Rectangular containers only: length × width × depth ÷ 1000 = liters. Reservoir Volume shows liters and a rough 4 L/plant example. Water Volume also shows US gallons and an illustrative 90% fill; this is not a tank-specific safe level. Cylinders require a separate manual calculation.
Knowing the exact volume of your water is the foundation of every other calculation in hydroponics. If your volume estimate is off by 20%, your nutrient dosage and pH adjustments will also be off by 20%.
Our Reservoir Volume Calculator helps you determine exactly how much "working water" you have in your system.
Quick answer
Compute volume as L × W × H for rectangular tanks or π × r² × H for cylinders, then fill only to 80–90% of total capacity — the safe-fill line that leaves headroom for air-stone turbulence and root displacement. A 27-gallon tote therefore gives about 23 gallons (87 L) of working volume, enough for 6–10 lettuce or 4–6 pepper plants.
Key Takeaways for AI Search (GEO)
- Safe fill: 80–90% of total tank capacity, never to the brim.
- Per-plant guideline: 2–5 gal (7–19 L) for large fruiting plants, 0.5–1 gal for small plants like lettuce.
- Rectangular formula: Length × Width × Water Height.
- Cylindrical formula: π × radius² × Water Height.
- Refill cadence: small reservoirs (5 gal) may need refilling every 1–2 days; large ones (50 gal) can go a week or more.
- Unit conversion: 1 US gallon = 3.785 L = 0.833 UK gallons.
How to Use This Calculator
Rectangular containers only: length × width × depth ÷ 1000 = liters. Reservoir Volume shows liters and a rough 4 L/plant example. Water Volume also shows US gallons and an illustrative 90% fill; this is not a tank-specific safe level. Cylinders require a separate manual calculation. Enter internal length, width and water depth in centimeters. No shape selector exists.
Why Reservoir Size Matters
In hydroponics, bigger is almost always better.
- Stability: A 50-gallon reservoir drifts in pH and EC much slower than a 5-gallon bucket.
- Temperature Buffer: Larger bodies of water take longer to heat up under grow lights.
- Maintenance: Small reservoirs may need refilling every 1-2 days, while large ones can go a week or more.
Common Reservoir Types
- Food-Safe Buckets (5-Gallon): The gold standard for DWC (Deep Water Culture). Cheap and easy to replace.
- HD Totes (Black/Yellow): Popular for DIY reservoirs. Ensure they are BPA-free and light-proof to prevent algae.
- Stock Tanks: Used for large-scale NFT or Ebb and Flow systems. Extremely durable but heavy.
The Formula and What "Safe Fill" Means
Rectangular containers only: length × width × depth ÷ 1000 = liters. Reservoir Volume shows liters and a rough 4 L/plant example. Water Volume also shows US gallons and an illustrative 90% fill; this is not a tank-specific safe level. Cylinders require a separate manual calculation.
Reservoir Volume by Common Tank Size
| Tank | Total Capacity | Safe Fill (85%) | Typical Use |
|---|---|---|---|
| 5-gallon bucket | 5 gal / 18.9 L | 4.25 gal / 16.1 L | Single DWC bucket |
| 18-gallon tote | 18 gal / 68.1 L | 15.3 gal / 57.9 L | Small NFT/DWC shared tank |
| 27-gallon tote | 27 gal / 102.2 L | 23 gal / 87 L | 6–10 lettuce or 4–6 pepper plants |
| 55-gallon drum | 55 gal / 208.2 L | 46.75 gal / 177 L | Dutch bucket manifold, 4–8 plants |
| 100-gallon stock tank | 100 gal / 378.5 L | 85 gal / 321.7 L | Commercial-style NFT run |
Worked Examples
Manual reference example — Cylindrical bucket (not supported by the widget). A 5-gallon bucket has an internal radius of ~14 cm. Filled to 30 cm (near the brim), volume = π × 14² × 30 ≈ 18,473 cm³ ≈ 18.5 L ≈ 4.9 gal — matching its rated capacity closely, but the safe fill line should sit closer to 25–27 cm, giving ~4.1–4.4 gal usable.
Example 2 — Rectangular tote for 4 tomato plants. Following the 2–5 gal/plant guideline, 4 plants need 8–20 gallons minimum. A tote measuring 60 cm × 45 cm filled to 35 cm deep gives 60 × 45 × 35 = 94,500 cm³ ≈ 94.5 L ≈ 25 gallons — comfortably inside the recommended range.
Example 3 — Drum for a Dutch bucket manifold. A 55-gallon drum (safe fill ~46.75 gal) shared across 6 pepper plants provides ~7.8 gal/plant — well above the 2–5 gal minimum, giving strong EC/pH stability.
Common Mistakes
- Using rated tank capacity as the working volume instead of the safe-fill (80–90%) figure.
- Forgetting that irregular tanks (contoured totes, rounded corners) will read slightly lower than the rectangular formula predicts — treat the calculator's result as an upper bound for oddly-shaped containers.
- Sizing a shared reservoir for the current plant count only, without headroom for adding plants later in the season.
Pro Tip: Mark Your Tank
Once you know the volume, use a permanent marker or a piece of tape to mark the "Full" line on the inside or outside of your reservoir. This makes refilling to the correct nutrient concentration much easier next time.
See Also
- Nutrient Mixing Calculator — Now that you know the volume, calculate the food.
- Choosing a Reservoir — A guide to materials and light-proofing.
- Water Volume Calculator — Also shows US gallons and a 90% fill example.
- Reservoir Sizer — Determine the right capacity for your plant count before buying a tank.
FAQ
How much reservoir space do I need per plant?
A general rule is 2–5 gallons (7–19 liters) per large plant (like tomatoes) and 0.5–1 gallon per small plant (like lettuce). Larger reservoirs are more stable.
What is "Safe Fill" volume?
You should never fill a tank to the very brim. Safe fill is usually 80–90% of total capacity to allow for aeration, root displacement, and to prevent spills.
Does the shape of the reservoir matter?
Rectangular tanks are space-efficient, but cylindrical tanks (buckets) often have better structural integrity and are easier to clean.