VPD Heatmap: Temperature Meets Humidity
One chart shows why the same humidity feels different at 18 °C and at 30 °C. Move the marker to your room and read the drying demand.
Vapour pressure deficit (VPD) combines temperature and humidity into one number: how strongly the air pulls water from a leaf. Drag the sliders to your room, or hover the map to read its value.
At these conditions
1.34kPa
Bands are common working ranges, not a prescription. Air movement, leaf temperature and the plant’s own stomata decide how much water it actually loses.
How it works
Saturation vapour pressure is calculated with the Tetens form used in plant science: eₛ = 0.6108 × exp(17.27T ÷ (T + 237.3)), in kPa with T in °C. Actual vapour pressure is that value multiplied by relative humidity over 100, and VPD is the gap between the two.
The grid runs from 12 °C to 34 °C across and 20% to 95% relative humidity up. Each point is coloured by its band, and the ring marks your room. Hover or tap the map to read its value; the marker stays on your slider settings so you can compare.
| Band | VPD | What it usually means indoors |
|---|---|---|
| Very low | under 0.2 kPa | Still, humid air; condensation risk on cool glass |
| Low | 0.2–0.4 kPa | Gentle demand, common in cool damp rooms |
| Gentle | 0.4–0.8 kPa | Comfortable for many tropical houseplants |
| Active | 0.8–1.2 kPa | Good airflow and active growth |
| High | 1.2–1.6 kPa | Mix dries faster; watch small pots |
| Very high | 1.6–2.5 kPa | Heated winter rooms; stress risk for thin leaves |
| Extreme | above 2.5 kPa | Very dry air; misting will not fix it |
These are air values. A leaf in bright light can run warmer than the room, which raises its own VPD; a leaf cooled by transpiration can run cooler. That is why the same room number can feel different to a plant under a lamp.
Frequently asked questions
What is VPD in plain terms?
The difference between how much water the air could hold at that temperature and how much it actually holds. A bigger gap means stronger drying demand.
What VPD should I aim for?
There is no single target for every houseplant. Many tropical foliage plants grow well somewhere around 0.4–1.2 kPa, but succulents tolerate drier air and ferns prefer the lower end. Use the chart to compare conditions rather than to chase a number.
Why is VPD so high in winter?
Heating raises the temperature without adding water, so relative humidity falls and the saturation gap grows. A room at 24 °C and 30% RH sits far to the right of a summer room at the same temperature and 60% RH.
Does a high VPD mean I should water more often?
Not automatically. It means the air is pulling harder, but the pot, the mix and the roots decide how fast water disappears. Check the soil before watering.
Why is leaf VPD different from air VPD?
The leaf has its own temperature. In strong light it can be several degrees warmer than the air, which raises the deficit at the leaf surface even when the room reading is unchanged.