Conversions & Units
Fahrenheit to Celsius Calculator
Converts Fahrenheit to Celsius, with Kelvin shown alongside
Updated May 27, 2026 · Live
What this tool does
Converts a temperature in Fahrenheit to its exact Celsius equivalent. The Kelvin value is shown alongside. The conversion follows the linear formula °C = (°F − 32) × 5/9 — exact, no rounding in the arithmetic.
Formula Used
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How the Fahrenheit to Celsius conversion works
The Fahrenheit and Celsius scales are related by a linear equation, not a single multiplication factor. The formula is °C = (°F − 32) × 5/9. Two adjustments happen in sequence: subtracting 32 accounts for the difference in where each scale places its zero point (0 °C is the freezing point of water, while 32 °F is that same freezing point on Fahrenheit's scale); multiplying by 5/9 scales the degree size, because a Fahrenheit degree is exactly 5/9 the size of a Celsius degree. The result is exact for any input — no rounding occurs in the arithmetic itself.
The exact formula and its components
°C = (°F − 32) × 5/9
The 32-degree offset: Fahrenheit set 32 °F as the freezing point of water and 212 °F as the boiling point at standard atmospheric pressure. Celsius set those same two physical events at 0 °C and 100 °C. The 180-Fahrenheit-degree span from 32 to 212 corresponds to 100 Celsius degrees, so the ratio of degree sizes is 100/180 = 5/9.
Working the formula for the freezing point: (32 − 32) × 5/9 = 0 × 5/9 = 0.00 °C. For body temperature: (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37.00 °C. For the standard room-temperature reference point of 68 °F: (68 − 32) × 5/9 = 36 × 5/9 = 20.00 °C.
What Celsius and Fahrenheit each measure
Both Celsius and Fahrenheit are temperature scales that assign numbers to the same physical states. Celsius is the SI-derived unit used in science, engineering, and everyday life in most of the world. Fahrenheit remains the primary scale for everyday weather and thermostat readings in the United States. Neither scale has a true zero (the absence of thermal energy); that role belongs to Kelvin, which the calculator also displays: K = °C + 273.15.
Key reference points (verified)
32 °F = 0.00 °C = 273.15 K — freezing point of water at standard pressure. 212 °F = 100.00 °C = 373.15 K — boiling point of water at standard pressure. −40 °F = −40.00 °C = 233.15 K — the one point where the two scales read the same value. 68 °F = 20.00 °C = 293.15 K — standard indoor comfort reference. 98.6 °F = 37.00 °C = 310.15 K — normal human body temperature. 0 °F = −17.78 °C = 255.37 K — lower reference point on Fahrenheit's original scale.
Worked examples
A thermostat set to 72 °F: (72 − 32) × 5/9 = 40 × 5/9 = 22.22 °C (295.37 K). A hot summer day at 95 °F: (95 − 32) × 5/9 = 63 × 5/9 = 35.00 °C (308.15 K). A cold-weather concrete pour threshold at 40 °F: (40 − 32) × 5/9 = 8 × 5/9 = 4.44 °C (277.59 K). A paint application lower limit at 50 °F: (50 − 32) × 5/9 = 18 × 5/9 = 10.00 °C (283.15 K).
Where this conversion appears in US construction practice
Thermostats and HVAC setpoints: US residential thermostats display °F; HVAC equipment manufacturers and ASHRAE standards express performance curves in both °F and °C. A heat-pump rated at 47 °F outdoor ambient is 8.33 °C — a figure that aligns with European and international spec sheets.
Concrete curing and cold-weather pours: ACI 306R (Cold Weather Concreting) sets the critical fresh-concrete threshold at 50 °F (10 °C) for placement and at 40 °F (4.44 °C) as the lower limit below which hydration slows materially. Project specifications citing these limits in one unit convert directly to the other via this formula.
Coatings, adhesives, and sealants: US product data sheets typically state application temperature ranges in °F. International product data sheets and ISO test reports state the same ranges in °C. A primer rated for application above 50 °F is rated above 10 °C; a two-part epoxy requiring a substrate above 60 °F requires above 15.56 °C.
Weather data: NOAA and National Weather Service publish temperatures in °F; many imported materials or European-standard test data reference °C. The same linear formula bridges both.
Precision and rounding
The calculator displays °C to two decimal places and Kelvin to two decimal places. The underlying arithmetic uses full floating-point precision. For most practical applications — thermostat settings, weather comparisons, material data sheets — two decimal places represents more resolution than the measurement uncertainty in the original temperature. For scientific or laboratory work where sub-degree precision matters, the formula itself carries no rounding error; display precision is the only limit.
Reverse direction
To convert Celsius back to Fahrenheit, the inverse is °F = (°C × 9/5) + 32. This calculator takes °F as the input; the arithmetic is exact in either direction.
Using this alongside other BuildMetricLab tools
The Fahrenheit-to-Celsius result feeds naturally into the BTU to kW calculator when cross-referencing HVAC performance data that mixes US and metric units. The room volume calculator and spray foam calculator both reference temperature ranges for material application. All BuildMetricLab tools run entirely in the browser — no sign-up, no data sent anywhere, and the formula is shown on the page so the math can be audited.
Sources & methodology
Multiplies the offset-adjusted input by the exact scale ratio: °C = (°F − 32) × 5/9. Kelvin is derived as K = °C + 273.15. Both relationships are defined constants — no estimation, no material assumption, and no rounding in the arithmetic. Results are displayed to two decimal places.
Frequently asked questions
Is the Fahrenheit-to-Celsius conversion exact?
Yes. The formula °C = (°F − 32) × 5/9 is a defined relationship between the two scales, not an approximation. The 32-degree offset and the 5/9 ratio are exact constants. The only source of uncertainty in any result is the precision of the temperature value entered.
Why does the formula subtract 32 before multiplying?
The two scales place zero at different physical points. Celsius sets 0 °C at the freezing point of water; Fahrenheit sets that same freezing point at 32 °F. Subtracting 32 first aligns the two scales at their common reference, then the 5/9 factor scales from Fahrenheit degree sizes to Celsius degree sizes.
At what temperature do Fahrenheit and Celsius read the same number?
At −40 degrees. Setting °C = °F gives °F = (°F − 32) × 5/9, which solves to °F = −40. Both scales display −40 at that point, which corresponds to 233.15 K.
What is Kelvin, and why is it shown alongside Celsius?
Kelvin is the SI base unit of thermodynamic temperature. It uses the same degree size as Celsius but starts at absolute zero (the point of minimum thermal energy) rather than at the freezing point of water. K = °C + 273.15, so 0 °C = 273.15 K and 100 °C = 373.15 K. Kelvin appears in thermodynamic equations, heat-transfer calculations, and some HVAC and building-science standards.
Does this replace professional advice for temperature-sensitive construction work?
The arithmetic is exact — no professional input changes the formula. Temperature-sensitive decisions on a construction site, such as whether conditions meet the cold-weather concreting thresholds in ACI 306R or the application range on a product data sheet, depend on the actual measured site temperature, not on the conversion itself.
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