TECHNICAL TOOL / 08

Dew Point Chart — Celsius & Fahrenheit

Look up dew or frost point from dry-bulb temperature and relative humidity for warehouses, cold rooms, greenhouses and electrical enclosures. Print either table for field reference.

QUICK LOOKUP

Find the row, then the RH column.

Each cell is the saturation temperature for that air state. Compare it with a measured surface temperature when screening condensation or frost. Scroll within the table on a phone.

Celsius table · −10 to 50°C
Fahrenheit table · 14 to 122°F
Calculate between table entries

Both tables are generated at build time. No JavaScript is needed to read or print them; use your browser’s Print command.

REFERENCE EXAMPLE

25°C / 60% RH → dew point: 16.70°C
Same air state in Fahrenheit: 77°F → 62.06°F

Reference pressure: 101.325 kPa. At fixed dry bulb and RH, this ideal-gas dew-point calculation does not depend on total pressure. Pressure is needed for humidity ratio and wet bulb.

01 / °C

Dew / frost point chart (°C)

Rows: dry bulb. Columns: RH. Cell values: °C. Below 0.01°C (32.018°F), the saturation inversion uses ice and the result is a frost point.

Dry bulb × relative humidity → dew / frost point
°C / RH	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%
-10	-33.6	-27.0	-22.9	-19.9	-17.6	-15.6	-14.0	-12.5	-11.2	-10.0
-9	-32.8	-26.1	-22.0	-19.0	-16.6	-14.7	-13.0	-11.5	-10.2	-9.0
-8	-31.9	-25.2	-21.1	-18.1	-15.7	-13.7	-12.0	-10.5	-9.2	-8.0
-7	-31.1	-24.3	-20.2	-17.2	-14.8	-12.8	-11.0	-9.5	-8.2	-7.0
-6	-30.3	-23.5	-19.3	-16.2	-13.8	-11.8	-10.1	-8.6	-7.2	-6.0
-5	-29.5	-22.6	-18.4	-15.3	-12.9	-10.8	-9.1	-7.6	-6.2	-5.0
-4	-28.6	-21.7	-17.5	-14.4	-11.9	-9.9	-8.1	-6.6	-5.2	-4.0
-3	-27.8	-20.8	-16.6	-13.5	-11.0	-8.9	-7.2	-5.6	-4.2	-3.0
-2	-27.0	-20.0	-15.7	-12.5	-10.0	-8.0	-6.2	-4.6	-3.3	-2.0
-1	-26.2	-19.1	-14.8	-11.6	-9.1	-7.0	-5.2	-3.7	-2.3	-1.0
0	-25.3	-18.2	-13.9	-10.7	-8.2	-6.1	-4.3	-2.7	-1.3	0.0
1	-24.6	-17.5	-13.1	-9.9	-7.3	-5.2	-3.4	-1.8	-0.4	1.0
2	-23.9	-16.7	-12.3	-9.1	-6.5	-4.4	-2.6	-1.0	0.5	2.0
3	-23.2	-15.9	-11.5	-8.3	-5.7	-3.5	-1.7	-0.1	1.5	3.0
4	-22.5	-15.2	-10.7	-7.4	-4.9	-2.7	-0.9	0.9	2.5	4.0
5	-21.7	-14.4	-9.9	-6.6	-4.0	-1.9	-0.0	1.8	3.5	5.0
6	-21.0	-13.7	-9.1	-5.8	-3.2	-1.0	0.9	2.8	4.5	6.0
7	-20.3	-12.9	-8.4	-5.0	-2.4	-0.2	1.9	3.8	5.5	7.0
8	-19.6	-12.1	-7.6	-4.2	-1.6	0.7	2.9	4.8	6.5	8.0
