TECHNICAL TOOL / 05

VPD Calculator and Leaf Vapor Pressure Deficit Chart

Use air temperature, RH and leaf offset to check the vapour-pressure difference between leaf and air.

RESULTS

Leaf VPD: 0.97 kPa
Stage reference: Vegetative / early flowering reference
Air VPD: 1.34 kPa
Leaf temperature: 24.0 °C

Enable JavaScript to calculate and select chart cells. Current results and the static chart show the example 26°C, 60% RH, −2°C leaf offset. Use the formulas below for manual calculation.

Interactive VPD chart

Select a cell to fill air temperature and RH. Changing leaf offset recalculates every cell; ● marks your current inputs. Swipe the table horizontally on mobile.

Propagation / early vegetative: 0.4–0.8 kPaVegetative / early flowering: 0.8–1.2 kPaMid / late flowering: 1.2–1.6 kPa

These are common reference ranges, not crop setpoints. Stages overlap; the growing team should confirm targets for cultivar, lighting and crop response. Values <0 indicate possible leaf condensation; 0–0.4 and >1.6 kPa are outside the stage bands. Source: VPD Chart — stage reference bands.

Static example: leaf offset −2°C; leaf VPD (kPa)
°C / RH	30%	35%	40%	45%	50%	55%	60%	65%	70%	75%	80%	85%	90%
16	1.05	0.96	0.87	0.78	0.69	0.60	0.51	0.42	0.33	0.23	0.14	0.05	-0.04
18	1.20	1.10	0.99	0.89	0.79	0.68	0.58	0.48	0.37	0.27	0.17	0.06	-0.04
20	1.36	1.25	1.13	1.01	0.89	0.78	0.66	0.54	0.43	0.31	0.19	0.08	-0.04
22	1.55	1.41	1.28	1.15	1.02	0.88	0.75	0.62	0.49	0.36	0.22	0.09	-0.04
24	1.75	1.60	1.45	1.30	1.15	1.00	0.85	0.70	0.56	0.41	0.26	0.11	-0.04
26	1.98	1.81	1.64	1.47	1.30	1.14	● 0.97	0.80	0.63	0.46	0.29	0.13	-0.04
28	2.23	2.04	1.85	1.66	1.47	1.28	1.09	0.90	0.72	0.53	0.34	0.15	-0.04
30	2.51	2.29	2.08	1.87	1.66	1.45	1.23	1.02	0.81	0.60	0.39	0.17	-0.04
32	2.82	2.58	2.34	2.10	1.87	1.63	1.39	1.15	0.91	0.68	0.44	0.20	-0.04
34	3.16	2.89	2.63	2.36	2.10	1.83	1.56	1.30	1.03	0.77	0.50	0.23	-0.03
36	3.54	3.24	2.94	2.65	2.35	2.05	1.75	1.46	1.16	0.86	0.57	0.27	-0.03

What is VPD?

Air VPD is saturation vapour pressure at air temperature minus actual vapour pressure. Leaf VPD uses saturation pressure at leaf temperature, then subtracts actual pressure in the surrounding air. Results are in kPa.

Formula and worked example

es(T) = 0.6108 × exp[17.27T / (T + 237.3)] kPa

ea = es(Tair) × RH/100; VPDleaf = es(Tair + ΔT) − ea

VPDair = es(Tair) − ea

The saturation-pressure equation follows FAO-56, Chapter 3, equation 11. Use °C inside the formula. FAO examples give 3.075 kPa at 24.5°C and 1.705 kPa at 15°C. This tool is limited to non-frozen growing conditions.

At 26°C, 60% RH and a 24°C leaf: air VPD is about 1.34 kPa; leaf VPD is about 0.97 kPa. Leaf temperature changes the interpretation, so keep the two results distinct.

How to measure leaf temperature

Use an infrared thermometer or thermal camera suited to plant surfaces. Follow its emissivity and distance instructions and keep the measurement spot on the leaf. Sample representative canopy leaves alongside shaded air-temperature and RH readings, including lights-on, lights-off and post-irrigation conditions. Do not substitute fixture, floor or wall temperatures.

High humidity and dehumidification capacity

Low leaf VPD means a small evaporative driving force; a negative value indicates possible leaf condensation at these inputs. Disease risk also depends on wetness duration, airflow and crop conditions. VPD is not a moisture-removal rate: kPa cannot be converted directly to L/day. Review irrigation and drainage, moisture-generation timing, HVAC removal, air exchange and equipment performance at actual conditions.