diameter to allow adequate water flow.
How long does a concrete planter need to cure before planting?
Concrete planters require a curing period of at least 28 days before planting because fresh concrete is highly alkaline with a pH around 12 to 13, which can burn plant roots. During the first seven days, keep the planter covered with plastic and mist it daily to ensure proper hydration curing. After the initial cure, soak the planter in water for several days, changing the water daily to leach out excess lime. You can test the pH of the soaking water using a simple aquarium test kit. Once the water pH drops below 8, the planter is safe for most plants. Applying a concrete sealer to the interior can speed up this process and provide an additional barrier between the alkaline concrete and plant roots.
How much does a finished concrete planter weigh?
Standard concrete weighs approximately 133 pounds per cubic foot, so even small planters can be surprisingly heavy. A typical 18 by 12 by 10 inch planter with 1.5 inch walls weighs roughly 40 to 50 pounds empty before adding soil and water. Filling the same planter with moist potting soil adds another 20 to 30 pounds. To reduce weight, you can substitute part of the aggregate with perlite, vermiculite, or even styrofoam beads to create a lighter mix. Hypertufa mixes using peat moss and perlite can reduce weight by 50 to 60 percent compared to standard concrete. Consider the final placement location and whether the surface can support the combined weight of planter, soil, water, and plant material.