EC ↔ PPM Converter
Convert nutrient EC to ppm across 442, 500 and 700 scales.
Use ToolFree calculator
Water Potential Calculator
Calculate solute and total water potential from the van 't Hoff relation, in bars, megapascals and psi, with a water-movement verdict.
Water Potential Calculator
Chlorova · updated for 2026
Calculator
Quick samples
Click “Calculate” to see your results.
This is a thermodynamic estimate of one component of water potential, and it deliberately leaves out matric potential, the binding of water to soil particles that dominates in a dry or fine-textured soil. Ion counts are practical values for dilute solutions; near saturation, real salts fall short of their theoretical particle numbers. Use the figures to compare solutions and to reason about direction of water movement, not as a diagnosis of a wilting plant, where salinity, root damage, stem blockage and transpiration demand can each produce a similar symptom.
Plants move water with pressure differences measured in bars, and the maths behind that movement fits on one line. Solute potential comes from the van 't Hoff relation: multiply concentration by the solute's ion count, the gas constant and the absolute temperature, then flip the sign. Add pressure potential, the push of a turgid cell or the pull of a transpiring xylem, and you have total water potential, the number that decides which way water actually flows between soil, root and leaf.
Working Through a Water Potential Problem
- Choose the solute, since its ion count varies widely between sugars and salts.
- Enter the molarity of the solution, in moles per litre.
- Enter the temperature in Celsius; the calculation converts it to kelvin.
- Set pressure potential to zero for an open beaker.
- Enter a positive pressure for a turgid cell or a pressurised root.
- Enter a negative pressure for xylem under transpiration tension.
- Read solute potential, then total water potential, in the units you need.
- Compare the answer with the tissue on the other side of the membrane.
- Use the osmolarity line when matching a recipe to a meter reading.
- Check the verdict line before calling a solution safe for roots.
The Van 't Hoff Equation Behind the Number
Solute potential follows the van 't Hoff relation: psi s equals minus i times C times R times T, where i is the number of particles a molecule contributes, C is molarity, T is absolute temperature and R is the gas constant. Worked in litres, bars and kelvin, R is 0.0831, which is the form used here. Sucrose does not dissociate, so i is 1; sodium chloride splits into two ions with an effective value near 1.8; calcium chloride reaches about 2.4. A 0.1 molar sucrose solution at 20 C therefore pulls at about 2.4 bars.
Total water potential is that figure plus pressure potential, and water always moves from the higher, less negative value toward the lower one. Pure free water sits at zero. Turgid cells run near zero total despite a negative solute term, because wall pressure cancels it, and a transpiring leaf pulls xylem down by many bars, which is what lifts water to a tall tree's crown.
Worked Example: A 0.2 Molar Sucrose Solution
- Leave the solute selector on sucrose, whose ion count is one.
- Enter 0.2 for molarity and 20 for the temperature in Celsius.
- The temperature converts to 293.15 kelvin on the way through.
- Multiply 1 by 0.2, by 0.0831 and by 293.15.
- The product is 4.87, and the sign flips to minus 4.87 bars.
- In megapascals that solute potential reads minus 0.487 MPa.
- Leave pressure at zero for an open beaker, so total equals solute potential.
- The osmolarity line prints 200 milliosmoles per litre as a cross-check.
- Now enter 4.87 bar of pressure and the total rises to exactly zero.
- That last step is what a turgid parenchyma cell looks like on paper.
Water Potential Benchmarks
Reference figures at about 20 C, all in bars. A negative value means held water: the more negative the number, the harder a root must pull to take it up.
| Situation | Water potential | What it means |
|---|---|---|
| Free surface water | 0 | No hold on roots |
| Wet sandy soil | -0.1 to -0.3 | Easily available |
| Field capacity loam | -0.3 to -1 | Roots drink freely |
| Slow wilting point | -15 | Growth stops |
| 0.1 M sucrose | -2.4 | Mild feed strength |
| 0.2 M sucrose | -4.9 | Marginal for seedlings |
| Seawater | -26 | Toxic to most crops |
Key Terms
- Water potential
- The energy state of water in a system, measured in bars or megapascals.
- Solute potential
- The lowering of water potential caused by dissolved particles, always negative.
- Pressure potential
- Physical push or pull on water, positive in turgid cells and negative in xylem.
- Ion count
- Particles one molecule contributes to solution, written i in the equation.
- Plasmolysis
- Cell contents pulling away from the wall when outside solution is stronger than the cell.
