Water Potential Calculator
Calculate water potential (psi) from osmotic potential and pressure potential components. Essential for plant physiology, soil science, and agricultural irrigation management. Includes solute concentration inputs.
Water Potential Calculator Tool
Why use this free water potential calculator?
Built with the features most competitors miss — deeper inputs, benchmark data, and actionable guidance alongside the core calculation.
How to use this water potential calculator
Water potential reference values
| System | Typical water potential | Notes |
|---|---|---|
| Pure water | 0 MPa | Reference point |
| Well-watered plant leaf | -0.1 to -0.3 MPa | High water status |
| Field capacity soil | -0.033 MPa | Optimal soil moisture |
| Plant at mild stress | -0.5 to -1.0 MPa | Reduced growth |
| Permanent wilting point | -1.5 MPa | Most crops cannot recover |
| 0.3 M sucrose (25C) | -0.74 MPa | Van't Hoff calculation |
| Seawater | ~-2.7 MPa | Too low for most crops |
How this calculator compares
LazyTools fills the gaps most competing tools leave open — deeper analysis, benchmark context, and actionable guidance alongside the core calculation.
| Feature | LazyTools | OmniCalculator | Bioninja.com | Biology textbook tools |
|---|---|---|---|---|
| 3 calculation modes | ✓ Yes | ✓ | Partial | ✗ |
| Van't Hoff osmotic calc | ✓ Yes | ✓ | ✓ | ✗ |
| Plant stress classification | ✓ Yes | ✗ | ✗ | ✗ |
| Ionisation constant presets | ✓ Yes | ✓ | ✗ | ✗ |
| Flow direction output | ✓ Yes | ✗ | ✗ | ✗ |
| Free, no registration | ✓ Yes | ✓ | ✓ | ✓ |
Water Potential Calculator: Complete Guide
Water potential is the fundamental concept governing water movement in plants, soil, and biological systems. It integrates the effects of solute concentration (osmotic potential) and physical pressure (pressure potential) into a single thermodynamic quantity that predicts the direction and magnitude of water flow.
The water potential equation
Total water potential (psi) = Osmotic potential (psi_s) + Pressure potential (psi_p). Pure water has psi = 0 MPa. Osmotic potential is always negative because dissolved solutes reduce the free energy of water molecules. Pressure potential is positive in turgid cells (turgor pressure) and negative in xylem under tension (tension pressure). The sum determines the total driving force for water movement.
The van't Hoff equation for osmotic potential
Psi_s = -iCRT. R = 0.00831 L MPa/(mol K) is the gas constant in appropriate units. T is temperature in Kelvin (C + 273.15). Example: 0.2 M sucrose (i=1) at 25C: psi_s = -(1 x 0.2 x 0.00831 x 298.15) = -0.495 MPa. Example: 0.2 M NaCl (i=2) at 25C: psi_s = -(2 x 0.2 x 0.00831 x 298.15) = -0.990 MPa. Note how a 1:1 salt at the same molar concentration produces twice the osmotic effect as a non-electrolyte.
Water potential in plant physiology
Water enters roots from soil because root cell water potential is lower (more negative) than soil water potential. It moves up the xylem driven by the tension created by transpiration at leaf surfaces. In leaves, water evaporates from cell walls into air spaces, creating a water potential gradient that pulls water up the plant. This process (the cohesion-tension mechanism) explains how trees can lift water to heights of 100 metres or more without a pump.
Soil water potential and plant water status
Field capacity (soil at optimal moisture after drainage): approximately -0.033 MPa. Permanent wilting point: approximately -1.5 MPa. Plant-available water is between these two values. When soil water potential falls below the plant's leaf water potential, water can no longer enter the plant even if it is present in the soil. Understanding this gradient explains why plants on certain soil types wilt faster despite the same water content.