Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin and Rankine with the correct offset arithmetic — the one conversion family where simple multiplication is wrong.
Temperature Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Temperature conversion requires both a scale factor and an offset, unlike most unit conversions. Celsius to Fahrenheit is °F = °C × 9/5 + 32, and Celsius to Kelvin is K = °C + 273.15. The offset is why you cannot convert a temperature by multiplying alone.
Formula
Celsius and Fahrenheit are interval scales with arbitrary zeros; Kelvin and Rankine are absolute scales starting at absolute zero.
Worked Example
- Start in Celsius. 25 °C.
- Apply the ratio. 25 × 9/5 = 45.
- Add the offset. 45 + 32 = 77 °F.
- Convert to Kelvin. 25 + 273.15 = 298.15 K.
- Convert to Rankine. 298.15 × 9/5 = 536.67 °R.
Metrology note. Temperature is the one common conversion where the offset matters. A temperature difference of 25 °C equals 45 °F, not 77 °F, because differences carry the ratio but not the offset. Mixing the two is a frequent error in thermal calculations.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Cooking and food safety
Recipes and food safety guidance cross between scales constantly. The critical thresholds — 4 °C for refrigeration and 74 °C for poultry — are 40 °F and 165 °F respectively, and getting them wrong has real consequences.
Scientific and engineering work
Thermodynamic calculations require absolute scales. Gas laws, efficiency calculations and radiative heat transfer all break if you substitute Celsius where Kelvin is required, because ratios of interval-scale values are meaningless.
Weather and travel
The two scales cross at −40°, which is the only temperature where Celsius and Fahrenheit read the same. For rough mental conversion, double the Celsius and add 30 — accurate to within a few degrees across normal weather ranges.
Methodology & Editorial Standards
This engine converts every input to Celsius first, then derives all four scales from that single value, which guarantees internal consistency across the output cards. Kelvin is offset from Celsius by exactly 273.15, and Rankine relates to Kelvin by exactly 9/5. Inputs below absolute zero are rejected with an explicit error rather than silently returning an unphysical result. Water state assumes standard atmospheric pressure of 101.325 kPa. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Temperature Converter — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32. So 25 °C × 1.8 = 45, plus 32 gives 77 °F. The addition of 32 is essential because the two scales place their zero points at different physical temperatures.
How do I convert Fahrenheit to Celsius?
Subtract 32 first, then multiply by 5/9. For 98.6 °F: 98.6 − 32 = 66.6, and 66.6 × 5/9 = 37 °C. Reversing the order of operations is the most common mistake.
At what temperature do Celsius and Fahrenheit meet?
At exactly −40°. It is the single point where the two scales coincide, which follows from solving x = 9x/5 + 32.
What is absolute zero?
The theoretical temperature at which particles have minimum thermal motion: 0 K, equal to −273.15 °C or −459.67 °F. It cannot be reached, only approached, as a consequence of the third law of thermodynamics.
Why does Kelvin not use the degree symbol?
Because it is an absolute thermodynamic scale, not an interval scale. Since 1967 the SI convention writes 295 K rather than 295 °K. Celsius and Fahrenheit retain the degree symbol as interval measures.
What is the Rankine scale?
An absolute scale using Fahrenheit-sized degrees, where 0 °R is absolute zero and water freezes at 491.67 °R. It appears in some US engineering and thermodynamics contexts, effectively as the Fahrenheit counterpart to Kelvin.
Why can't I convert a temperature difference the same way?
Because a difference carries the scale factor but not the offset. A rise of 10 °C is a rise of 18 °F, not 50 °F. Convert differences using the ratio 9/5 alone, with no addition of 32.
What is normal body temperature?
Conventionally 37 °C or 98.6 °F, though the true population average is slightly lower, around 36.6 °C, and varies with time of day, measurement site and individual. The 98.6 figure is an artefact of over-precise conversion from a rounded 37 °C.
At what temperature does water boil?
100 °C or 212 °F at standard atmospheric pressure. Boiling point falls with altitude — roughly 1 °C per 300 m — which is why high-altitude cooking requires longer times or a pressure cooker.
Which countries use Fahrenheit?
The United States and a small number of territories including the Cayman Islands, Palau and Liberia. Essentially every other country uses Celsius for everyday temperature, with Kelvin used universally in science.
How do I estimate the conversion mentally?
Double the Celsius and add 30 for a quick Fahrenheit estimate: 20 °C gives 70 against the exact 68 °F. For the reverse, subtract 30 and halve. Accuracy degrades at extremes but is fine for weather.
Is 0 °C the same as 32 °F exactly?
Yes, exactly. Both denote the freezing point of water at standard pressure, and the conversion carries no rounding error since both scale definitions are exact.
Is this temperature converter free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.