Transpiration Rate Calculator
Enter water loss, leaf area, and time to compute the transpiration rate (g·m⁻²·h⁻¹) and convert it to mmol·m⁻²·s⁻¹, quantifying leaf water loss intensity.
Input Data
Results
At a glance:Transpiration rate is the amount of water a plant loses per unit leaf area per unit time, quantifying the strength of transpiration. Transpiration is water taken up by roots, moved upward through xylem, and finally lost mainly as vapor through leaf stomata to the atmosphere; the resulting 'transpirational pull' is the main driving force for long-distance upward transport of water and minerals, and evaporative cooling protects leaves from heat. Experimentally, rate = water loss ÷ (leaf area × time): measure weight loss over a period by weighing (potometer or whole-pot weighing), divide by the transpiring leaf area and time to get g·m⁻²·h⁻¹. Plant physiology more often uses the molar unit mmol·m⁻²·s⁻¹ — divide g·m⁻²·h⁻¹ by water molar mass 18 g/mol and convert per hour (3600 s). Transpiration rate is affected by light, temperature, air humidity (vapor-pressure deficit), wind, and soil water: strong light, high temperature, dry air, and wind open stomata and raise evaporative drive, increasing rate; water deficit or high humidity closes stomata, lowering it. Thus it is an important index for assessing plant water relations and varietal drought tolerance.
Formula
Transpiration rate: rate(g·m⁻²·h⁻¹) = loss(g) ÷ (area(m²) × time(h)).
Molar conversion: rate(mmol·m⁻²·s⁻¹) = rate(g·m⁻²·h⁻¹) ÷ 18 × 1000 ÷ 3600.
Water molar mass 18 g/mol; 3600 s per hour.
$$E_{(g\,m^{-2}\,h^{-1})} = \frac{W_{loss}}{A \times t}$$$$E_{(mmol\,m^{-2}\,s^{-1})} = \frac{E_{(g\,m^{-2}\,h^{-1})} \times 1000}{18 \times 3600}$$How to Use
- Enter the water loss measured over a period (g, e.g. weight loss from weighing).
- Enter the transpiring leaf area (m²) and measurement time (h).
- The tool returns the rate in g·m⁻²·h⁻¹ and converts to mmol·m⁻²·s⁻¹.
Two common units and magnitudes of transpiration rate
| Condition | g·m⁻²·h⁻¹ | mmol·m⁻²·s⁻¹ (approx) |
|---|---|---|
| Low (shade, high humidity) | about 18 | about 0.28 |
| Medium (typical daytime) | about 50 | about 0.77 |
| High (bright, dry, windy) | about 130 | about 2.01 |
| Very high (hot-dry stress) | about 260 | about 4.01 |
Values are indicative magnitudes varying with species, stomatal state, and environment. mmol·m⁻²·s⁻¹ ≈ g·m⁻²·h⁻¹ × 0.01543 (i.e. ÷18×1000÷3600).
Case Studies
Weighing-method leaf transpiration
A potted plant weighed over 2 h lost 5 g, measured leaf area 0.05 m².
Rate = 5 ÷ (0.05 × 2) = 5 ÷ 0.1 = 50 g·m⁻²·h⁻¹.
Molar = 50 ÷ 18 × 1000 ÷ 3600 ≈ 0.77 mmol·m⁻²·s⁻¹, a typical daytime magnitude.
Rises after moving to bright dry conditions
Same plant moved to bright, dry, breezy conditions lost 13 g in 2 h, leaf area still 0.05 m².
Rate = 13 ÷ (0.05 × 2) = 130 g·m⁻²·h⁻¹ ≈ 2.01 mmol·m⁻²·s⁻¹.
About 2.6× the previous rate, reflecting wider stomata and faster water loss as evaporative drive rises.
FAQ
What are the common units of transpiration rate?
Teaching and simple experiments use 'water mass lost per unit leaf area per unit time', e.g. g·m⁻²·h⁻¹ (or g·cm⁻²·h⁻¹); plant physiology uses molar flux mmol·m⁻²·s⁻¹. They convert: take g·m⁻²·h⁻¹, divide by water molar mass 18 g/mol, and convert per hour (3600 s) to per second mmol, i.e. mmol·m⁻²·s⁻¹ ≈ g·m⁻²·h⁻¹ × 1000 ÷ 18 ÷ 3600. This calculator outputs both.
How to measure water loss?
The most common is gravimetric weighing: weigh a whole potted plant or a leaf-on potometer at start and end; the weight loss approximates transpired water (cover soil to exclude soil evaporation). Porometers or infrared gas analyzers directly measure stomatal conductance and transpiration flux. This calculator accepts water loss (g) from any method with its leaf area and time.
How to estimate leaf area?
Small leaves: trace on grid paper and count squares, scan and use image-analysis software, or a dedicated leaf-area meter. In the field use specific leaf area (SLA) or leaf area index (LAI) to convert. Convert to m² (1 m² = 10000 cm²) and count only the leaf area actually transpiring, for a correct per-area rate.
What factors change transpiration rate?
Mainly environment and stomatal state: stronger light opens stomata and speeds transpiration; higher temperature, drier air (larger VPD), and wind raise evaporative drive and speed it; high humidity or calm air lowers it. Under soil water deficit or stress, plants close stomata to conserve water, lowering the rate. Fix other conditions when comparing treatments.
What does transpiration do for the plant?
Three roles: (1) generates transpirational pull, the main driving force lifting water and dissolved minerals from roots; (2) evaporative cooling protects leaves from heat damage; (3) accompanies stomatal opening for gas exchange (CO₂ uptake for photosynthesis). But excessive transpiration causes water deficit, so plants balance CO₂ gain and water conservation via stomatal movement.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.