Seven categories of the units chemistry actually uses — temperature, pressure, volume, mass, amount of substance, energy and concentration — with the underlying formula displayed for every conversion.
K = °C + 273.15 • °F = °C × 9/5 + 32
The conversions students need most
Conversion
Formula / factor
Where it appears
°C → K
K = °C + 273.15
every gas law problem
°C → °F
°F = °C × 9/5 + 32
everyday comparisons
atm → kPa
× 101.325
gas laws with SI units
atm → mmHg (torr)
× 760
pressure measurements
mL → L
÷ 1000
molarity and dilution
cal → J
× 4.184
thermochemistry
⚠ Common mistake: Temperature conversions add or subtract — they don't multiply like other units. And gas-law temperatures must be kelvin: 25 °C is 298 K, not 25 K.
Why chemistry prefers SI units
Working in one unit system keeps constants clean: R = 8.314 J/(mol·K) only works with pascals, cubic metres and kelvin — or its cousin 0.0821 with atm, L and K. Most calculation errors trace back to a unit sneaking in from the wrong system, so convert first, calculate second.
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Frequently asked questions
▸How do I convert Celsius to Kelvin?
Add 273.15: 25 °C = 298.15 K. Kelvin never goes negative — 0 K (−273.15 °C) is absolute zero.
▸Is a torr the same as a mmHg?
For all practical purposes, yes — they differ by less than one part in seven million. Both are 1/760 of an atmosphere.
▸What's the difference between a calorie and a Calorie?
The food “Calorie” (capital C) is actually a kilocalorie: 1 Cal = 1000 cal = 4184 J.