About this tool
The two scales meet at -40 (the only point where °C = °F). Celsius is based on water freezing (0) and boiling (100). Fahrenheit puts the same freezing point at 32 and boiling at 212 - a holdover from Daniel Fahrenheit's 1724 calibration. Kelvin starts at absolute zero (-273.15°C).
How it works
Type any temperature and choose its scale. The calculator instantly displays equivalents in all three scales.
Common unit conversion factors
| Type | Conversion | Memory aid |
|---|---|---|
| Length | 1 inch = 2.54 cm (exact) | Always exact since 1959 international yard agreement |
| Length | 1 mile = 1.60934 km | 5 miles ~ 8 km |
| Length | 1 foot = 0.3048 m (exact) | |
| Mass | 1 lb = 0.453592 kg | 2.2 lb per kg |
| Mass | 1 stone (UK) = 6.35 kg = 14 lb | British weight, also Ireland |
| Volume | 1 US gal = 3.785 L | US gallon is smaller than UK |
| Volume | 1 UK gal = 4.546 L | Imperial gallon, ~20% larger than US |
| Volume | 1 fl oz US = 29.57 mL | |
| Temperature | F = C x 9/5 + 32 | 0 C = 32 F; 100 C = 212 F |
| Temperature | K = C + 273.15 | Absolute zero = 0 K |
| Energy | 1 kcal = 4.184 kJ | Food labels use kcal even when they say 'calorie' |
| Power | 1 hp (metric) = 735.5 W | Engine horsepower |
| Pressure | 1 atm = 101.325 kPa = 14.696 psi | Atmospheric at sea level |
SI prefixes
| Prefix | Symbol | Power | Example |
|---|---|---|---|
| yotta | Y | 10^24 | Mass of the Sun ~2 YT |
| zetta | Z | 10^21 | Global data created annually |
| exa | E | 10^18 | Atomic bonds in 1 mole |
| peta | P | 10^15 | Petabyte = 1000 TB |
| tera | T | 10^12 | Terabyte |
| giga | G | 10^9 | Billion |
| mega | M | 10^6 | Million |
| kilo | k | 10^3 | Thousand |
| centi | c | 10^-2 | Centimeter, hundredth |
| milli | m | 10^-3 | Millisecond, millimeter |
| micro | µ | 10^-6 | Bacterial size |
| nano | n | 10^-9 | Wavelength of visible light |
| pico | p | 10^-12 |
Imperial vs metric: where each is used
Three countries still officially use imperial: United States, Liberia, Myanmar. The UK uses a mix (metric for trade, imperial for road distance, beer pints). Everywhere else is metric, though horsepower, inches for screens, and feet-and-inches for height are common globally even in metric countries.
The formula explained
This calculator uses the following formula:
C → F: F = C × 9/5 + 32; F → C: C = (F - 32) × 5/9; K = C + 273.15
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.
Frequently asked questions
Quick mental conversion C → F?
Double C and add 30 (rough). Exact: × 1.8 + 32. 25°C ≈ 80°F (exact 77°F).
Why -40 is the same in both?
It's the equation cross-over: -40 × 9/5 + 32 = -40. The only fixed point.
What's a comfortable room temp?
21-22°C (70-72°F) is the international comfort range. Below 18°C feels cold; above 26°C feels warm.
Body temperature?
Normal: 36.5-37.5°C (97.7-99.5°F). Fever: above 38°C (100.4°F). Hypothermia: below 35°C (95°F).
Why is 1 US gallon different from 1 UK gallon?
The US gallon (3.785 L) is based on the wine gallon of 1707. The UK / Imperial gallon (4.546 L) was redefined in 1824 as the volume of 10 lb of water at 62°F. They've diverged ever since.
Is Celsius or Fahrenheit better for weather?
Celsius scales 0-100 over freezing-to-boiling of water, useful for science. Fahrenheit scales roughly 0-100 over 'very cold' to 'very hot' for human experience, useful for daily weather. Most of the world uses Celsius; the US uses Fahrenheit.
How many significant figures does this conversion preserve?
Most factors used are exact (defined by international agreement) or accurate to 6+ significant figures. Display rounding may show fewer digits. Underlying math uses full IEEE 754 precision.
Are pound (mass) and pound-force the same?
On Earth at sea level, approximately yes. Strictly: 1 lb mass weighs 1 lbf force in standard gravity. In space or precise engineering, the distinction matters. Most everyday usage conflates them.
What's the difference between metric ton, short ton, and long ton?
Metric ton (tonne) = 1,000 kg (universal). Short ton (US) = 2,000 lb = 907 kg. Long ton (UK historical) = 2,240 lb = 1,016 kg. Shipping uses metric tons; US road weight limits use short tons.
How accurate is the Celsius ↔ Fahrenheit Converter?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Celsius ↔ Fahrenheit Converter free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Celsius ↔ Fahrenheit Converter on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Celsius ↔ Fahrenheit Converter work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Celsius ↔ Fahrenheit Converter?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Celsius ↔ Fahrenheit Converter?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another celsius ↔ fahrenheit tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.