9	-18.9	-11.4	-6.8	-3.4	-0.8	1.7	3.8	5.7	7.4	9.0
10	-18.2	-10.6	-6.0	-2.6	0.1	2.6	4.8	6.7	8.4	10.0
11	-17.5	-9.9	-5.2	-1.8	1.0	3.5	5.7	7.7	9.4	11.0
12	-16.8	-9.1	-4.5	-1.0	1.9	4.5	6.7	8.7	10.4	12.0
13	-16.1	-8.4	-3.7	-0.2	2.8	5.4	7.7	9.6	11.4	13.0
14	-15.4	-7.6	-2.9	0.6	3.8	6.4	8.6	10.6	12.4	14.0
15	-14.7	-6.9	-2.1	1.5	4.7	7.3	9.6	11.6	13.4	15.0
16	-14.0	-6.2	-1.4	2.4	5.6	8.2	10.5	12.6	14.4	16.0
17	-13.3	-5.4	-0.6	3.3	6.5	9.2	11.5	13.5	15.3	17.0
18	-12.6	-4.7	0.2	4.2	7.4	10.1	12.5	14.5	16.3	18.0
19	-11.9	-3.9	1.0	5.1	8.4	11.1	13.4	15.5	17.3	19.0
20	-11.2	-3.2	1.9	6.0	9.3	12.0	14.4	16.4	18.3	20.0
21	-10.5	-2.5	2.8	6.9	10.2	12.9	15.3	17.4	19.3	21.0
22	-9.8	-1.7	3.6	7.8	11.1	13.9	16.3	18.4	20.3	22.0
23	-9.1	-1.0	4.5	8.7	12.0	14.8	17.2	19.4	21.3	23.0
24	-8.4	-0.3	5.4	9.6	12.9	15.8	18.2	20.3	22.3	24.0
25	-7.7	0.5	6.2	10.5	13.9	16.7	19.1	21.3	23.2	25.0
26	-7.1	1.3	7.1	11.4	14.8	17.6	20.1	22.3	24.2	26.0
27	-6.4	2.1	8.0	12.3	15.7	18.6	21.1	23.3	25.2	27.0
28	-5.7	3.0	8.8	13.2	16.6	19.5	22.0	24.2	26.2	28.0
29	-5.0	3.8	9.7	14.0	17.5	20.5	23.0	25.2	27.2	29.0
30	-4.4	4.6	10.5	14.9	18.4	21.4	23.9	26.2	28.2	30.0
31	-3.7	5.4	11.4	15.8	19.4	22.3	24.9	27.1	29.2	31.0
32	-3.0	6.3	12.3	16.7	20.3	23.3	25.8	28.1	30.1	32.0
33	-2.3	7.1	13.1	17.6	21.2	24.2	26.8	29.1	31.1	33.0
34	-1.7	7.9	14.0	18.5	22.1	25.1	27.7	30.1	32.1	34.0
35	-1.0	8.7	14.8	19.4	23.0	26.1	28.7	31.0	33.1	35.0
36	-0.3	9.5	15.7	20.3	23.9	27.0	29.7	32.0	34.1	36.0
37	0.4	10.3	16.6	21.2	24.8	27.9	30.6	33.0	35.1	37.0
38	1.1	11.2	17.4	22.0	25.8	28.9	31.6	33.9	36.1	38.0
39	1.9	12.0	18.3	22.9	26.7	29.8	32.5	34.9	37.1	39.0
40	2.6	12.8	19.1	23.8	27.6	30.7	33.5	35.9	38.0	40.0
41	3.4	13.6	20.0	24.7	28.5	31.7	34.4	36.8	39.0	41.0
42	4.1	14.4	20.8	25.6	29.4	32.6	35.4	37.8	40.0	42.0
43	4.9	15.2	21.7	26.5	30.3	33.5	36.3	38.8	41.0	43.0
44	5.6	16.0	22.5	27.4	31.2	34.5	37.3	39.8	42.0	44.0
45	6.4	16.8	23.4	28.2	32.1	35.4	38.2	40.7	43.0	45.0
46	7.1	17.7	24.2	29.1	33.1	36.3	39.2	41.7	44.0	46.0
47	7.9	18.5	25.1	30.0	34.0	37.3	40.1	42.7	44.9	47.0
48	8.6	19.3	25.9	30.9	34.9	38.2	41.1	43.6	45.9	48.0
49	9.3	20.1	26.8	31.8	35.8	39.1	42.0	44.6	46.9	49.0
50	10.1	20.9	27.6	32.7	36.7	40.1	43.0	45.6	47.9	50.0

02 / °F

Dew / frost point chart (°F)

Rows: dry bulb. Columns: RH. Cell values: °F. Below 0.01°C (32.018°F), the saturation inversion uses ice and the result is a frost point.

Dry bulb × relative humidity → dew / frost point
°F / RH	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%
14	-28.5	-16.5	-9.2	-3.9	0.4	3.9	6.9	9.5	11.9	14.0
16	-26.8	-14.8	-7.4	-2.0	2.2	5.8	8.8	11.5	13.9	16.0
18	-25.2	-13.0	-5.6	-0.2	4.1	7.7	10.8	13.4	15.8	18.0
20	-23.5	-11.3	-3.8	1.7	6.0	9.6	12.7	15.4	17.8	20.0
22	-21.9	-9.5	-2.0	3.5	7.9	11.5	14.6	17.4	19.8	22.0
24	-20.2	-7.8	-0.2	5.4	9.8	13.4	16.6	19.3	21.8	24.0
26	-18.6	-6.0	1.6	7.2	11.7	15.3	18.5	21.3	23.8	26.0
28	-16.9	-4.3	3.4	9.1	13.5	17.3	20.5	23.2	25.7	28.0
30	-15.3	-2.6	5.2	10.9	15.4	19.2	22.4	25.2	27.7	30.0
32	-13.6	-0.8	7.0	12.8	17.3	21.1	24.3	27.2	29.7	32.0
34	-12.2	0.7	8.6	14.4	19.0	22.8	26.0	28.9	31.5	34.0
36	-10.7	2.3	10.2	16.0	20.6	24.5	27.7	30.6	33.4	36.0
38	-9.3	3.8	11.8	17.6	22.3	26.1	29.4	32.4	35.3	38.0
40	-7.9	5.3	13.4	19.2	23.9	27.8	31.1	34.3	37.3	40.0
42	-6.4	6.8	14.9	20.9	25.6	29.5	32.9	36.3	39.3	42.0
44	-5.0	8.3	16.5	22.5	27.2	31.1	34.9	38.2	41.3	44.0
46	-3.6	9.8	18.1	24.1	28.8	32.9	36.8	40.2	43.2	46.0
48	-2.1	11.4	19.6	25.7	30.5	34.8	38.7	42.1	45.2	48.0
50	-0.7	12.9	21.2	27.3	32.1	36.7	40.6	44.1	47.2	50.0
52	0.7	14.4	22.7	28.9	34.0	38.6	42.5	46.0	49.2	52.0
54	2.1	15.9	24.3	30.5	35.8	40.4	44.5	48.0	51.1	54.0
56	3.5	17.3	25.8	32.0	37.6	42.3	46.4	49.9	53.1	56.0
58	4.9	18.8	27.4	33.8	39.5	44.2	48.3	51.9	55.1	58.0
60	6.3	20.3	28.9	35.6	41.3	46.1	50.2	53.8	57.1	60.0
62	7.7	21.8	30.5	37.4	43.2	48.0	52.1	55.8	59.0	62.0
64	9.1	23.3	32.0	39.2	45.0	49.9	54.0	57.7	61.0	64.0
66	10.5	24.8	33.7	41.0	46.9	51.7	55.9	59.7	63.0	66.0
68	11.9	26.2	35.4	42.8	48.7	53.6	57.9	61.6	65.0	68.0
70	13.3	27.7	37.2	44.6	50.5	55.5	59.8	63.6	66.9	70.0
72	14.6	29.2	38.9	46.4	52.4	57.4	61.7	65.5	68.9	72.0
74	16.0	30.6	40.6	48.2	54.2	59.2	63.6	67.4	70.9	74.0
76	17.4	32.1	42.4	50.0	56.0	61.1	65.5	69.4	72.9	76.0
78	18.7	33.7	44.1	51.7	57.9	63.0	67.4	71.3	74.8	78.0
80	20.1	35.4	45.8	53.5	59.7	64.9	69.3	73.3	76.8	80.0
82	21.5	37.0	47.5	55.3	61.5	66.8	71.2	75.2	78.8	82.0
84	22.8	38.7	49.3	57.1	63.4	68.6	73.2	77.2	80.7	84.0
86	24.2	40.3	51.0	58.9	65.2	70.5	75.1	79.1	82.7	86.0
88	25.5	41.9	52.7	60.7	67.0	72.4	77.0	81.0	84.7	88.0
90	26.9	43.6	54.4	62.4	68.9	74.2	78.9	83.0	86.7	90.0
92	28.2	45.2	56.1	64.2	70.7	76.1	80.8	84.9	88.6	92.0
94	29.5	46.9	57.9	66.0	72.5	78.0	82.7	86.9	90.6	94.0
96	30.9	48.5	59.6	67.8	74.4	79.9	84.6	88.8	92.6	96.0
98	32.2	50.1	61.3	69.6	76.2	81.7	86.5	90.8	94.6	98.0
100	33.7	51.8	63.0	71.3	78.0	83.6	88.4	92.7	96.5	100.0
102	35.2	53.4	64.7	73.1	79.8	85.5	90.3	94.6	98.5	102.0
104	36.7	55.0	66.4	74.9	81.7	87.3	92.2	96.6	100.5	104.0
106	38.2	56.6	68.1	76.7	83.5	89.2	94.2	98.5	102.4	106.0
108	39.7	58.3	69.8	78.4	85.3	91.1	96.1	100.5	104.4	108.0
110	41.2	59.9	71.5	80.2	87.1	92.9	98.0	102.4	106.4	110.0
112	42.7	61.5	73.3	82.0	88.9	94.8	99.9	104.3	108.4	112.0
114	44.2	63.1	75.0	83.7	90.8	96.7	101.8	106.3	110.3	114.0
116	45.7	64.7	76.7	85.5	92.6	98.5	103.7	108.2	112.3	116.0
118	47.2	66.4	78.4	87.3	94.4	100.4	105.6	110.2	114.3	118.0
120	48.7	68.0	80.1	89.0	96.2	102.3	107.5	112.1	116.2	120.0
122	50.2	69.6	81.8	90.8	98.0	104.1	109.4	114.0	118.2	122.0

03 / SITE USE

Compare the dew point with the coldest surface.

Read the dew point for the measured air, then measure the surface itself. The margin between the two decides whether water can form; air temperature alone does not.

Coating and painting

Coating specifications commonly require the steel surface to stay at least 3°C (5°F) above the dew point during surface preparation and application; ISO 8502-4 describes the measurement. The coating data sheet governs.

Cold pipes, ducts and coils

A chilled surface below the room dew point sweats unless insulation and its vapour barrier keep the outer face warmer. Use the table to set the insulation surface target, then verify on site.

Cabinets, cold rooms and warehouses

Night cooling, door openings and cold product raise condensation risk. When the dew-point margin is small, lower the air dew point with dehumidification rather than relying on heating alone.

04 / METHOD & LIMITS

Calculation method and sources

pᵥ = RH × pws(Tdry) / 100
pws(Tdew/frost) = pᵥ

Saturation pressure and its inverse follow ASHRAE Fundamentals, Chapter 1, and PsychroLib. Saturation uses ice at or below 0.01°C and liquid water above. Subzero RH must use the same ice convention; RH reported over supercooled liquid water needs conversion first. Tables round to 0.1 degree.

ASHRAE Fundamentals (2017), Chapter 1 · PsychroLib

What this tool does not decide

A chart does not establish surface temperature, future condensation, sensor uncertainty or equipment capacity. Frost-point values do not describe supercooled-water dew point. The linked dew-point calculator uses a different NPL Magnus convention; compare methods before comparing subzero results.

NEXT / PROJECT REVIEW

Bring the operating conditions to an equipment review.

Share inlet and target air states, room volume, outdoor airflow, moisture sources and installation constraints.

Review project requirements