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Hectopascal to Millimeter of Water Column Converter

Convert hectopascals to millimeters of water column with our free online pressure converter.

Quick Answer

1 Hectopascal = 10.197162 millimeters of water column

Formula: Hectopascal × conversion factor = Millimeter of Water Column

Use the calculator below for instant, accurate conversions.

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All conversion formulas on UnitsConverter.io have been verified against NIST (National Institute of Standards and Technology) guidelines and international SI standards. Our calculations are accurate to 10 decimal places for standard conversions and use arbitrary precision arithmetic for astronomical units.

Last verified: February 2026Reviewed by: Sam Mathew, Software Engineer

Hectopascal to Millimeter of Water Column Calculator

How to Use the Hectopascal to Millimeter of Water Column Calculator:

  1. Enter the value you want to convert in the 'From' field (Hectopascal).
  2. The converted value in Millimeter of Water Column will appear automatically in the 'To' field.
  3. Use the dropdown menus to select different units within the Pressure category.
  4. Click the swap button (⇌) to reverse the conversion direction.
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How to Convert Hectopascal to Millimeter of Water Column: Step-by-Step Guide

Converting Hectopascal to Millimeter of Water Column involves multiplying the value by a specific conversion factor, as shown in the formula below.

Formula:

mmH₂O = hPa × 10.1972

Example Calculation:

1 hPa = 10.197162 mmH₂O

Disclaimer: For Reference Only

These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.

Not for professional use. Results should be verified before use in any critical application. View our Terms of Service for more information.

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What is a Hectopascal and a Millimeter of Water Column?

1 hPa = 100 Pa = 0.1 kPa = 1 mbar (exactly) = 0.001 bar = 0.0145 PSI = 0.000987 atm = 0.75 mmHg

Why Hectopascal?

The hectopascal perfectly bridges SI and traditional meteorology:

  • SI compliance: Uses SI base unit (Pascal)
  • Backward compatibility: 1 hPa = 1 mbar exactly (no conversion needed)
  • Convenient scale: Typical atmospheric pressure ~1,000 hPa (manageable numbers)
  • Global adoption: WMO (World Meteorological Organization) standard since 1980s

1 mmH₂O = 9.807 Pa = 0.09807 mbar = 0.001422 PSI = 0.0736 mmHg = 0.03937 inH₂O

Key relationships:

  • 25.4 mmH₂O = 1 inH₂O
  • 13.6 mmH₂O ≈ 1 mmHg
  • 102 mmH₂O ≈ 1 kPa

Physical meaning: Pressure exerted by 1-millimeter column of water at 4°C

Note: The Hectopascal is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Millimeter of Water Column belongs to the imperial/US customary system.

History of the Hectopascal and Millimeter of Water Column

: The Transition from Millibar

The Evolution of Pressure Units

  • Pre-1900s: Mercury column (mmHg) standard for all pressure
  • 1900s-1980s: Meteorologists adopted millibar (mbar = 1/1000 bar)
    • Why: Sea level pressure ~1,013 mbar (convenient round number)
    • Advantage: Easy to work with 3-4 digit numbers
  • 1980s: International metric shift pushed for SI units
    • Problem: 1 mbar = 100 Pa (awkward multiplier)
    • Solution: 1 hectopascal = 100 Pa = 1 mbar (exact equivalence!)
  • 1980s-present: Smooth transition using hPa = mbar equivalence
    • No conversion confusion (same number, new name)
    • Allows decades of data to be "relabeled" without recalculation

Why This Worked

The brilliance: WMO chose hectopascal specifically because:

  1. 100 Pa = 1 hPa (clean, easy power of 10)
  2. 1 hPa = 1 mbar (exact, no confusion)
  3. Sea level pressure stayed ~1,013 hPa (same numbers as before)
  4. All existing meteorology infrastructure could transition seamlessly

Result: Weather services worldwide switched from mbar to hPa with zero data loss.

Common Uses and Applications: hectopascals vs millimeters of water column

Explore the typical applications for both Hectopascal (imperial/US) and Millimeter of Water Column (imperial/US) to understand their common contexts.

