Hydroponic Reservoir Size Calculator

Why reservoir size matters in hydroponics:

An undersized reservoir swings in pH, spikes in nutrient concentration, and follows room temperature up and down. All three stress plants. A properly sized reservoir stays stable between changes and gives you a buffer for the day you cannot get to it.

General rule of thumb:

Minimum reservoir = Number of plants × Gallons per plant

Gallons per plant by system type:

DWC and Kratky are the odd ones out. There is no central tank to size in either: the number you get is the total across all your buckets, and each plant sits in its own. The calculator says so when you pick them. Plumb your DWC buckets together into a recirculating loop and it becomes a shared reservoir again, at which point the single-tote figure applies.

Worked example:

12 tomato plants in an Ebb and Flow system at 2.5 gal/plant:

• Minimum reservoir = 12 × 2.5 = 30 gallons
• Recommended (add 25% buffer): 37.5 gallons → use a 40-gallon tote

This rule sizes for stability, not for thirst

Here is where the rule of thumb runs out, and it is worth being blunt about it. Gallons per plant is about keeping pH and EC steady, not about how much the plants drink. Those two only agree while the plants are small.

Take that same worked example. Twelve mature, fruiting tomatoes drink somewhere between 12 and 24 gallons a day in warm conditions. Against a 37.5-gallon reservoir that is a third to two thirds of the tank every single day, and the standard advice is that anything past a quarter per day means the reservoir is too small. So 37.5 gallons is a sensible tote for twelve seedlings and an undersized one for twelve plants in full fruit.

Watch your water level rather than trusting the arithmetic. If it drops more than 25% in a day, size up. The calculator now works out that daily draw for you and says when it crosses the line.

Temperature control:

Larger reservoirs are easier to keep cool. Root zone temperature should stay between 65–72°F (18–22°C). Above 75°F, dissolved oxygen drops and root rot (pythium) risk skyrockets. In warm environments, add a water chiller or use frozen water bottles.

Practical tips:

• Use opaque containers. Light in the tank means algae, and algae competes for oxygen.
• Never let the reservoir run dry. Exposed roots die within hours, not days.
• Top off evaporated water with plain pH-adjusted water rather than nutrient solution, which is what prevents salt buildup.
• Full reservoir changes every 7–14 days keep things stable.