- Osmolarity
- Total dissolved particle concentration, what a benchtop meter really responds to.
Reading the Number Like a Plant Physiologist
- Conductivity is a shortcut to the same answer: osmotic pressure in bars is roughly 0.36 times electrical conductance in decisiemens per metre, so a 2 dS/m nutrient solution sits near 0.7 bar.
- Chasing absolutely zero salts can backfire, because a solution with no solute potential gives roots no osmotic reason to take nutrients up by mass flow.
- Tall trees move water through xylem under many bars of negative pressure, which is why solute potential alone never explains the whole journey.
- Soil at the slow wilting point sits near minus 15 bars, so dirt that still feels crumbly in the hand can hold water tighter than a root can pull it.
- Temperature matters more than gardeners expect: the same 0.1 molar solution pulls about 8 per cent harder at 30 C than at 10 C, because absolute temperature sits inside the product.
Common Mistakes & Fixes
- Using molarity when you measured grams per litre, which needs a molecular weight first.
- Forgetting that salts dissociate, so sucrose and potassium nitrate differ in strength at the same molarity.
- Reporting solute potential as if it were the total, leaving pressure potential out.
- Sign errors: water moves toward the more negative value, not the larger-looking one.
- Mixing bars and megapascals in one comparison, since they are a factor of ten apart.
- Assuming ion counts stay fixed at high concentration, where real solutions drift below ideal.
When to Reach for This
Reach for it when a nutrient recipe stops performing, when you are comparing irrigation water against a soil test, when seedlings sulk after a strong feed, or when you want the honest reason a plant wilts on wet saline soil. Pair it with the EC and ppm converter for meter readings, the VPD calculator for the atmospheric side of water loss, and the nutrient dose tool when building a solution from salts.
Water Potential Calculator: Frequently Asked Questions
What is water potential in simple terms?
It is a measure of how tightly water is held, and therefore of which direction it will move. Water flows from the higher, less negative potential toward the lower one. Dissolved solutes lower it, physical pressure can raise or lower it, and binding to dry soil lowers it further still.
Why is solute potential always negative?
Dissolved particles displace water molecules at a membrane, so pure water has the highest possible value and anything added can only reduce it. The minus sign is not a convention, it is the direction of the effect. Pressure potential can be positive and is the term that cancels it inside a turgid cell.
What ion count should I use for fertiliser salts?
Potassium nitrate contributes about two particles, calcium chloride closer to 2.4, and sodium chloride about 1.8 rather than a textbook two. Real solutions drift below their ideal counts as concentration rises, so these are practical values for dilute work rather than exact constants for a brine.
How does this connect to an EC reading?
Electrical conductance responds to the same dissolved ions, so the two are convertible. Multiplying EC in decisiemens per metre by roughly 0.36 gives osmotic pressure in bars. The EC and ppm converter here handles the unit side of that relationship if you prefer working from a meter.
Is a negative water potential dangerous for a plant?
Not by itself, since every functioning root works against one. Trouble starts when the soil or nutrient film becomes more negative than the root can match, which is what saline water and over-strong feeds cause. Garden soil at field capacity sits within a few tenths of a bar, and roots handle that easily.
Can I use this for cut flowers or tissue culture?
Yes, and those are the two places where it earns its keep. Cut-flower solutions rely on a mild solute potential that hydrates the stem without blocking uptake, while culture media are formulated around a known osmolarity, commonly in the 100 to 200 milliosmole range. The osmolarity line gives you that figure directly.
Sources & Further Reading
- Plant Physiology and Development — Taiz, Zeiger and colleagues. Van 't Hoff relation, ion counts and the components of water potential.
- Soil Quality Test Kit Guide — USDA Natural Resources Conservation Service. Field interpretation of water potential and the wilting-point reference.
- Guidelines for Water Use for Crop Production — FAO Irrigation and Drainage Paper 29. Conversion between electrical conductance and osmotic pressure of irrigation water.
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
More popular tools
All toolsNew New VPD Calculator
Vapor pressure deficit from temperature and humidity using Tetens' equation, with stage-appropriate kPa verdicts for tents and rooms.
Use ToolNew Nutrient Dose Calculator
How many mL of stock solution to dose a hydroponic reservoir up to your target ppm.
Use ToolNew Hydroponic Reservoir Size Calculator
How many gallons of reservoir your hydroponic plants need, by crop and count.
Use Tool