inch of water to millimeter of water converter

Convert inch of water to millimeter of water in the Pressure family. See a sample calculation, unit definitions, and the interactive converter.

Example conversion: 1 inH2O

25.4 millimeter of water

This example is calculated from the same unit registry used by the interactive converter.

Formula and unit definitions

Convert SI, atmospheric, imperial, water-column and engineering pressure units.

target value = source value × source factor ÷ target factor

Source unit: inch of water (inH2O), factor 249.08891, offset 0.

Target unit: millimeter of water (mmH2O), factor 9.80665, offset 0.

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Important note

The calculation is performed locally from the listed unit definitions. Display results are rounded for readability; high-precision and regulated work should use the governing standard and appropriate significant figures.

For high-precision, regulated or safety-critical use, check the underlying standard and its required precision. Historical and approximate units are identified in their names or family descriptions.

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