Boil-Water Time and Boiling Point at Altitude

ScopeThis calculator computes the standard air pressure at a site's elevation from the U.S. Standard Atmosphere, the boiling point of water from the NIST Antoine constants, and the boil time each federal agency prescribes. The agencies disagree about where the longer boil starts: the EPA says 3 minutes above 5,000 ft and the CDC says 3 minutes above 6,500 ft, so a notice reprinted from the two sources gives two answers to a responder in between. A relief camp at 6,000 ft sits at 81.20 kPa (11.78 psia), where water boils at 201.0 degF, about 11 degrees below sea level; the EPA says 3 minutes and the CDC says 1. The calculator names which agency says which and recommends 3 minutes anywhere in the split, because the longer boil costs fuel and nothing else. The same method reproduces 212.0 degF at sea level, which is the check that the constants are right, and gives 193.7 degF at 10,000 ft. The boiling point also serves a reader checking that a boil is real or running a cooking or pasteurizing step. It is for a standard atmosphere and plain water: weather moves it by a degree or so, and a low-pressure storm lowers it further. Boiling does not remove fuel, chemicals, or toxins and can concentrate them, and the calculator does not decide when a boil-water notice may be lifted, which follows the utility's coliform sampling and the primacy agency's decision. It does not compute fuel for boiling. The EPA and CDC guidance, the local health department, and the utility's public notice govern.

Formula or tableP (kPa) = 101.325 x (1 - 6.87535e-6 x h_ft)^5.25588; T (K) = 1663.125 / (5.08354 - log10(P_bar)) + 45.622; EPA boil 1 minute, 3 minutes above 5,000 ft; CDC boil 1 minute, 3 minutes above 6,500 ft; recommended = the longer of the two.

Source and editionEPA "Emergency Disinfection of Drinking Water" and CDC "How to Make Water Safe in an Emergency" boil guidance, by name; the U.S. Standard Atmosphere (1976) troposphere pressure-altitude relation; the NIST Chemistry WebBook Antoine constants for water (A 5.08354, B 1663.125, C -45.622, 344-373 K). The local health department and the utility's boil-water notice govern.

Free accessFederal guidance and public-domain standard-atmosphere and NIST constants, evaluated directly; nothing is reproduced beyond the constants.

What governsEstimate. Operator of record and primacy agency govern.

API field nameselevation, elevation_unit, pressure_kpa, pressure_psia, boiling_point_k, boiling_point_c, boiling_point_f, epa_boil_min, cdc_boil_min, recommended_boil_min

• Standard atmosphere the 1976 troposphere relation; actual barometric pressure moves the boiling point about a degreeU.S. Standard Atmosphere (1976)
• Antoine constants for water fit over 344-373 K, which spans -1,500 ft to 20,000 ft within a fraction of a degreeNIST Chemistry WebBook
• The longer boil in the split 3 minutes between 5,000 and 6,500 ft, where the EPA already requires it and the CDC does not yetEPA and CDC boil guidance
• EPA metric figure the EPA page prints its threshold as 5,000 ft (1,000 meters), but 1,000 m is only 3,281 ft; the feet figure is used, so a metric reader between 1,000 and 1,524 m sees 1 minute hereEPA, Emergency Disinfection of Drinking Water