Printable

Dew Point Chart

This dew point chart lists the dew point for every air temperature and relative humidity, in °F and °C. Print it or save it as a PDF to keep beside your hygrometer.

Dew point chart (°C)Dew point in °C. The outlined cell is closest to your conditions.vpdcalculator.com

Air ↓ RH →	30%	35%	40%	45%	50%	55%	60%	65%	70%	75%	80%	85%	90%	95%
10°C 50°F	−6.8	−4.7	−3.0	−1.4	0.1	1.4	2.6	3.7	4.8	5.8	6.7	7.6	8.4	9.2
11°C 52°F	−5.9	−3.8	−2.1	−0.5	1.0	2.3	3.5	4.7	5.7	6.7	7.7	8.6	9.4	10.2
12°C 54°F	−5.0	−3.0	−1.2	0.5	1.9	3.3	4.5	5.6	6.7	7.7	8.7	9.6	10.4	11.2
13°C 55°F	−4.1	−2.1	−0.3	1.4	2.8	4.2	5.4	6.6	7.7	8.7	9.6	10.5	11.4	12.2
14°C 57°F	−3.3	−1.2	0.6	2.3	3.8	5.1	6.4	7.5	8.6	9.6	10.6	11.5	12.4	13.2
15°C 59°F	−2.4	−0.3	1.5	3.2	4.7	6.0	7.3	8.5	9.6	10.6	11.6	12.5	13.4	14.2
16°C 61°F	−1.5	0.6	2.4	4.1	5.6	7.0	8.2	9.4	10.5	11.6	12.6	13.5	14.4	15.2
17°C 63°F	−0.7	1.4	3.3	5.0	6.5	7.9	9.2	10.4	11.5	12.5	13.5	14.5	15.3	16.2
18°C 64°F	0.2	2.3	4.2	5.9	7.4	8.8	10.1	11.3	12.5	13.5	14.5	15.4	16.3	17.2
19°C 66°F	1.1	3.2	5.1	6.8	8.4	9.8	11.1	12.3	13.4	14.5	15.5	16.4	17.3	18.2
20°C 68°F	1.9	4.1	6.0	7.7	9.3	10.7	12.0	13.2	14.4	15.4	16.4	17.4	18.3	19.2
21°C 70°F	2.8	5.0	6.9	8.6	10.2	11.6	12.9	14.2	15.3	16.4	17.4	18.4	19.3	20.2
22°C 72°F	3.6	5.8	7.8	9.5	11.1	12.5	13.9	15.1	16.3	17.4	18.4	19.4	20.3	21.2
23°C 73°F	4.5	6.7	8.7	10.4	12.0	13.5	14.8	16.1	17.2	18.3	19.4	20.3	21.3	22.2
24°C 75°F	5.4	7.6	9.6	11.3	12.9	14.4	15.8	17.0	18.2	19.3	20.3	21.3	22.3	23.1
25°C 77°F	6.2	8.5	10.5	12.2	13.9	15.3	16.7	18.0	19.1	20.3	21.3	22.3	23.2	24.1
26°C 79°F	7.1	9.4	11.4	13.2	14.8	16.3	17.6	18.9	20.1	21.2	22.3	23.3	24.2	25.1
27°C 81°F	8.0	10.2	12.3	14.1	15.7	17.2	18.6	19.9	21.1	22.2	23.3	24.3	25.2	26.1
28°C 82°F	8.8	11.1	13.1	15.0	16.6	18.1	19.5	20.8	22.0	23.1	24.2	25.2	26.2	27.1
29°C 84°F	9.7	12.0	14.0	15.9	17.5	19.0	20.4	21.7	23.0	24.1	25.2	26.2	27.2	28.1
30°C 86°F	10.5	12.9	14.9	16.8	18.4	20.0	21.4	22.7	23.9	25.1	26.2	27.2	28.2	29.1
31°C 88°F	11.4	13.7	15.8	17.7	19.4	20.9	22.3	23.6	24.9	26.0	27.1	28.2	29.2	30.1
32°C 90°F	12.3	14.6	16.7	18.6	20.3	21.8	23.3	24.6	25.8	27.0	28.1	29.2	30.1	31.1
33°C 91°F	13.1	15.5	17.6	19.5	21.2	22.7	24.2	25.5	26.8	28.0	29.1	30.1	31.1	32.1
34°C 93°F	14.0	16.4	18.5	20.4	22.1	23.7	25.1	26.5	27.7	28.9	30.1	31.1	32.1	33.1
35°C 95°F	14.8	17.2	19.4	21.3	23.0	24.6	26.1	27.4	28.7	29.9	31.0	32.1	33.1	34.1

Dew point chart (°F)Dew point in °F. The outlined cell is closest to your conditions.vpdcalculator.com

