What is water potential (Ψ)?

Water potential (Ψ) quantifies the potential energy of water relative to pure water at standard conditions and determines the direction of water movement. It is the sum of solute potential (Ψs, usually negative) and pressure potential (Ψp, often positive in turgid cells).

What units are used for water potential?

Common units include megapascals (MPa), kilopascals (kPa), and bar. 1 MPa = 1000 kPa ≈ 10 bar. Pure water has Ψ = 0 MPa; plant tissues are typically negative, e.g., -0.3 to -1.5 MPa.

How do I estimate solute potential (Ψs)?

Use the van't Hoff equation: Ψs = -iCRT, where i is the ionization factor, C is molarity (mol/L), R is the gas constant (0.008314 MPa·L·mol⁻¹·K⁻¹), and T is temperature in Kelvin. The calculator provides a helper to compute Ψs from these inputs.

💧 Water Potential Calculator

Calculate total water potential Ψ from solute potential (Ψs) and pressure potential (Ψp). Switch units, and use the built-in helper to estimate Ψs from molarity.

Units

Enter Potentials

Solute Potential (Ψs) [MPa]

Typically negative (e.g., -0.3 to -1.5 MPa).

Pressure Potential (Ψp) [MPa]

Usually positive in turgid cells, near 0 in xylem.

Results

Ψs

-0.800

MPa

Ψp

0.300

MPa

Total Ψ

-0.500

MPa

Interpretation

Moderately negative. Common in active transpiration or mild drought.

Equation: Ψ = Ψs + Ψp

Typical Water Potentials

Pure water at standard conditions: 0 MPa
Healthy leaf (midday): -1.0 to -2.0 MPa
Xylem sap: near 0 to negative (often < -0.2 MPa)
Plant wilting point: around -1.5 MPa
Very dry soils can be below -3 MPa