This calculator will not tell you exactly how to build your aquarium, nor will it tell you directly what material thickness or type you should choose. The way it works is that you must create your own plans, and then input your numbers into the calculator, which will tell you if your plans are good or not. You will also get numbers that tell you how overbuilt or underbuilt you structure is. This will help you to tinker around and find a way to build that is both safe and economical.

The calculator evaluates your structure in these two ways:

1. Strength safety factor

The calculator finds the maximum stress your wall/bottom will be exposed to from bending caused by static water pressure. The strength safety factor is how strong the material is, compared to the maximum stress. For example: 100% means the wall will barely hold. Less than 100% means it will probably break, and more than 100% means will hold with some margin. The higher the safety factor, the better. The calculator will tell you that anything below 300% is too low to be safe. The result is then marked in red color and an error message with an explanation is displayed. But There is no absolute answer to how much stronger than the estimated stress the aquarium should be. You can make your own choice. Note that this is only stress from bending, which is where the stress is normally the highest. But aquariums can fail in other ways too: If the silicone, glue, screws or other fasteners holding the structure together aren't strong enough, or used wrong, the aquarium can form cracks or tear seams. These are common failure modes in glass aquariums and plywood aquariums. These failure modes are outside the scope of the calculator.

2. Deflection (bending/bowing)

For many materials, like plywood and acrylic, strength isn't the limiting factor. Before the material breaks, it will bend too much. Bending both looks bad and causes uneven stress on the seams, which can cause structural failure. Bending, or deflection, is given as the maximum distance that the plate has deflected sideways as a result of static water pressure. It is also given as a percentage of the smallest dimension of the plate. This is because larger deflection is acceptable for larger aquariums. If the deflection percentage is 0.5% or more, the calculator marks it as too big. The mathematical model used for calculating deflection here does not support deflection that is larger than half the thickness of the plate. So if that is the case, the deflection is also marked as too big, and the strength is marked as red, since the results are unreliable.

Sometimes the calculator specifies that the wall must be supported on 4 sides. Here is what that means: We may think of a typical aquarium as having four walls and a bottom, but no top. But from a structural perspective this can be misleading. If you look at a large glass aquarium you may see strips of glass glued along the upper edges and even across from front to back. I call this the "top framing and bracing". This has a very important function: It is there to make sure that the top acts a 6th "wall" in terms of supporting the other walls, holding them in place, and make sure they don't bend at the top. When the calculator specifies "supported on 4 sides", it is assumed that such a top is in place and that the top edge of your wall is firmly held in place, and held straight, by it. If this is not the case then the calculator may give wrong results.

The design philosophy behind this calculator is to always err on the side of caution. This means that it will tell you that your materials are exposed to more stress than they actually are, rather then less. In some cases, like with glass, this is not a big problem since the material is simple and standardized. With plywood on the other hand, there may be large deviations between how much actual bending there is compared to what the calculator says. But the real structure should always bend less than what the calculator says. It is very difficult to know the exact properties of plywood. This is particularly true for thick plywood with few plies, for example 5 plies. Such plywood is much stronger and stiffer in the grain direction of the outer plies. As an extreme example, 15 millimeter (approx. 5/8 in), 5-ply spruce plywood is more than 3 times stiffer in that direction. The calculator assumes the worst case scenario, which is typical in plywood aquarium building, where the grains on the outer plies go along the length on the aquarium. But if the plywood is used optimally, it may be 3 times more resistant to bending than assumed by the calculator.

When specifying the dimensions it is important to know where to measure and the restrictions that apply. Measure from where the window opening starts to where it ends. So it is not the size of the glass plate itself that is measured if the glass is set in a frame, but the opening. Also, the calculator assumes that the water level goes to the top of the opening. If you plan on having a water level higher than that, you must pretend that the window is taller and goes to the water surface when calculating. Otherwise you may end up with glass that is too weak.

The calculator can't find exactly how this type of wall behaves. Instead it divides the problem into two separate calculations. None of these give an exact picture of the behavior of the wall, but together they should give a good enough description.

Calculation 1 - Plywood, between studs

This calculation looks at a section of plywood between two studs. Assuming that the studs are completely rigid it calculates the deflection and stress on the plywood as if it was a plywood wall. This will tell you if the plywood is strong/stiff enough to do it's job between the studs. The actual stress/bending of the plywood will be higher than shown here as the studs are in reality also bending from water pressure. It is important that the plywood is firmly fixed to the studs, preferrably with both screws and glue, as this improves the bending characteristics of the plywood. Known weakness: When the distance between the studs becomes very small the calculator will find that the safety factor is much lower than it really is. This issue starts when the section of plywood is more than twice as tall as it is wide. Be aware of this. It is a weakness in the algorithm I am using and not much I can do about for now. It will also exaggerate the bending.

Calculation 2 - Studs, stiffened by plywood

This calculation pretends that a stud and the plywood that "belongs" to it, the section of plywood that goes half the way to the next stud on both sides, form a T-shaped vertical beam. It then calculates the bending of this beam, based on water pressure and as if it was detached from anything else on each side.

This is very similar to a studded plywood wall. So please read that section first. Also, note the following: A wall without top support must be considered more advanced structurally than other aquarium walls. The reason is that there will be a moment (also known as torque) in the lower anchoring point. This may cause very high stresses that will splitt the lower seam open. Some structure that counters the moment is needed. Proper fasteners are also important. Make sure you understand these principles before building. In the case of a free standing wall maximum deflection is given at the water surface. This means that the wall will deflect more than show by the calculator, above the water surface, even if it doesn't bend above the surface. The higher above the surface it goes, the more deflection.

All calculations allow you to specify your own values for strength and stiffness in case the values provided don't fit the materials you are using. Most manufacturers provide these numbers for their products. They are highly standardized numbers for building materials. The values are given as pressures. They are not just called "strength" and "stiffness" however:

Strength

A material has different types of strength. The one that the calculator needs is called "Strength in bending", "Bending strength" or "Flexural strength". Sometimes the words "characteristic" or "average" is added. "Characteristic" has a bit safety included in the measurement method and is safer. The values provided with the calculator are characteristic.

Stiffness

"Stiffness" is my word. What you will find in the literature is "Modulus of elasticity in bending". "Bending modulus" or "Flexural modulus".

I am not a structural engineer or anything similar to that. I work with IT, and this calculator is just my hobby. So be aware.

Contact

If you have questions please ask them at the Facebook group: DIY Plywood aquarium builds, if you use Facebook, or at www.monsterfishkeepers.com where I am user JonOlavCalc. Please let me know if you find any bugs in the calculator, or find that the results don't fit with other data that you may have found.