Hydroponic Reservoir Size Calculator
How many gallons of reservoir your hydroponic plants need, by crop and count.
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Kratky Method Calculator
Reservoir size in gallons and liters for passive Kratky hydroponics, by crop and plant count, with a bucket count suggestion.
Kratky Method Calculator
Chlorova · updated for 2026
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Gallon-per-plant figures are standing-water rules of thumb, not physics, and real demand swings with temperature, humidity, light and how mature the plant gets, so a hot south window drinks far faster than a cool shelf. The reserve percentage only budgets for top-offs, and Kratky still needs the initial root-zone contact and an air space as the level drops. Solution strength, pH and water quality are separate questions this tool does not touch, so mix nutrients to the crop label, and treat the vessel size here as the minimum comfortably bigger than the plant ever needs.
Kratky is the strangest and simplest way to grow hydroponically: a jar of nutrient solution, a net cup, and no pump, no timer and no moving parts at all, because the roots drink from a static bath and an air gap feeds them oxygen as the level falls. The only design decision is vessel size, and this Kratky calculator makes it from crop and plant count, converting the standing-water rule of thumb into gallons, liters and a suggested number of five-gallon buckets, plus a top-off reserve so the bath never drops too far.
Sizing Kratky Jars in Steps
- Pick the crop so the tool applies its correct gallon-per-plant standing-water rule.
- Enter how many plants the whole batch will hold in total.
- Set a top-off reserve percentage above the bare starting solution.
- Read the straight, unreserved solution volume in gallons on the panel first.
- Check the reserved volume, since that is your real target fill line.
- Use the bucket count to pick vessels you already own or can buy.
- Mix nutrient solution to label strength and check the level every few days.
- When in doubt size the jar up, as a dry vessel kills a Kratky crop fast.
How Vessel Volume Is Worked Out
The starting point is a per-crop standing-water allowance, roughly one gallon for lettuce, half that for small herbs and more for big leafy crops, multiplied simply by the number of plants. That product is the minimum nutrient solution the batch should hold at the start, and the crop picker supplies only this one number.
The reserve adds a safety margin on top, calculated as the solution volume multiplied by one plus the reserve percent over one hundred, because a vessel exactly the size of a plant's thirst runs dry the day a heat wave hits. Dividing the reserved gallons by five translates the plan into common five-gallon buckets, rounded up since partial buckets do not exist, and multiplying gallons by 3.785 gives the metric reading for anyone mixing by liters.
Worked Example: Four Lettuces in Buckets
- You want four leaf lettuces on a shelf under a light.
- Lettuce carries a one-gallon-per-plant standing-water rule.
- Bare solution is 4 times 1, so exactly 4 gallons.
- You set a 25 percent top-off reserve for hot days.
- Reserved volume becomes 4 times 1.25, so 5 gallons.
- Dividing by five buckets means 1 bucket for the batch.
- In metric the bare 4 gallons reads as 15.1 liters.
- So one five-gallon bucket with four net-cup lids runs the batch.
- Fill to the reserve line, leaving a genuine air gap under the lid.
- Check the level after a hot week and top off with plain water.
- That single bucket grows four full heads with no pump or electricity at all.
Crop Volumes at a Glance
Standing-water allowances per plant that the crop picker applies, with the harvest window each one supports on a single fill plus a sensible reserve. Bigger numbers mean steadier levels and less frequent top-offs for the same plant count.
| Crop | Gal per plant | Typical cycle |
|---|---|---|
| Leaf lettuce | 1.0 | 4-6 weeks |
| Kale | 1.5 | 2-4 months |
| Swiss chard | 1.25 | 2-3 months |
| Basil | 0.75 | 6-8 weeks |
| Small herbs | 0.5 | 4-6 weeks |
| Spinach | 0.75 | 5-7 weeks |
Key Terms
- Kratky method
- Passive hydroponics where roots sit in still solution and an air gap feeds oxygen.
