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Water Potential Calculator

📅Last updated: December 6, 2025
✓Reviewed by: LumoCalculator Team

Calculate water potential (Ψ) for plant cells and solutions. Determine solute potential from molarity and temperature, add pressure potential, and predict the direction of water movement.

Calculate Water Potential

Ψ = Ψs + Ψp

Molarity in mol/L

van't Hoff factor (1 for non-electrolytes)

Turgor pressure (0 for open system)

Water Potential Results

Water Potential (Ψ)
-0.9395 MPa
= -9.3948 bar
➡️ Water flows OUT of cell

Normal - Cell in typical hydration state

Solute Potential (Ψs)
-1.2395 MPa
Always ≤ 0
Pressure Potential (Ψp)
+0.30 MPa
Turgor pressure
Input Parameters
Molarity:0.5 M
Temperature:25.0°C (298.1 K)
i-factor:1
💡 Analysis

Moderate negative water potential. Normal range for most plant cells in soil conditions.

Calculation Steps

Formula:
Ψ = Ψs + Ψp, where Ψs = -iCRT
T(K) = 25°C + 273.15 = 298.15 K
Ψs = -(1)(0.5 M)(0.008314 L·MPa/mol·K)(298.15 K) = -1.2395 MPa
Ψ = -1.2395 MPa + 0.3000 MPa = -0.9395 MPa

Related Calculators

Water Potential Formula

Main Equation

Ψ = Ψs + Ψp
Ψ = Water Potential (MPa or bar)
Ψs = Solute Potential (always ≤ 0)
Ψp = Pressure Potential (turgor)

Solute Potential Formula

Ψs = -iCRT
i = Ionization constant (van't Hoff factor)
C = Molarity (mol/L)
R = Gas constant (0.00831 L·MPa/mol·K)
T = Temperature (Kelvin)

Components of Water Potential

🧪 Solute Potential (Ψs)

Also called osmotic potential

  • • Always negative or zero
  • • More solute = more negative
  • • Pure water: Ψs = 0
  • • Lowers water's free energy
⬆️ Pressure Potential (Ψp)

Physical pressure on water

  • • Usually positive in cells (turgor)
  • • Zero in open systems
  • • Negative in xylem (tension)
  • • Cell wall provides back-pressure
📉 Matric Potential (Ψm)

Water bound to surfaces

  • • Important in soil
  • • Adhesion to particles
  • • Usually negative
  • • Often small in solutions
🏔️ Gravitational Potential (Ψg)

Effect of gravity on water

  • • ~0.01 MPa per meter height
  • • Important in tall trees
  • • Usually neglected in cells
  • • Adds to Ψ at ground level

Common Ionization Constants

Solutei valueDissociationType
Sucrose, Glucose1No dissociationNon-electrolyte
NaCl, KCl2→ Na⁺ + Cl⁻Strong electrolyte
CaCl₂, MgCl₂3→ Ca²⁺ + 2Cl⁻Strong electrolyte
Na₂SO₄, K₂SO₄3→ 2Na⁺ + SO₄²⁻Strong electrolyte
AlCl₃4→ Al³⁺ + 3Cl⁻Strong electrolyte

Note: Actual i values may be slightly lower due to ion pairing at high concentrations.

Water Movement Direction

Water flows from HIGH Ψ → LOW Ψ
(Down the water potential gradient)
High Ψ
More water available
→
Low Ψ
Less water available
🌱 Hypotonic Environment

External Ψ > Cell Ψ

Water enters cell → Cell swells (turgid)

🥀 Hypertonic Environment

External Ψ < Cell Ψ

Water leaves cell → Cell shrinks (plasmolysis)

Typical Water Potential Values

SystemΨ (MPa)Description
Pure Water0Reference point
Moist Soil-0.01 to -0.3Well-watered conditions
Plant Root Cells-0.3 to -0.8Absorbs from soil
Leaf Cells-0.5 to -2.0Drives transpiration
Dry Soil-1.5 to -6.0Plant stress begins
Atmosphere (50% RH)~-100Very negative

Frequently Asked Questions

What is water potential?
Water potential (Ψ, psi) is a measure of the potential energy of water in a system compared to pure water. It determines the direction of water movement - water flows from high to low water potential. In plant biology, water potential is crucial for understanding osmosis, turgor pressure, and plant water relations. Pure water has Ψ = 0 MPa.
How do you calculate water potential?
Water potential is calculated using Ψ = Ψs + Ψp, where Ψs is solute potential and Ψp is pressure potential. Solute potential is calculated as Ψs = -iCRT, where i is the ionization constant, C is molarity, R is the gas constant (0.00831 L·MPa/mol·K), and T is temperature in Kelvin. Adding dissolved solutes always lowers (makes more negative) the water potential.
What is the ionization constant (i)?
The ionization constant (van't Hoff factor, i) represents how many particles a solute produces when dissolved. Non-electrolytes like sucrose have i = 1 (don't dissociate). NaCl has i = 2 (dissociates into Na⁺ and Cl⁻). CaCl₂ has i = 3 (Ca²⁺ and 2Cl⁻). Higher i values create more negative solute potential for the same molarity.
What is pressure potential?
Pressure potential (Ψp) is the physical pressure exerted on water. In plant cells, it's the turgor pressure from the cell wall pushing against the cell contents. Ψp is usually positive in turgid plant cells (0.3-1.0 MPa), zero in flaccid cells or open containers, and can be negative in xylem vessels under tension.
How does water move in plants?
Water moves from regions of higher water potential to lower water potential (down the water potential gradient). In plants: soil (highest Ψ) → root cells → xylem → leaf cells → atmosphere (lowest Ψ, very negative). This gradient drives the transpiration stream that pulls water up through the plant.
What happens when water potential equals zero?
When Ψ = 0, the system is at the same potential as pure water. In plant cells, this typically means Ψs (negative) + Ψp (positive) = 0, so the cell is turgid with pressure potential balancing solute potential. If a cell is placed in pure water, it will gain water until equilibrium or until the cell wall prevents further expansion.