Direct answer
Celsius and Fahrenheit are equal at −40 degrees
−40°C equals exactly −40°F. This is the only temperature where the Celsius and Fahrenheit scales display the same numerical value.
At temperatures above −40, the Fahrenheit number is greater than the Celsius number. At temperatures below −40, the Fahrenheit number is lower than the Celsius number.
The equal point can be confirmed by substituting −40 into the Celsius-to-Fahrenheit formula.
Enter values above and below −40 to compare the scales.
Finding the intersection
How to solve for the equal temperature
Begin with the standard Celsius-to-Fahrenheit formula:
At the equal point, Fahrenheit and Celsius have the same numerical value.
Replace Fahrenheit with Celsius because the two values are equal:
Subtract 9C/5 from both sides:
This becomes −4C/5 = 32.
Multiply both sides by −5/4:
Therefore, −40°C equals −40°F.
Formula check
Verifying −40 with the conversion formula
−40 × 9/5 = −72.
The Fahrenheit result matches the Celsius input.
The same result appears when using the decimal form: −40 × 1.8 + 32 = −40.
Scale structure
Why do Celsius and Fahrenheit cross at −40?
The two scales have different zero points and different degree sizes. Celsius uses a larger degree interval, while Fahrenheit uses a smaller interval and begins with a 32-degree offset at the Celsius freezing point.
Equals a 1.8°F change
Different zero points
Values become identical
Because the Fahrenheit value changes faster numerically, the two linear scales meet once and then separate again.
Nearby temperatures
Celsius and Fahrenheit values around −40
| Celsius | Fahrenheit | Relationship |
|---|---|---|
| −60°C | −76°F | Fahrenheit number is lower |
| −50°C | −58°F | Fahrenheit number is lower |
| −45°C | −49°F | Fahrenheit number is lower |
| −40°C | −40°F | Values are equal |
| −35°C | −31°F | Fahrenheit number is higher |
| −30°C | −22°F | Fahrenheit number is higher |
| −20°C | −4°F | Fahrenheit number is higher |
Although one numerical value may appear higher or lower, each pair represents the same physical temperature.
Changes between temperatures
The scales are equal only as readings, not intervals
At −40, the displayed values match. The degree intervals remain different. A change of 1°C still equals a change of 1.8°F.
| Starting temperature | Change | Ending temperature |
|---|---|---|
| −40°C | Increase by 1°C | −39°C |
| −40°F | Equivalent increase by 1.8°F | −38.2°F |
The scales intersect at one point, but their intervals remain in a 9-to-5 relationship.
Mistakes to avoid
Common questions about the equal point
Are Celsius and Fahrenheit equal at positive 40?
No. Positive 40°C equals 104°F. Only negative 40 produces the same numerical value.
Are there multiple equal temperatures?
No. The scales are linear and have different slopes, so they intersect at only one point.
Does −40 represent absolute zero?
No. Negative 40 is simply the intersection of the Celsius and Fahrenheit scales. Absolute zero is much colder.
Does −40°C feel different from −40°F?
No. They describe exactly the same physical temperature.
Continue reading
Related Celsius-to-Fahrenheit guides
Questions and answers
Frequently asked questions
At what temperature are Celsius and Fahrenheit equal?
Celsius and Fahrenheit are equal at negative 40 degrees.
Is positive 40 Celsius equal to 40 Fahrenheit?
No. Positive 40 degrees Celsius equals 104 degrees Fahrenheit.
Why do the scales meet at negative 40?
Their different zero points and degree intervals cause the two linear scales to intersect at negative 40.
Are Celsius and Fahrenheit degree sizes equal at negative 40?
No. The readings match, but a change of one Celsius degree still equals a change of 1.8 Fahrenheit degrees.
Convert a temperature
Use the Celsius to Fahrenheit Converter
Enter any positive, negative, whole, or decimal Celsius temperature to calculate Fahrenheit instantly.
Open Celsius to Fahrenheit ConverterLast updated: July 27, 2026