Water Potential Calculator
Calculate Ψw, Ψs, and Ψp for biology and environmental science applications
How It Works
What is Water Potential?
Water potential (Ψw) is a measure of the potential energy of water per unit volume relative to pure water in reference conditions. It determines the direction and rate of water movement in plants, soils, and cells.
Main Components of Water Potential
Total water potential is the sum of its main components:
Where:
- Ψp = Pressure potential (physical pressure)
- Ψs = Solute potential (osmotic potential)
- Ψg = Gravitational potential (usually negligible in cells)
- Ψm = Matric potential (important in soils, negligible in cells)
For most cellular applications, we simplify to:
Solute Potential Formula
Solute potential (Ψs) is calculated using the formula:
Where:
- i = Ionization constant (number of particles formed per molecule)
- M = Molar concentration (mol/L)
- R = Universal gas constant (0.0831 liter·bar/mol·K)
- T = Temperature in Kelvin (K = °C + 273)
Example Problem
A plant cell has a pressure potential (Ψp) of 0.45 MPa and a solute potential (Ψs) of -0.35 MPa. What is the total water potential?
Ψw = 0.45 MPa + (-0.35 MPa) = 0.10 MPa
This means the cell’s water potential is 0.10 MPa, which would cause water to move into the cell if placed in pure water (Ψw = 0).
Units Used in Water Potential
Water potential is typically measured in:
- Megapascals (MPa) – most common in plant physiology (1 MPa = 10 bar)
- Kilopascals (kPa) – 1 MPa = 1000 kPa
- Bars – older unit but still used (1 bar = 0.1 MPa)
- Atmospheres (atm) – 1 atm ≈ 0.101 MPa
Why is Solute Potential Negative?
Solute potential (Ψs) is always negative or zero because:
- It represents the reduction in water potential due to dissolved solutes
- Pure water has Ψs = 0 (maximum potential)
- Adding solutes reduces the free energy of water molecules
- The negative sign indicates water moves from higher (less negative) to lower (more negative) water potential
Frequently Asked Questions
Water potential (Ψw) is a measure of the potential energy of water per unit volume relative to pure water in reference conditions. It determines the direction and rate of water movement in biological systems. Water always moves from areas of higher (less negative) water potential to areas of lower (more negative) water potential.
To calculate solute potential (Ψs), use the formula: Ψs = –i × M × R × T, where i is the ionization constant, M is molar concentration, R is the gas constant (0.0831 L·bar/mol·K), and T is temperature in Kelvin.
To calculate total water potential (Ψw), add the pressure potential (Ψp) and solute potential (Ψs): Ψw = Ψp + Ψs.
Our calculator automates these calculations in both basic and advanced modes.
Solute potential is negative because dissolved solutes reduce the free energy of water molecules compared to pure water (which has Ψs = 0). The more solutes present, the more negative the solute potential becomes. This negative value indicates that water will move from areas of less negative (higher) water potential to more negative (lower) water potential.
The most common units for water potential are:
- Megapascals (MPa) – standard in plant physiology (1 MPa = 10 bar)
- Kilopascals (kPa) – 1 MPa = 1000 kPa
- Bars – older unit but still used (1 bar = 0.1 MPa)
- Atmospheres (atm) – 1 atm ≈ 0.101 MPa
Our calculator allows you to select your preferred unit for results.
High water potential (less negative): Indicates cells or tissues are well-hydrated. In soil, high Ψw means water is readily available to plants.
Low water potential (more negative): Indicates water stress. Cells may be flaccid (low Ψp) or have high solute concentration (low Ψs). In soil, low Ψw means water is less available to plants.
Water always moves from high (less negative) to low (more negative) Ψw, driving water uptake in roots and transpiration in leaves.