Air ↓ RH →	30%	35%	40%	45%	50%	55%	60%	65%	70%	75%	80%	85%	90%	95%
50°F 10°C	19.8	23.5	26.7	29.5	32.1	34.5	36.7	38.7	40.6	42.4	44.1	45.7	47.2	48.6
52°F 11°C	21.6	25.2	28.5	31.4	34.0	36.4	38.6	40.6	42.5	44.3	46.0	47.6	49.2	50.6
54°F 12°C	23.3	27.0	30.3	33.2	35.8	38.2	40.5	42.5	44.5	46.3	48.0	49.6	51.1	52.6
56°F 13°C	25.1	28.8	32.1	35.0	37.7	40.1	42.3	44.4	46.4	48.2	49.9	51.6	53.1	54.6
58°F 14°C	26.8	30.5	33.9	36.8	39.5	41.9	44.2	46.3	48.3	50.1	51.9	53.5	55.1	56.6
60°F 16°C	28.5	32.3	35.6	38.6	41.3	43.8	46.1	48.2	50.2	52.1	53.8	55.5	57.1	58.6
62°F 17°C	30.3	34.1	37.4	40.4	43.2	45.7	48.0	50.1	52.1	54.0	55.8	57.4	59.0	60.6
64°F 18°C	32.0	35.8	39.2	42.3	45.0	47.5	49.9	52.0	54.0	55.9	57.7	59.4	61.0	62.5
66°F 19°C	33.7	37.6	41.0	44.1	46.8	49.4	51.7	53.9	55.9	57.9	59.7	61.4	63.0	64.5
68°F 20°C	35.5	39.4	42.8	45.9	48.7	51.2	53.6	55.8	57.9	59.8	61.6	63.3	65.0	66.5
70°F 21°C	37.2	41.1	44.6	47.7	50.5	53.1	55.5	57.7	59.8	61.7	63.5	65.3	66.9	68.5
72°F 22°C	38.9	42.9	46.4	49.5	52.4	55.0	57.4	59.6	61.7	63.6	65.5	67.2	68.9	70.5
74°F 23°C	40.6	44.6	48.2	51.3	54.2	56.8	59.2	61.5	63.6	65.6	67.4	69.2	70.9	72.5
76°F 24°C	42.4	46.4	50.0	53.1	56.0	58.7	61.1	63.4	65.5	67.5	69.4	71.2	72.9	74.5
78°F 26°C	44.1	48.2	51.7	54.9	57.9	60.5	63.0	65.3	67.4	69.4	71.3	73.1	74.8	76.4
80°F 27°C	45.8	49.9	53.5	56.8	59.7	62.4	64.9	67.2	69.3	71.4	73.3	75.1	76.8	78.4
82°F 28°C	47.5	51.7	55.3	58.6	61.5	64.2	66.7	69.1	71.2	73.3	75.2	77.0	78.8	80.4
84°F 29°C	49.2	53.4	57.1	60.4	63.4	66.1	68.6	71.0	73.2	75.2	77.2	79.0	80.7	82.4
86°F 30°C	51.0	55.2	58.9	62.2	65.2	67.9	70.5	72.9	75.1	77.1	79.1	81.0	82.7	84.4
88°F 31°C	52.7	56.9	60.6	64.0	67.0	69.8	72.4	74.7	77.0	79.1	81.0	82.9	84.7	86.4
90°F 32°C	54.4	58.7	62.4	65.8	68.8	71.6	74.2	76.6	78.9	81.0	83.0	84.9	86.7	88.4
92°F 33°C	56.1	60.4	64.2	67.6	70.7	73.5	76.1	78.5	80.8	82.9	84.9	86.8	88.6	90.4
94°F 34°C	57.8	62.2	66.0	69.4	72.5	75.3	78.0	80.4	82.7	84.8	86.9	88.8	90.6	92.3

How a dew point feels

The US National Weather Service’s scale for outdoor comfort. In a grow room the surface margin above matters more.

55 °F (13 °C) or lower
Dry and comfortable
55–65 °F (13–18 °C)
Sticky, muggy evenings
65 °F (18 °C) or higher
Oppressive

Using the chart

Compare the dew point with your coldest surface. If a wall, window or leaf can fall to that temperature, water condenses on it, and mould and leaf diseases follow. Keep a few degrees of margin, especially at lights-off when the room cools.

For one exact number, and the margin for a surface you measured, use the dew point calculator.

Formula (Magnus, same constants as Tetens)

γ  = ln(RH/100) + 17.27·T / (T + 237.3)
Td = 237.3·γ / (17.27 − γ)

Using a dew point chart in the grow room

Each row is an air temperature, 10–35 °C on the Celsius table or 50–94 °F on the Fahrenheit table, and each column is a relative humidity from 30% to 95%. The number where they meet is the dew point: the temperature at which water starts to condense out of that air.

A worked example: a room at 26 °C (78.8 °F) and 60% RH has a dew point of 17.6 °C (63.7 °F). On the Fahrenheit table, 80 °F at 60% gives 64.9 °F. Any leaf, wall or window that cools to that temperature gets wet.

