Last updated: November 19, 2025
Water potential calculator explained
Total water potential (Psi) is the sum of multiple components: osmotic, pressure, matric, gravitational, humidity, and air pressure. This calculator adds whichever components you enter so you can see how a mesophyll cell or soil core reaches its net Psi.
How the conversion works
Water potential is an additive state function:
Each term can be positive or negative. For example, the pressure potential (Psi_p) in turgid cells is positive, while osmotic (Psi_s) and matric (Psi_m) potentials are usually negative.
Units and conversions
| Component | Typical range | Notes |
|---|---|---|
| Osmotic (Psi_s) | -2 to 0 MPa | Driven by solute concentration. |
| Pressure (Psi_p) | -0.5 to +1 MPa | Positive inside turgid cells, negative in xylem tension. |
| Matric (Psi_m) | -0.01 to -2 MPa | Soil texture and water films. |
| Gravitational (Psi_g) | +/-0.01 MPa per meter | Important in tall trees. |
| Humidity/Vapor (Psi_h) | negative at low RH | Vapor pressure deficit effect. |
| Air (Psi_a) | near 0 MPa | Deviations occur in pressurized chambers. |
You can enter values in MPa, kPa, or bar; the calculator converts and sums them consistently.
Worked examples
- Turgid mesophyll cell
Psi_s = -0.8 MPa, Psi_p = 0.6 MPa, Psi_m = -0.1 MPa.
- Soil profile with gravity term
Dry soil with Psi_s = -0.05 MPa, Psi_m = -1.2 MPa, plus -0.1 MPa gravitational effect 10 m above ground.
Tips and pitfalls
- Keep units consistent; MPa is standard in plant physiology (1 MPa approx 10 bar).
- Matric potential dominates in dry soils; do not ignore Psi_m when modeling root uptake under drought.
- Positive pressure potential occurs in living cells but not in xylem conduits under tension.
- Use the gravitational term for tall crops or trees when modeling water columns.