Degrees Fahrenheit to degrees Celsius converter

A Fahrenheit reading often needs conversion for a metric recipe or weather comparison. Subtract the scale's offset before applying the degree-size ratio. An oven's marked setting and its actual measured temperature can differ even when the unit conversion is exact.

Subtract 32 and multiply by 5/9: (98.6 °F − 32) × 5/9 = 37 °C.

Subtract 30 and halve the result. It is exact at 50 °F (10 °C) and gives 20 °C for 70 °F instead of 21.1 °C; the error grows by about 0.5 °C for every 9 °F away from 50.

At −40: −40 °F is exactly −40 °C, because solving F = C × 9/5 + 32 with F = C gives C = −40.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “98.6 °F to °C” is checked against Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions).

NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B; NIST Handbook 44, Appendix C: General tables of units of measurement; BIPM, The International System of Units (SI Brochure), 9th edition.