Enter your coldest surface under “Colour the cells for a surface at” and the chart does the comparison for you. Cells turn red where that surface gets wet, and amber where it is within 2 °C (3.6 °F) of the dew point. Your own conditions are outlined. For one exact number, or to work out humidity from a dew point, use the dew point calculator.

Humidity dew point chart: reading across a row

Move along a row and you can see how much humidity matters. At 26 °C the dew point is 14.8 °C (58.6 °F) at 50% RH, 17.6 °C at 60% and 20.1 °C (68.2 °F) at 70%. Each 10% of humidity raises it by roughly 2.5–3 °C here, bringing condensation that much closer to your coldest surface.

Moving down a column is different. When the room cools, the water in the air does not change, so the dew point stays put while humidity climbs. That room at 26 °C and 60% reaches about 81% when it cools to 21 °C at lights-off, and its dew point is still 17.6 °C. To check lights-off, find your daytime dew point on the chart, then compare it with your night-time temperature. If the night temperature comes within a couple of degrees of it, expect wet leaves.

Dew point comfort and plant safety

The comfort scale on this page comes from the US National Weather Service and is written for people outdoors: 55 °F (13 °C) or lower feels dry, 55–65 °F (13–18 °C) sticky, and 65 °F (18 °C) or higher oppressive. A room at 26 °C and 60% RH, ideal for veg under LEDs, sits in the sticky band with its 17.6 °C dew point.

Plants do not care how the air feels to you. What matters for them is the gap between the dew point and the coldest surface. A dew point of 20 °C is fine if every surface stays above 24 °C, and a problem if leaves cool to 21 °C at night. Keep the margin in view, not the comfort label.

Printing the dew point chart PDF

Press Print / Save PDF and choose Save as PDF in the print dialog to keep a file. The menu and controls are left out, and the Celsius and Fahrenheit tables each print on their own landscape page. Choose °C or °F under Tables to print just one, or download either table as a PNG.

Reading the wrong table. Check the heading says °C or °F before you read a number off a printed sheet.
Off the chart. Conditions outside 10–35 °C (50–94 °F) or 30–95% RH are not outlined. Use the calculator for those.
Readings from a psychrometer. The chart needs relative humidity. The wet bulb calculator turns dry and wet bulb readings into humidity and dew point.
Stopping at the dew point. Plants respond to VPD as well. The VPD chart prints the same way.

FAQ

About the dew point chart

How do I use a dew point chart?

Find your air temperature in the left column, then move across to your relative humidity; that cell is the dew point. At 26 °C and 60% RH it is 17.6 °C (63.7 °F). Then compare it with your coldest surface, because anything at or below that temperature gets wet.

How do I find dew point on a psychrometric chart?

Find your dry bulb temperature on the bottom axis and go up to the curve for your relative humidity. From that point, move straight left until you meet the saturation curve, the 100% line. The temperature there is the dew point. This table gives the same answer without plotting anything.

What is a normal dew point?

There is no single normal value. Outdoors, the US National Weather Service calls 55 °F (13 °C) or lower dry and comfortable. A veg room at 26 °C and 60% RH has a dew point of 17.6 °C (63.7 °F). Indoors, what matters is how close it comes to your coldest surface.

What dew point is comfortable?

55 °F (13 °C) or lower feels dry and comfortable, according to the US National Weather Service. From 55 to 65 °F (13–18 °C) the air feels sticky, and 65 °F (18 °C) or higher is oppressive. The scale is for people outdoors; in a grow room the surface margin matters more.

What is worse, humidity or dew point?

Neither; they measure different things. The dew point shows how much water is actually in the air. Relative humidity shows how full the air is at its current temperature, so it changes as the room warms or cools. Air at 26 °C and 60% RH cooled to 21 °C reads 81%, with the same dew point.

Can I save the dew point chart as a PDF?

Yes. Press Print / Save PDF, then choose Save as PDF in your browser's print dialog. The menu and controls are left out, and the Celsius and Fahrenheit tables print on separate landscape pages. You can also print only one unit, or download either table as a PNG image.

What dew point is too high for a grow room?

It depends on your coldest surface, not one number. Keep that surface at least 2 °C (3.6 °F) above the dew point; 4 °C (7.2 °F) is safer. At 26 °C and 70% RH the dew point is 20.1 °C, so leaves that cool to 21 °C at lights-off sit less than 1 °C above it.

How do I read a dew point chart?

Find your air temperature in the left column and your relative humidity across the top. The number where they meet is the dew point: any surface at or below that temperature gets wet.

Sources

The formulas and target ranges on this page come from these references.

University of Arizona, AZMet: dew point from temperature and relative humidity. azmet.arizona.edu/azmet/dewpoint.html
Allen, Pereira, Raes & Smith (1998). Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, chapter 3, equations 11 and 19. www.fao.org/4/x0490e/x0490e07.htm
US National Weather Service, La Crosse office. Why dew point vs humidity: 55 °F or less dry and comfortable; 55–65 °F sticky with muggy evenings; 65 °F or more oppressive. www.weather.gov/arx/why_dewpoint_vs_humidity

Tools