How to Convert Celsius, Fahrenheit, and Kelvin

Fahrenheit degrees are smaller: a 1 °C change equals a 1.8 °F change. The scales also zero in different places, so you need a shift and a scale factor.

Formulas

°F = (°C × 9/5) + 32

°C = (°F − 32) × 5/9

9/5 is 1.8; 5/9 is about 0.555…

Worked examples

0 °C → (0 × 9/5) + 32 = 32 °F (water freezes at standard pressure).

100 °C → (100 × 9/5) + 32 = 212 °F (water boils at standard pressure).

20 °C roomy indoor air → (20 × 1.8) + 32 = 68 °F.

72 °F → (72 − 32) × 5/9 = 40 × 5/9 ≈ 22.2 °C.

−40 is the meeting point: −40 °C = −40 °F.

Skip the arithmetic with the online temperature converter. For other unit families, the free unit converter covers length, mass, and more.

Kelvin uses the same step size as Celsius but starts at absolute zero. There is no degree sign in modern SI style: write 300 K, not 300 °K.

Formulas

K = °C + 273.15

°C = K − 273.15

K = (°F + 459.67) × 5/9

°F = (K × 9/5) − 459.67

Worked examples

25 °C → 25 + 273.15 = 298.15 K.

0 K → −273.15 °C ≈ −459.67 °F (absolute zero).

373.15 K → 100 °C → 212 °F.

Human body ~37 °C → ~310 K.

When Kelvin appears

Gas laws, color temperature of lights, electronics, and climate science papers use K so ratios and absolute temperatures stay meaningful. Doubling Kelvin is a real “twice the absolute temperature” statement; doubling Celsius is not.

Rankine (°R) is an absolute scale with Fahrenheit-sized degrees. Engineers in some U.S. contexts still use it.

°R = °F + 459.67

°R = K × 9/5

Point	°C	°F	K
Absolute zero	−273.15	−459.67	0
Water freezes	0	32	273.15
Cool room	20	68	293.15
Body (approx.)	37	98.6	310.15
Water boils	100	212	373.15

Memorize freeze/boil anchors and room ~20 °C / 68 °F; convert the rest with the tool when precision matters.

The temperature converter includes reference points and side-by-side scales so you can sanity-check lab or recipe numbers quickly.

Same formulas — different stakes and typical ranges.

Weather band

• Heat wave ~35–40 °C → about 95–104 °F
• Mild spring ~15 °C → 59 °F
• Hard freeze ~−10 °C → 14 °F

Forecasts rarely need 0.01° precision. Being within 1 °C / 2 °F is enough for clothing and travel plans.

Kitchen band

• Oven 180 °C ≈ 356 °F (often labeled 350 °F in U.S. recipes — close enough for many home ovens)
• Oven 200 °C ≈ 392 °F (commonly rounded to 400 °F in casual conversions — know when your recipe is strict)
• Candy or oil stages can need tighter control than cake recipes

Fan/convection ovens and altitude change real cooking results even when the conversion is perfect. Convert the number, then still watch the food.

Health-related body metrics use different tools: see the free BMI calculator, BMI guide, and calorie needs guide — those are not temperature problems, but they show up in the same “numbers for your body” workflow.

Absolute zero (0 K) is the lower limit of thermodynamic temperature — molecular motion at the quantum minimum for a perfect system. You will not hit it in a home freezer; the number still matters because Kelvin and Rankine count upward from that floor.

Practical takeaway

You can convert negative Celsius and Fahrenheit freely. You should not see negative Kelvin in normal data. If a spreadsheet spits out −40 K, a unit or formula is wrong.

Delta temperatures

A change of 10 °C is a change of 10 K and 18 °F. For differences only, °C and K increments match; °F increments are 1.8× larger. People often mess up “increase the bath by 5 degrees” when recipes hop languages mid-paragraph.

Need “what percent is this rise of that range?” for experiments — the percentage calculator and percentage guide help with relative change, not the temperature transform itself.

Type a value in any supported scale and read the others at once. Use formulas on-screen when you’re learning; use batch tables when you have a list of setpoints.

1. Open the free temperature converter.
2. Enter the temperature you know (weather, recipe, or lab reading).
3. Select or confirm the input unit (°C, °F, K, or °R).
4. Read the converted values on the other scales.
5. Check a reference point (freezing, boiling, body) to catch unit mix-ups.
6. For recipes, round the way your oven dial allows (often 5 °F or 5 °C steps).
7. For science notes, keep more decimals and write the unit every time.

Field habit

Always write the unit next to the number in shared docs. “Set to 180” without °C/°F is how trays burn.

These errors show up in homework, travel, and kitchens every week.

• Using ×2 instead of ×1.8 — rough mental math that drifts several degrees
• Forgetting ±32 — scaling without the offset (20 × 1.8 = 36, not 68 °F)
• Treating K like °C without +273.15
• Writing negative Kelvin from a bad spreadsheet chain
• Rounding oven temps twice — convert once, then match the nearest real dial mark
• Mixing delta and absolute — “add 10 °F” is not “add 10 °C”
• Assuming boiling is always 100 °C — altitude and purity shift boiling point

This guide is educational. Medical decisions need clinical thermometers and professional care, not kitchen conversions.