Common Uses for hectopascals

Meteorology: Weather forecasts, atmospheric pressure reporting (worldwide standard). Aviation: Altimeter settings, flight level pressure (ICAO standard). Oceanography: Atmospheric pressure corrections for sea level measurements. Climatology: Historical pressure data, pressure trends. Medical: Altitude acclimatization, pressure chambers, hyperbaric medicine.

When to Use millimeters of water column

Medical Ventilation: Mechanical ventilators, respiratory therapy. CPAP/BiPAP: Sleep apnea treatment pressure settings. Precision Instruments: Low differential pressure measurements. European HVAC: Clean room pressurization (metric countries).

Additional Unit Information

About Hectopascal (hPa)

How do I convert hPa to PSI?

Formula: PSI = hPa × 0.0145

Examples:

  • 1,013 hPa = 14.7 PSI (sea level)
  • 1,000 hPa = 14.5 PSI
  • 900 hPa = 13.05 PSI
  • 800 hPa = 11.6 PSI

hPa to PSI converter →

Is hPa the same as mbar?

Yes, exactly! 1 hPa = 1 mbar

Why two names?:

  • Millibar (mbar): Traditional meteorology unit (1900s-1980s)
  • Hectopascal (hPa): SI-compliant name (1980s-present)
  • Same value, just renamed for SI consistency

Conversion: None needed! They're identical.

Modern usage: Weather services worldwide now use hPa, but many pilots and mariners still say "millibar."

What is normal atmospheric pressure in hPa?

Standard sea level: 1,013.25 hPa (exactly, by definition)

Typical range: 980-1,050 hPa depending on weather

Pressure variations:

  • Fair weather (high): 1,020-1,040 hPa
  • Normal range: 1,000-1,020 hPa
  • Stormy (low): 980-1,000 hPa
  • Hurricane: <980 hPa

Regional/seasonal: Varies ±30 hPa from standard

How does pressure change with altitude?

Rough approximation: Pressure decreases ~12 hPa per 100m near sea level

More accurate (ISA model):

| Altitude | Pressure (hPa) | % of Sea Level | |----------|---------------|----------------| | 0m | 1,013 | 100% | | 500m | 955 | 94% | | 1,000m | 899 | 89% | | 2,000m | 795 | 78% | | 3,000m | 701 | 69% | | 5,000m | 540 | 53% | | 8,848m (Everest) | 313 | 31% |

Formula (approximate): P = 1013 × (1 - altitude/44,300)^5.255

What hPa is considered high or low pressure?

High pressure (anticyclone):

  • Strong high: >1,030 hPa (stable, clear weather)
  • Moderate high: 1,020-1,030 hPa (fair weather)

Normal pressure: 1,005-1,020 hPa

Low pressure (cyclone):

  • Moderate low: 990-1,005 hPa (cloudy, possible rain)
  • Strong low: 970-990 hPa (rain, wind)
  • Very low: <970 hPa (storms, gales)

Extreme low: <920 hPa (major hurricanes/typhoons)

Context: Depends on region and season. 1,000 hPa may be "low" in winter anticyclone, "high" in tropical region.

About Millimeter of Water Column (mmH₂O)

How do I convert mmH₂O to cmH₂O?

Simple: Divide by 10

1 cmH₂O = 10 mmH₂O

Examples (medical ventilators often use cmH₂O):

  • 100 mmH₂O = 10 cmH₂O (PEEP setting)
  • 200 mmH₂O = 20 cmH₂O (peak pressure)
  • 50 mmH₂O = 5 cmH₂O (CPAP setting)
  • 150 mmH₂O = 15 cmH₂O (typical ventilator)

Medical context: "PEEP of 5" means 5 cmH₂O = 50 mmH₂O

How do I convert mmH₂O to Pascals or mbar?

Formula:

  • Pa = mmH₂O × 9.807
  • mbar = mmH₂O × 0.09807

Examples:

  • 100 mmH₂O = 981 Pa ≈ 9.8 mbar (CPAP)
  • 200 mmH₂O = 1,961 Pa ≈ 19.6 mbar (ventilator)
  • 50 mmH₂O = 490 Pa ≈ 4.9 mbar (low PEEP)
  • 10 mmH₂O = 98 Pa ≈ 1 mbar (clean room)

Quick approximation: 100 mmH₂O ≈ 1 kPa

mmH₂O to Pascal converter →

What is normal CPAP pressure in mmH₂O?

