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Rotary Evaporator Calculator: Bath Temperature, Vacuum & Boiling Point
Choose a solvent and get the bath temperature, vacuum (mbar) and coolant temperature for your rotary evaporator – for the recommended setting or any bath temperature you choose. Also finds the boiling point of 45 solvents at any vacuum, lists every solvent at one bath temperature, and sizes the chiller and vacuum pump for the evaporation rate you need. Results in the units you use – mbar, hPa, Torr, mmHg, kPa, bar, atm, psi or inHg, and °C, °F or K. Based on the 20/20 rule and published vapour-pressure data.
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How it works
Coolant temperature = vapour temperature − 20 °C (raised for solvents that would freeze)
Vacuum = vapour pressure of the solvent at the vapour temperature
This “20/20 rule” (also called the Δ20 or 20/40/60 rule) keeps a steady distillation: the bath supplies heat 20 °C above the boiling point, and a condenser 20 °C colder than the vapour recovers almost all of it. Vapour pressures come from published correlations (DIPPR, NIST and the thermo database) and were checked against each solvent’s normal boiling point.
Worked example
Ethyl acetate with a 50 °C bath
Vapour temperature 30 °C. The vapour pressure of ethyl acetate at 30 °C is about 160 mbar, so set the vacuum to 160 mbar and the coolant to 10 °C or colder. Lower the vacuum slowly from about 210 mbar to avoid bumping.
Tips for faster, safer evaporation
- Fill the flask no more than half full and use a speed that gives a thin film.
- Lower the vacuum in steps; sudden low pressure causes bumping and foaming.
- High-boiling solvents (DMF, DMSO, NMP) need a pump that reaches 5–30 mbar and leak-tight joints.
- Peroxide-forming ethers (THF, diethyl ether, dioxane) must never be evaporated to dryness.
- For corrosive solvents and acids use a chemically resistant (PTFE) diaphragm pump.
Sizing a chiller and vacuum pump
The Chiller & pump sizing tab works out how much heat the condenser must remove for the evaporation rate you need: heat load (W) = moles evaporated per second × heat of vaporisation. It recommends a chiller with 1.5 times that capacity at the coolant temperature, and a pump whose ultimate vacuum is about half the working vacuum. Example: removing 1.5 L of ethanol per hour needs about 300 W of condensing capacity, so choose a chiller of at least 500 W at 10 °C.
Prefer a printable chart? See the Rotary Evaporator Solvent Library with a 1-page PDF.
Frequently asked questions
What is the 20/40/60 rule?
A guideline for rotary evaporation: bath 60 °C, vapour 40 °C, coolant 20 °C – each step 20 °C apart. The same 20 °C steps work at other temperatures, e.g. bath 50 °C, vapour 30 °C, coolant 10 °C, which is gentler on heat-sensitive samples.
Why is my solvent not evaporating?
Usually the vacuum is not low enough for the bath temperature (check for leaks and pump capacity), or the flask is rotating too slowly. Compare your gauge reading with the vacuum shown here.
Should I use absolute or gauge pressure?
This calculator uses absolute pressure in mbar (0 = perfect vacuum, 1013 = atmosphere). If your gauge reads relative (e.g. −0.9 bar), convert it with the Lab Unit Converter.
How big a chiller do I need for my rotary evaporator?
It depends on how fast you evaporate and which solvent. Water needs about 680 W per litre per hour, ethanol about 200 W and dichloromethane about 125 W. Add 50 % for losses and check the chiller capacity at your coolant temperature. Use the Chiller & pump sizing tab for your solvent and rate.
How accurate are the vacuum values?
Within about 5 % of published vapour-pressure data for most solvents in the usual range (often within 1–2 %). Real systems differ slightly because of dissolved samples and gauge accuracy, so fine-tune by watching the condensation rate.