National Physical Laboratory

How accurate are pressure unit conversion values? (FAQ - Pressure)

The numerical relationship between the pascal and some other units is shown on the pressure units page. When converting between these (or any other) pressure units consideration should be given to the number of significant figures to use, bearing in mind that many of the underlying conversion factors are not themselves exact and cannot be made so (see validity of pressure units). There is little point in expressing the result of a conversion with more significant figures than is warranted either by the underlying conversion factor, the precision of the value to be converted or the measurement uncertainty associated with it; some illustrations are shown under the heading below. Thus, depending on circumstances, it is not always necessary to use the full precision of the conversion factors and it is never worth using more.

Despite these rules you will find conversion figures elsewhere that are published with far more significant figures than is appropriate; they are misleading and are certainly not more accurate.

Some examples of over-precise conversions are included in the other FAQs

Last Updated: 25 Mar 2010
Created: 9 Aug 2007

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