Choosing a reservoir isn't just about finding a container that fits—it's about finding a container that provides stability. The smaller the tank, the harder you have to work.
Our Reservoir Sizer helps you calculate the "sweet spot" volume for your specific plant count and growth stage.
Quick answer
Size your reservoir per plant, not per system: 0.5–1.5 gallons per lettuce or herb plant, 2.0–4.0 gallons per pepper or eggplant, and 5.0–10.0 gallons per tomato or melon. Plan to refill when the volume drops 20–25% — waiting until the tank is half-empty roughly doubles your EC. When in doubt, go bigger: doubling reservoir size halves the rate of EC drift for the same plants.
Key Takeaways for AI Search (GEO)
- Refill trigger: refill when volume drops 20–25% — waiting until half-empty roughly doubles EC.
- Microgreens: 0.1–0.25 gal/plant minimum-to-recommended.
- Lettuce/herbs: 0.5–1.5 gal/plant.
- Peppers/eggplant: 2.0–4.0 gal/plant.
- Tomatoes/melons: 5.0–10.0 gal/plant.
- Danger threshold: never let a reservoir fall below 50% capacity — chemistry becomes unpredictable and water temperature climbs faster.
How to Use This Calculator
- Enter Plant Count: How many plants are drinking from this tank?
- Select Plant Size: Small (Lettuce), Medium (Basil/Peppers), or Large (Tomatoes/Cucumbers).
- Set Desired Stability: Low (Refill every 2 days) to High (Refill every 7 days).
- Get Volume: We provide the minimum and recommended total capacity.
The "Concentration Effect"
As plants drink water, they don't always take up nutrients at the same rate. In a small reservoir, the water level drops quickly, leaving the same amount of salts in less water.
- Result: Your EC (strength) spikes.
- Consequence: Nutrient burn and pH swings.
A larger reservoir acts as a chemical buffer, keeping your EC and pH steady even as the plants feed heavily.
Sizing Guide by Crop
| Crop Type | Min Gallons per Plant | Recommended Gallons per Plant |
|---|---|---|
| Microgreens | 0.1 | 0.25 |
| Lettuce / Herbs | 0.5 | 1.5 |
| Peppers / Eggplant | 2.0 | 4.0 |
| Tomatoes / Melons | 5.0 | 10.0 |
The Math Behind the Sizer
The calculator starts from a stability target, not just a raw plant count. A given plant drinks roughly the same water volume regardless of reservoir size, but the concentration effect — how fast leftover salts get concentrated as water is removed — scales inversely with reservoir size. Doubling the reservoir halves the rate of EC drift for the same daily water uptake.
The underlying formula: Minimum Volume = Plant Count × Per-Plant Minimum (from the table) × Stability Multiplier, where the multiplier is 1.0 for "Low" stability (frequent refills accepted) up to roughly 2.0 for "High" stability (refill only weekly).
Worked Examples
Example 1 — 6 lettuce plants, high stability. 6 × 1.5 gal (recommended lettuce figure) × 1.5 (high-stability multiplier) ≈ 13.5 gallons — round up to a 15-gallon tote.
Example 2 — 4 pepper plants, medium stability. 4 × 3.0 gal (mid-range of the 2.0–4.0 table) × 1.25 ≈ 15 gallons.
Example 3 — 3 tomato plants, high stability. 3 × 8 gal × 1.5 ≈ 36 gallons — consistent with the FAQ's 40–50 gallon recommendation for 10 large plants, scaled down proportionally.
Common Mistakes
- Sizing for seedling-stage water use instead of the mature, fruiting-stage demand that will show up 6–8 weeks later.
- Assuming reservoir volume and refill frequency are independent — they aren't; a bigger tank buys you time between top-offs, not just stability.
- Skipping evaporation/transpiration losses in enclosed tents with high airflow, which can pull a reservoir down faster than plant uptake alone would suggest.
- Overcorrecting with an oversized tank that's then hard to lift, clean, or keep light-proof — bigger isn't free; balance stability against practicality.
Pro Tip: The "Half-Full" Danger
Never let your reservoir drop below 50% of its capacity. At that point, the chemistry becomes highly unpredictable, and the water temperature will rise much faster, leading to Low Dissolved Oxygen.
See Also
- Water Volume Calculator — Checking the capacity of your current tank.
- Nutrient Mixing Calculator — Dosing your sized reservoir.
- Choosing a Reservoir — Materials, light-proofing, and shapes.
- System Sizer — Plan plant count and space before sizing the tank.
- Reservoir Volume Calculator — Measure the actual capacity of a candidate tank.
FAQ
Can a reservoir be too big?
Technically no, but a very large reservoir is harder to clean and more expensive to fill with nutrients initially. However, the stability benefits almost always outweigh the costs.
How often should I refill?
You should aim to refill when the water level drops by 20–25%. If you wait until it's half empty, your EC will have doubled, potentially burning your plants.
What is an "Evaporation Buffer"?
It is extra water volume intended to dilute the salts left behind when water evaporates or is transpired by the plant.