Fahrenheit to Celsius Formula

To convert Fahrenheit to Celsius use the formula: °C = (°F − 32) × 5/9. Subtract 32 to align the zero points, water freezes at 32°F and 0°C. Multiply by 5/9 because the Fahrenheit scale has 180 degrees between freezing and boiling while Celsius has 100, a ratio of 100/180 = 5/9. NIST SP 811 (2008) confirms these as exact definitions: 0 °C = 32 °F, 100 °C = 212 °F.

Apply the formula in order: subtract 32, then multiply by 5, then divide by 9. Or multiply by 5/9 in one step. Never reverse the order.

Warm Day: 68°F to Celsius

68 − 32 = 36. 36 × 5 = 180. 180 ÷ 9 = 20. Result: 68 °F = 20 °C. The exact conversion per NIST SP 811.

Below Freezing: −4°F to Celsius

−4 − 32 = −36. −36 × 5 = −180. −180 ÷ 9 = −20. Result: −4 °F = −20 °C. The negative sign carries through every step. This is the only point where the shortcut described below breaks badly.

Oven Temperature: 350°F to Celsius

350 − 32 = 318. 318 × 5 = 1590. 1590 ÷ 9 = 176.666... °C. Round for practical use: 177 °C (or 180 °C for most recipe conversions). The exact NIST value is 176.666... °C.

To convert f to c in your head quickly: subtract 30 then divide by 2. For 68°F: 68 − 30 = 38, halve to 19°C. The exact answer is 20°C, close enough for weather. The shortcut approximates 32 as 30 and 5/9 as 0.5. It works reasonably between 50°F and 80°F (10°C to 27°C), where error stays under 3°F. Outside that range the error grows fast. At 0°F the shortcut gives −15°C; the real value is −17.8°C. At 100°F it gives 35°C; the real value is 37.8°C. Never use the shortcut for cooking or medical decisions.

The f to c formula is linear: °C = (°F − 32) × 5/9. That is the fahrenheit to celsius equation. Write it as C = (F − 32) ÷ 9 × 5 if that helps you remember the order. The 5/9 factor is exact; 1 °F = 5/9 °C by definition (NIST SP 811). For the reverse direction, celsius to fahrenheit, use F = (C × 9/5) + 32.

When you convert a temperature interval, not a reading, drop the 32. A rise of 9 °F is a rise of 5 °C. Multiply or divide by the ratio only: Δ°C = Δ°F × 5/9. For example, an oven that cycles 25 °F above and below its set point varies by 25 × 5/9 = 13.9 °C. The BIPM SI Brochure (9th edition, 2019) defines the degree Celsius as equal to the kelvin, so the same rule applies for kelvin intervals: ΔK = Δ°C.

Three errors account for nearly all wrong answers. One: adding 32 before multiplying. Starting from °F, you subtract 32 first, the correct order is subtract, then multiply. Two: forgetting the negative sign on sub‑zero temperatures. −10°F minus 32 is −42, not 22. Three: rounding the 5/9 factor to 0.6 instead of using 5/9 exactly. 5/9 is 0.5555..., using 0.6 introduces a 9% error that compounds. Always keep full precision until the final step, then round only for display.

The Celsius scale is an offset of the kelvin, the SI base unit. Per the BIPM SI Brochure: t/°C = T/K − 273.15. So 0 °C = 273.15 K exactly. To convert Fahrenheit to kelvin, first get Celsius, then add 273.15. The Rankine scale is the Fahrenheit absolute scale: 0 °Ra = −459.67 °F. A Rankine degree is the same size as a Fahrenheit degree. These scales matter in thermodynamics and engineering but are rarely needed for weather or cooking.