Typical CPAP range: 40-140 mmH₂O (4-14 cmH₂O)

By severity:

  • Mild sleep apnea: 40-60 mmH₂O (4-6 cmH₂O)
  • Moderate sleep apnea: 60-100 mmH₂O (6-10 cmH₂O)
  • Severe sleep apnea: 100-140 mmH₂O (10-14 cmH₂O)

Starting pressure: Usually 40 mmH₂O (4 cmH₂O), titrated upward

Maximum typical: 200 mmH₂O (20 cmH₂O) for severe cases

Note: CPAP machines usually display in cmH₂O, not mmH₂O

  • Setting of "8" = 8 cmH₂O = 80 mmH₂O

What is PEEP in ventilators?

PEEP = Positive End-Expiratory Pressure

Pressure maintained in lungs at end of exhalation (prevents alveolar collapse)

Normal PEEP: 50 mmH₂O (5 cmH₂O) Moderate PEEP: 50-100 mmH₂O (5-10 cmH₂O) High PEEP: 100-200 mmH₂O (10-20 cmH₂O)

Used for:

  • Oxygenation improvement
  • Preventing lung collapse
  • ARDS (Acute Respiratory Distress Syndrome)
  • Pulmonary edema

Too high: Can cause lung damage (barotrauma) Too low: Alveoli collapse, poor oxygenation

Example ventilator settings:

  • Peak pressure: 200 mmH₂O (20 cmH₂O)
  • PEEP: 50 mmH₂O (5 cmH₂O)
  • Tidal volume: 500 mL

Why use mmH₂O instead of kPa or mmHg?

Medical respiratory context:

mmH₂O preferred:

  • Appropriate scale: Ventilator pressures 50-300 mmH₂O (vs 0.5-3 kPa - awkward decimals)
  • Historical: Water manometers originally used
  • Precision: 1 mmH₂O resolution appropriate for therapy adjustments
  • International standard: ISO, medical device standards use cmH₂O/mmH₂O
  • Clinical familiarity: Respiratory therapists trained in cmH₂O

Not mmHg because:

  • mmHg too large (1 mmHg = 13.6 mmH₂O)
  • mmHg used for blood pressure, not airway pressure
  • Less precise for ventilator adjustments

Not kPa because:

  • Requires decimal precision (8 cmH₂O = 0.78 kPa)
  • Less intuitive for clinicians
  • Though some modern ventilators offer kPa display option

Conversion: 100 mmH₂O = 10 cmH₂O = 0.98 kPa = 7.4 mmHg

Conversion Table: Hectopascal to Millimeter of Water Column

Hectopascal (hPa)Millimeter of Water Column (mmH₂O)
0.55.099
110.197
1.515.296
220.394
550.986
10101.972
25254.929
50509.858
1001,019.716
2502,549.291
5005,098.581
1,00010,197.162

People Also Ask

How do I convert Hectopascal to Millimeter of Water Column?

To convert Hectopascal to Millimeter of Water Column, enter the value in Hectopascal in the calculator above. The conversion will happen automatically. Use our free online converter for instant and accurate results. You can also visit our pressure converter page to convert between other units in this category.

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What is the conversion factor from Hectopascal to Millimeter of Water Column?

The conversion factor depends on the specific relationship between Hectopascal and Millimeter of Water Column. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.

Can I convert Millimeter of Water Column back to Hectopascal?

Yes! You can easily convert Millimeter of Water Column back to Hectopascal by using the swap button (⇌) in the calculator above, or by visiting our Millimeter of Water Column to Hectopascal converter page. You can also explore other pressure conversions on our category page.

Learn more →

What are common uses for Hectopascal and Millimeter of Water Column?

Hectopascal and Millimeter of Water Column are both standard units used in pressure measurements. They are commonly used in various applications including engineering, construction, cooking, and scientific research. Browse our pressure converter for more conversion options.

For more pressure conversion questions, visit our FAQ page or explore our conversion guides.

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Verified Against Authority Standards

All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.

NIST Reference on Constants, Units and Uncertainty

National Institute of Standards and Technology — Fundamental physical constants and unit conversions

BIPM SI Units

Bureau International des Poids et Mesures — International System of Units (SI) definitions and standards

ISO 80000

International Organization for Standardization — Quantities and units international standards series

Last verified: February 19, 2026