Relative Water Content Calculator
Enter fresh weight, dry weight, and turgid (saturated) weight to compute relative water content RWC = (Fresh − Dry) ÷ (Turgid − Dry) × 100%, quantifying leaf water deficit and drought tolerance.
Input Data
Results
At a glance:Leaf relative water content (RWC) is the most practical and widespread index of plant water status, answering 'what fraction of this leaf's fully saturated water does it currently hold?'. Defined as RWC = (Fresh weight FW − Dry weight DW) ÷ (Turgid weight TW − Dry weight DW) × 100%. The three weights each have a clear operational definition: fresh weight (FW) is the leaf weight weighed immediately after sampling, the actual current water state; turgid weight (TW, also saturated weight) is the leaf floated on water (or petiole in water) in low light and cool conditions for several hours until fully turgid, then blotted and weighed — the 'fully watered' reference point; dry weight (DW) is the same sample oven-dried to constant weight at about 70–80°C. The numerator (FW − DW) is the water currently held, the denominator (TW − DW) is the maximum water the leaf can hold when saturated; their ratio is the water fraction relative to saturation. The closer RWC to 100%, the closer to saturation, higher turgor, better physiology; when drought, root water uptake cannot keep up with transpiration, or VPD is high, water deficit deepens and RWC falls. Most crops show clear inhibition of photosynthesis, stomatal conductance, and growth when RWC drops to about 70–80%, and severe, even irreversible damage below that; drought-tolerant varieties' advantage is partly maintaining function at lower RWC. Compared with simple 'moisture content' (water as a fraction of fresh or dry weight), RWC uses saturation as a common baseline, allowing comparison of deficit across leaf ages and species, making it the core measurement in drought breeding, water-stress physiology, and irrigation decisions. Note the three weights must come from the same sample, avoid over-saturation or hypoxia when saturating, dry to constant weight, and turgid weight must exceed dry weight or the formula is invalid.
Formula
Relative water content: RWC = (Fresh − Dry) ÷ (Turgid − Dry) × 100%.
Numerator = current water (FW − DW); denominator = max saturated water (TW − DW).
Condition: turgid weight TW must be greater than dry weight DW.
$$RWC = \frac{FW - DW}{TW - DW} \times 100\%$$How to Use
- Weigh the leaf fresh weight FW immediately after sampling.
- Float the leaf until fully turgid, blot, and weigh turgid weight TW.
- Oven-dry the same sample to constant weight, weigh dry weight DW; the tool returns RWC; closer to 100% means more water.
General interpretation of RWC (relative; varies by species)
| RWC range | Water status | Physiological performance |
|---|---|---|
| 90 – 100% | Adequate | High turgor, open stomata, normal photosynthesis |
| 80 – 90% | Mild deficit | Most crops near normal; sensitive ones slightly affected |
| 70 – 80% | Moderate deficit | Stomata close; photosynthesis and growth clearly drop |
| < 70% | Severe deficit | Wilting, metabolic damage, possible irreversible harm |
Thresholds vary greatly with crop drought tolerance; tolerant varieties maintain function at lower RWC. Values are for relative comparison only.
Case Studies
RWC from three weights
A leaf: fresh FW = 2.5 g, turgid TW = 3.0 g, dry DW = 0.5 g.
RWC = (2.5 − 0.5) ÷ (3.0 − 0.5) × 100% = 2.0 ÷ 2.5 × 100% = 80%.
A mild-to-moderate deficit; sensitive crops should be watered and compared with post-irrigation samples.
RWC drops under drought treatment
Well-watered leaf: FW = 2.8, TW = 3.0, DW = 0.5 → RWC = (2.3 ÷ 2.5) × 100% = 92%.
After several days without irrigation, same plant: FW = 2.15, TW = 3.0, DW = 0.5 → RWC = (1.65 ÷ 2.5) × 100% = 66%.
RWC fell from 92% to 66%, severe deficit; stomata mostly closed, photosynthesis greatly reduced — a signal to irrigate promptly.
FAQ
How to measure turgid weight TW accurately?
Float the fresh leaf or leaf disc on distilled water (or stand the petiole in water) in low light and cool conditions to take up water, usually 4–6 hours or until weight stops increasing; blot the surface with tissue paper before weighing. Too short under-saturates and underestimates TW; too long or anaerobic may damage tissue and give abnormal weight. Use consistent floating time and conditions across the batch for fair comparison.
How does RWC differ from moisture content?
Moisture content is usually water as a fraction of fresh or dry weight, not referenced to 'saturation', and is hard to compare across samples because leaf thickness and age differ. RWC uses saturated weight as a common baseline, measuring 'of the water it can hold when fully saturated, how much is left now' — directly reflecting deficit degree, so it is more meaningful than moisture content in water-stress research and drought screening.
How low is RWC before it is 'severely short of water'?
No universal hard threshold; it varies by crop. Empirically most crops are normal above 80% RWC, begin stomatal closure and growth inhibition at 70–80%, and are in severe stress below 70%. Drought-tolerant varieties hold function at lower RWC. Interpret by comparing with the same species' well-watered control rather than the absolute value alone.
Why is the denominator (Turgid − Dry) not just Turgid?
Because we compare the amount of water, not the total weight including dry matter. Dry weight is the leaf's dry biomass, unchanged by hydration; (Turgid − Dry) is the maximum water the leaf holds when saturated, and (Fresh − Dry) is current water. Subtracting dry matter from both gives the pure 'relative saturation of water'.
What are the sampling precautions?
Take mature leaves of the same position and similar age, free of disease spots, and sample at a consistent time (e.g. morning) to reduce within-day variation; weigh fresh weight promptly after picking to avoid water loss; all three weights must come from the same sample. For batch comparison fix the saturation time, drying temperature, and constant-weight criterion, and ensure turgid weight exceeds dry weight, otherwise the result is invalid.
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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.