Understanding Centimeters of Water to Centimeters of Mercury Conversion
A centimeter of water (cmH2O) is the pressure from a 1 cm water column, common in ventilation and low-pressure medicine, while a centimeter of mercury (cmHg) uses a mercury column of the same height. Because mercury is about 13.6 times denser than water, a centimeter of water equals only a fraction of a centimeter of mercury. This conversion helps compare respiratory pressures measured in water against blood pressure or barometric readings measured in mercury.
Conversion Formula
To convert centimeters of water to centimeters of mercury, multiply by this factor:
Step-by-Step Example
Convert 25 centimeters of water to centimeters of mercury.
How to Convert Centimeters of Water to Centimeters of Mercury
Translate a water-column pressure into an equivalent mercury-column height using the fluids' density ratio.
- Note the cmH2O value: Start with the pressure in centimeters of water.
- Multiply by 0.07355591: This factor converts water-column height into mercury-column height.
- Report the result: For example, 25 cmH2O × 0.07355591 = 1.83890 cmHg.
- Reverse if needed: Multiply the cmHg value by 13.5951 to return to cmH2O.
centimeters of water to centimeters of mercury conversion table
| centimeters of water (cmH2O) | centimeters of mercury (cmHg) |
|---|---|
| 0 | 0 |
| 1 | 0.07355591 |
| 2 | 0.1471118 |
| 3 | 0.2206677 |
| 4 | 0.2942237 |
| 5 | 0.3677796 |
| 6 | 0.4413355 |
| 7 | 0.5148914 |
| 8 | 0.5884473 |
| 9 | 0.6620032 |
| 10 | 0.7355591 |
| 15 | 1.103339 |
| 20 | 1.471118 |
| 25 | 1.838898 |
| 30 | 2.206677 |
| 40 | 2.942237 |
| 50 | 3.677796 |
| 60 | 4.413355 |
| 70 | 5.148914 |
| 80 | 5.884473 |
| 90 | 6.620032 |
| 100 | 7.355591 |
| 150 | 11.03339 |
| 200 | 14.71118 |
| 250 | 18.38898 |
| 300 | 22.06677 |
| 400 | 29.42237 |
| 500 | 36.77796 |
| 600 | 44.13355 |
| 700 | 51.48914 |
| 800 | 58.84473 |
| 900 | 66.20032 |
| 1000 | 73.55591 |
| 2000 | 147.1118 |
| 3000 | 220.6677 |
| 4000 | 294.2237 |
| 5000 | 367.7796 |
| 10000 | 735.5591 |
| 25000 | 1838.898 |
| 50000 | 3677.796 |
| 100000 | 7355.591 |
| 250000 | 18388.98 |
| 500000 | 36777.96 |
| 1000000 | 73555.91 |
What is the centimeter of water?
The centimeter of water (cmH2O) is a unit of pressure equal to the pressure exerted by a one-centimeter-high column of water under standard conditions. It is used heavily in medicine and respiratory therapy, where small pressures are common.
Definition
The conventional centimeter of water is defined as the pressure of a 1 cm column of water with a density of 1000 kg/m³ under standard gravity (9.80665 m/s²):
This is the conventional value. Because water density varies with temperature, more precise variants exist (for example, cmH2O at 4 °C uses water's maximum density of about 999.972 kg/m³, giving roughly 98.064 Pa).
Origin and History
The unit comes directly from the water manometer, an instrument in which pressure is read as the height difference of a water column. Water was chosen for low pressures because it produces a taller, more readable column than mercury (water is about 13.6 times less dense). The convention fixes water density and gravity so the unit has a single agreed value independent of local conditions.
Law and Notable Facts
The centimeter of water is not an SI unit but is permitted in medical contexts, where it remains standard for measuring airway, ventilator, and cerebrospinal-fluid pressures. Roughly 1 cmH2O equals 0.7356 mmHg, so about 1.36 cmH2O make up 1 mmHg.
Real-World Examples and Conversions
- Mechanical ventilators typically deliver positive end-expiratory pressure (PEEP) of 5 to 10 cmH2O.
- Normal cerebrospinal-fluid opening pressure in a lumbar puncture is roughly 10 to 18 cmH2O.
- 1 cmH2O ≈ 98.0665 Pa ≈ 0.098 kPa.
- A standard atmosphere equals about 1033.2 cmH2O.
What is the centimeter of mercury?
The centimeter of mercury (cmHg) is a unit of pressure equal to the pressure exerted by a one-centimeter-high column of mercury under standard conditions. It is a scaled-up form of the millimeter of mercury (torr) used in some scientific and industrial settings.
Definition
The conventional centimeter of mercury is based on a 1 cm column of mercury with a density of 13,595.1 kg/m³ under standard gravity (9.80665 m/s²):
Exactly, 1 cmHg = 1333.22387415 Pa, which is ten times the millimeter of mercury (1 mmHg ≈ 133.322 Pa). This equals 10 torr under the conventional definition.
Origin and History
The unit descends directly from Torricelli's mercury barometer of 1643, where atmospheric pressure was read as the height of a mercury column. Mercury's high density makes the column compact and easy to read. The conventional value fixes mercury's density (its value at 0 °C) and standard gravity so the unit is independent of temperature and location.
Law and Notable Facts
The centimeter of mercury is not an SI unit; the SI unit of pressure is the pascal. It relates to the standard atmosphere as 1 atm = 76 cmHg exactly (by the historical 760 mmHg definition). The millimeter of mercury (mmHg), essentially identical to the torr, remains standard in medicine for blood pressure.
Real-World Examples and Conversions
- 1 cmHg = 10 mmHg (10 torr) ≈ 1.33322 kPa.
- One standard atmosphere equals exactly 76 cmHg.
- A blood pressure of 120/80 mmHg corresponds to 12/8 cmHg.
- A vacuum of 5 cmHg absolute is about 6.7 kPa, roughly 93% below atmospheric pressure.
Frequently Asked Questions
How many centimeters of mercury equal one centimeter of water?
One centimeter of water equals 0.07355591 centimeters of mercury, reflecting mercury's roughly 13.6-times-greater density.
How do I convert centimeters of mercury back to centimeters of water?
Multiply the cmHg value by 13.5951, the reciprocal of 0.07355591.
Why compare water and mercury columns at all?
Medical ventilation reports pressures in cmH2O while blood pressure and barometry use mercury, so converting lets clinicians and engineers reconcile the two scales.
Does the density ratio explain the factor?
Yes; the factor 0.07355591 is essentially the ratio of water's density to mercury's, about 1/13.6.
What is 68 cmH2O in centimeters of mercury?
68 cmH2O equals about 5.00180 cmHg (68 × 0.07355591).
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Complete centimeters of water conversion table
| Unit | Result |
|---|---|
| pascals (Pa) | 98.0665 Pa |
| kilopascals (kPa) | 0.0980665 kPa |
| megapascals (MPa) | 0.0000980665 MPa |
| hectopascals (hPa) | 0.980665 hPa |
| millibar (mbar) | 0.980665 mbar |
| bar (bar) | 0.000980665 bar |
| torr (torr) | 0.7355592 torr |
| meters of water @ 4°C (mH2O) | 0.01 mH2O |
| millimeters of mercury (mmHg) | 0.7355591 mmHg |
| standard atmospheres (atm) | 0.0009678411 atm |
| technical atmospheres (at) | 0.001 at |
| centimeters of mercury (cmHg) | 0.07355591 cmHg |
| pounds per square inch (psi) | 0.01422334 psi |
| kilopound per square inch (ksi) | 0.00001422334 ksi |
| Inches of mercury (inHg) | 0.02895902 inHg |