- Standing water
- The unmoving nutrient bath a Kratky vessel holds for the whole crop.
- Net cup
- The slotted plastic cup holding clay pebbles and the plant above the solution.
- Air gap
- The oxygenated space between the lid and the falling water level, the method's engine.
- Top-off
- Adding plain or weak solution as the level drops, which the reserve budgets for.
Lessons from Static-Solution Growers
- Size up rather than down, because the failure mode of Kratky is a dry vessel on a hot week, and an oversized jar costs only solution you would have dumped anyway.
- The air gap is the whole trick, so a level that never drops leaves roots drowned and a level that drops too fast leaves them parched, which is why mature crops want bigger baths than seedlings do.
- Temperature matters more than volume in disguise, since warm solution holds less oxygen and grows algae in light-leaking jars, so shade the vessel and the same gallon stretches further.
- Small herbs genuinely thrive in upcycled containers, and the half-gallon rule is why basil and parsley succeed in milk jugs where lettuce would sulk.
- Top off with plain water between harvests but re-mix fresh solution when the bath gets low and salty, because the plant takes nutrients and water unevenly and concentration drifts.
Common Mistakes & Fixes
- Sizing the jar for the small seedling instead of the mature plant it becomes in six weeks.
- Sealing the roots fully submerged with no air gap, which drowns the plant in its own success.
- Using transparent jars that green the solution with algae instead of opaque or painted buckets.
- Skipping the reserve, then finding the bath bone dry during one unseasonable heat spell.
- Reusing old solution indefinitely until salts build up, pH drifts, and leaf tips burn.
When to Use This (and When Not To)
Use it for passive single-plant or bucket systems on the counter, shelf or balcony, where simplicity is the point and soil-garden speed is not expected. It is the wrong tool for flowing systems, so size a recirculating tank with the hydroponic reservoir calculator instead, dose that tank with the nutrient-dose tool, and remember fruiting crops generally outgrow Kratky's static bath well before harvest.
Kratky Method Calculator: Frequently Asked Questions
How much water does a Kratky lettuce need?
About one gallon of standing solution per head, plus a reserve for top-offs, so four lettuces plan for roughly five gallons. Small herbs manage on half that, while kale and chard want a gallon and a half each because their cycles run longer.
Can I reuse the same jar for the next crop?
Yes, after a scrub with dilute bleach or hydrogen peroxide, since leftovers of old solution and root films invite algae and disease. A clean jar also lets you judge the new batch's level honestly instead of reading stained plastic.
Why does vessel size matter if the plant only drinks slowly?
Because the reservoir both feeds the plant and maintains the air gap, and a small jar empties or drops past the roots before harvest. The gallon rule is really a guarantee that level and gap stay right for the whole cycle.
Do tomatoes work in Kratky jars?
Dwarf tomatoes occasionally, in several gallons and with frequent top-ups, but most fruiting crops outgrow passive baths. They drink too much and too long for static solution, so a recirculating system with a pump is the honest answer for tomatoes.
What is the top-off reserve for?
It is headroom so the level never crashes below the roots between your checks, typically a quarter to a half again the base volume. Heat waves and big leaves make plants drink far faster than the calm-week baseline, and the reserve absorbs exactly those surprises.
How often should I check a Kratky vessel?
A glance every two or three days is enough early on, and daily during a heat spell or once the canopy fills out. The level tells you everything: falling fast means the plant is thriving, and a sudden stop often means roots have clogged rather than thirst ending.
Sources & Further Reading
- The Kratky Method: Passive Hydroponics — University of Hawaii, Professor Bernard Kratky. Origin of the static-solution method and its root-zone air-gap requirement.
- Soilless Culture by Nutrient Solution: Non-Circulating Systems — Penn State Extension. Basis for per-plant solution volumes in passive containers.
- Hydroponic Production for Home Growers — Oregon State University Extension. Underpins the vessel sizing, algae and top-off guidance here.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated