The “inch of mercury” (inHg) is a unit of pressure commonly used in aviation, meteorology, and refrigeration. It originates from the barometric pressure measurement practice using a column of mercury. This unit became a staple due to its early adoption in weather forecasting and its accurate and consistent measurements. An interesting fact is that an inch of mercury is approximately equal to 33.86 millibars or 3386 Pascals, but what truly distinguishes it is its historical significance in the development of modern atmospheric science.
In the 17th century, Evangelista Torricelli, a student of Galileo, first discovered that atmospheric pressure could sustain a column of mercury, laying the groundwork for this unit. His invention, the barometer, revolutionized how we understand and predict weather, marking the inch of mercury as more than just a unit—it’s a nod to the evolution of scientific inquiry and practice.
The Pascal, named after the French mathematician and physicist Blaise Pascal, is the SI unit of pressure. It is equivalent to one newton per square meter, symbolized as Pa. Despite its relatively recent introduction in 1971, the Pascal honors the pioneering work of Pascal in fluid mechanics and pressure, such as Pascal’s Law, which explains how pressure applied to a confined fluid is transmitted undiminished in all directions. Interestingly, a single Pascal is a tiny unit of pressure, roughly equal to the pressure exerted by a dollar bill resting flat on a table. This illustrates why everyday atmospheric pressure is typically measured in kilopascals (kPa), as one standard atmosphere is about 101.3 kPa.
Inches of mercury | Pascals |
---|---|
0
inHg
|
0
Pa
|
1
inHg
|
3386.38895
Pa
|
2
inHg
|
6772.77791
Pa
|
3
inHg
|
10159.16686
Pa
|
4
inHg
|
13545.55581
Pa
|
5
inHg
|
16931.94477
Pa
|
6
inHg
|
20318.33372
Pa
|
7
inHg
|
23704.72267
Pa
|
8
inHg
|
27091.11163
Pa
|
9
inHg
|
30477.50058
Pa
|
10
inHg
|
33863.88953
Pa
|
11
inHg
|
37250.27849
Pa
|
12
inHg
|
40636.66744
Pa
|
13
inHg
|
44023.05639
Pa
|
14
inHg
|
47409.44535
Pa
|
15
inHg
|
50795.8343
Pa
|
16
inHg
|
54182.22325
Pa
|
17
inHg
|
57568.61221
Pa
|
18
inHg
|
60955.00116
Pa
|
19
inHg
|
64341.39011
Pa
|
20
inHg
|
67727.77907
Pa
|
21
inHg
|
71114.16802
Pa
|
22
inHg
|
74500.55697
Pa
|
23
inHg
|
77886.94593
Pa
|
24
inHg
|
81273.33488
Pa
|
25
inHg
|
84659.72383
Pa
|
26
inHg
|
88046.11278
Pa
|
27
inHg
|
91432.50174
Pa
|
28
inHg
|
94818.89069
Pa
|
29
inHg
|
98205.27964
Pa
|
30
inHg
|
101591.6686
Pa
|
31
inHg
|
104978.05755
Pa
|
32
inHg
|
108364.4465
Pa
|
33
inHg
|
111750.83546
Pa
|
34
inHg
|
115137.22441
Pa
|
35
inHg
|
118523.61336
Pa
|
36
inHg
|
121910.00232
Pa
|
37
inHg
|
125296.39127
Pa
|
38
inHg
|
128682.78022
Pa
|
39
inHg
|
132069.16918
Pa
|
40
inHg
|
135455.55813
Pa
|
41
inHg
|
138841.94708
Pa
|
42
inHg
|
142228.33604
Pa
|
43
inHg
|
145614.72499
Pa
|
44
inHg
|
149001.11394
Pa
|
45
inHg
|
152387.5029
Pa
|
46
inHg
|
155773.89185
Pa
|
47
inHg
|
159160.2808
Pa
|
48
inHg
|
162546.66976
Pa
|
49
inHg
|
165933.05871
Pa
|
50
inHg
|
169319.44766
Pa
|
51
inHg
|
172705.83662
Pa
|
52
inHg
|
176092.22557
Pa
|
53
inHg
|
179478.61452
Pa
|
54
inHg
|
182865.00348
Pa
|
55
inHg
|
186251.39243
Pa
|
56
inHg
|
189637.78138
Pa
|
57
inHg
|
193024.17034
Pa
|
58
inHg
|
196410.55929
Pa
|
59
inHg
|
199796.94824
Pa
|
60
inHg
|
203183.3372
Pa
|
61
inHg
|
206569.72615
Pa
|
62
inHg
|
209956.1151
Pa
|
63
inHg
|
213342.50406
Pa
|
64
inHg
|
216728.89301
Pa
|
65
inHg
|
220115.28196
Pa
|
66
inHg
|
223501.67092
Pa
|
67
inHg
|
226888.05987
Pa
|
68
inHg
|
230274.44882
Pa
|
69
inHg
|
233660.83778
Pa
|
70
inHg
|
237047.22673
Pa
|
71
inHg
|
240433.61568
Pa
|
72
inHg
|
243820.00463
Pa
|
73
inHg
|
247206.39359
Pa
|
74
inHg
|
250592.78254
Pa
|
75
inHg
|
253979.17149
Pa
|
76
inHg
|
257365.56045
Pa
|
77
inHg
|
260751.9494
Pa
|
78
inHg
|
264138.33835
Pa
|
79
inHg
|
267524.72731
Pa
|
80
inHg
|
270911.11626
Pa
|
81
inHg
|
274297.50521
Pa
|
82
inHg
|
277683.89417
Pa
|
83
inHg
|
281070.28312
Pa
|
84
inHg
|
284456.67207
Pa
|
85
inHg
|
287843.06103
Pa
|
86
inHg
|
291229.44998
Pa
|
87
inHg
|
294615.83893
Pa
|
88
inHg
|
298002.22789
Pa
|
89
inHg
|
301388.61684
Pa
|
90
inHg
|
304775.00579
Pa
|
91
inHg
|
308161.39475
Pa
|
92
inHg
|
311547.7837
Pa
|
93
inHg
|
314934.17265
Pa
|
94
inHg
|
318320.56161
Pa
|
95
inHg
|
321706.95056
Pa
|
96
inHg
|
325093.33951
Pa
|
97
inHg
|
328479.72847
Pa
|
98
inHg
|
331866.11742
Pa
|
99
inHg
|
335252.50637
Pa
|
100
inHg
|
338638.89533
Pa
|
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