Water-Potential Components Need the Same Units, Datum and Sign Convention; illustrative quantitative diagram
Define the quantity before calculating
A water-potential sum combines components only when they use compatible units and reference conventions. A gravitational height needs a stated datum. An unqualified positive number for a component normally treated as negative can change the meaning of the total.
Use Water Potential Calculator with the units and the basis stated on its form. Check what each input represents before carrying the output into another record.
Read the model explicitly
Ideal osmotic term = −iCRT, R = 0.008314462618 L·MPa/(mol·K). Gravity ≈ 0.00980665 MPa per meter; sum with entered pressure and matric components.
The Gardening and crops calculators group sits within Biology calculators. Keep a measured observation distinguishable from a hypothetical input when moving between its tools.
A worked unit check
| Input as labeled on the tool | Illustrative value |
|---|---|
| Pressure component (MPa) | 0 |
| Matric component (MPa; zero or negative) | 0 |
| Height relative to chosen datum (m) | 0 |
| Ideal solute concentration (mol/L) | 0.1 |
| van ’t Hoff factor for the model | 1 |
| Solution temperature (°C) | 25 |
| Output from the illustrative inputs | Value | Unit |
|---|---|---|
| Ideal osmotic component | -0.247896 | MPa |
| Gravitational component | 0 | MPa |
| Sum of entered / modeled components | -0.247896 | MPa |
Record the original input and its unit with this result. Recalculate when the measurement basis changes; changing a unit label after calculation does not convert the quantity. More decimal places do not remove sampling or measurement uncertainty.
A related quantity
VPD Calculator (Vapor Pressure Deficit) addresses a related part of the task. Check its own input definition before carrying a number across.
Continue with Ideal Osmotic Potential Is a Model, Not a Measurement of Soil Water Status for the other part of this measurement question.
Return to GreenCalc for other subjects. Keep raw observations, chosen assumptions and calculated quantities identifiable so a later revision does not overwrite the basis of the original record.
By GreenCalc · Updated 2026-11-06