Temperature Converter
Convert any temperature between Celsius, Fahrenheit, and Kelvin using the exact thermodynamic relationships between scales. Useful for cooking, weather comparison, science work, and international travel.
Last updated: May 2026
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About this calculator
Temperature is measured on three primary scales, and exact conversions between any pair are well-defined. Celsius (°C) is anchored at the freezing (0 °C) and boiling (100 °C) points of water at standard atmospheric pressure. Fahrenheit (°F) places freezing at 32 °F and boiling at 212 °F, a 180-degree interval that makes each Fahrenheit degree 5/9 the size of a Celsius degree. Kelvin (K) is the SI absolute thermodynamic scale, anchored at absolute zero (0 K = −273.15 °C), with each Kelvin step equal in size to one Celsius degree. The six conversion formulas are: °C → °F: F = C × 9/5 + 32; °F → °C: C = (F − 32) × 5/9; °C → K: K = C + 273.15; K → °C: C = K − 273.15; °F → K: K = (F − 32) × 5/9 + 273.15; K → °F: F = (K − 273.15) × 9/5 + 32. The conversion at −40 is unique: −40 °C = −40 °F exactly, the only temperature where Celsius and Fahrenheit read the same. Edge cases: Kelvin cannot be negative (absolute zero is the floor); values below 0 K are unphysical and should be rejected. Inputs near absolute zero (below ~1 K) are valid but enter quantum-thermodynamic regimes where the ideal-gas-based intuitions break down. For everyday accuracy, keep two decimal places; for scientific work, six. Note the older Rankine scale (°R, used in some US engineering contexts) is Fahrenheit's absolute counterpart and is not covered here.
How to use
Example 1 — body temperature to Celsius. Convert 98.6 °F (the traditional human body temperature reference) to Celsius. Step 1: F − 32 = 98.6 − 32 = 66.6. Step 2: × 5/9 = 66.6 × 0.5556 = 37.0 °C. Verify by reversing: 37 × 9/5 + 32 = 66.6 + 32 = 98.6 °F — matches. Example 2 — boiling water to Kelvin. Convert 100 °C to Kelvin: K = 100 + 273.15 = 373.15 K. Now convert that same Kelvin value to Fahrenheit using the K→F path: F = (373.15 − 273.15) × 9/5 + 32 = 100 × 9/5 + 32 = 180 + 32 = 212 °F. Cross-verify by the direct C→F path: 100 × 9/5 + 32 = 212 °F — identical. The three scales are internally consistent: any route through any chain of conversions returns the same final number, provided you preserve enough decimal precision.
Frequently asked questions
What is the easiest way to estimate Fahrenheit-to-Celsius conversions in my head?
A useful mental shortcut for everyday weather temperatures: subtract 30 from Fahrenheit, then divide by 2. So 80 °F → (80 − 30)/2 = 25 °C (exact: 26.7 °C, error 1.7 °C). 50 °F → (50 − 30)/2 = 10 °C (exact: 10 °C, exact match). The shortcut is accurate to within ±2 °C for temperatures between 0 °F and 100 °F, which covers most weather scenarios. For more precision use the exact formula: subtract 32, multiply by 5/9. For Celsius to Fahrenheit, reverse the shortcut: double, then add 30. 20 °C → 40 + 30 = 70 °F (exact: 68 °F, error 2 °F). These shortcuts are unsuitable for cooking, science, or medical use where 1–2 degree errors matter.
Why does Kelvin start at absolute zero, and what does that mean physically?
Kelvin is an absolute thermodynamic scale with its zero point at absolute zero, the temperature at which the entropy of a perfect crystal reaches zero and molecular kinetic energy is minimized (not exactly zero — quantum zero-point motion still exists). Absolute zero is 0 K = −273.15 °C = −459.67 °F, and it is unattainable in practice (the third law of thermodynamics rules out cooling to exactly 0 K in a finite number of steps). Because Kelvin has no negative values, ratios are physically meaningful: 600 K is twice as hot as 300 K in energetic terms, while 60 °C is not meaningfully twice 30 °C. This is why gas laws (PV = nRT), blackbody radiation, and statistical mechanics all use Kelvin. The size of one Kelvin equals one Celsius degree, so converting between them is just an offset of 273.15.
At what temperature does Celsius equal Fahrenheit, and is that a useful fact?
−40 °C = −40 °F exactly — the unique temperature where the two scales coincide. Solving F = C × 9/5 + 32 with F = C gives C − C × 9/5 = 32, so −C × 4/5 = 32, hence C = −40. It is a useful sanity check: if your conversion of a cold temperature does not approach this fixed point as values decrease, you have a formula error. The −40 point also matters in cold-weather engineering — many specifications (lubricants, electronics, vehicles) are rated to −40, partly because that single number works in either unit system. No equivalent exact-match exists for Celsius and Kelvin (they have the same scale step but a 273.15 offset) or for Fahrenheit and Kelvin.
What are common mistakes when converting temperatures?
The most frequent mistake is using the wrong direction of the formula — applying (F − 32) × 5/9 in the C → F direction, or forgetting the +32 in the C → F direction. Sign errors in subtractions with negative inputs are also common (e.g., −20 °F → (−20 − 32) × 5/9 = −28.9 °C, but writing 12 × 5/9 = 6.7 °C is a frequent slip). People also confuse Kelvin offsets, sometimes using 273 instead of 273.15, which introduces a small but real 0.15-degree error in scientific work. Mixing up Rankine (Fahrenheit's absolute scale) with Kelvin in engineering contexts produces large errors. Finally, applying the body-temperature shortcut to extreme cold or heat without falling back to the exact formula can introduce 5–10 °C errors near −40 °F or +120 °F.
When should I NOT use this calculator?
Avoid using a generic temperature converter when working with specialized thermodynamic scales like Rankine (°R, the absolute Fahrenheit scale) or Réaumur (°Ré, used historically in parts of Europe) — those need different formulas. For scientific work where precision below 0.01 °C matters (e.g., calorimetry, NIST-traceable measurements), use a calculator that preserves all decimal places of 273.15 and avoids floating-point intermediate rounding. Do not use a single-value conversion for temperature differences in equations: a 10 °C change equals an 18 °F change (because of the 9/5 ratio), but adding 32 makes no sense — convert temperature deltas with multiplication only, not the full formula. For cooking conversions where ovens have ±5–15 °C accuracy anyway, exact decimals are overkill. Finally, do not rely on it for cold-chain logistics or medical refrigeration where regulatory standards may specify exact units and tolerances.