Quick example
Make 1,000 g of 30 °C water
With cold water at 20 °C and hot water at 80 °C, the target is one-sixth of the way from cold to hot. Use 166.67 g hot water and 833.33 g cold water.
Energy balance
Why the formula works
For two amounts of the same substance, heat lost by warmer water equals heat gained by cooler water in an ideal insulated mix. Water's specific heat cancels from both sides, leaving a weighted average of the two starting temperatures.
Hot amount
Total × (target − cold) ÷ (hot − cold).
Cold amount
Total amount minus the calculated hot amount.
Celsius or Fahrenheit
Either works because the calculation uses differences on one consistent scale.
Weight units
Any listed unit works when the same unit is used for both portions.
Idealized result
Containers and rooms exchange heat
The calculation assumes only the two water portions exchange heat. A cold bowl, warm kettle, evaporation, transfer time, and thermometer error can shift the actual result. Mix, measure, and make a small adjustment when temperature is important.
Method & source
How the blend is calculated
The method applies conservation of thermal energy for two samples of the same material, as described in OpenStax Physics: Heat and Heat Transfer. No density conversion is needed because both amounts use the same weight unit.
Common questions
Water temperature mixing FAQ
Can I use Fahrenheit?
Yes. Enter all three temperatures in Fahrenheit and select that scale.
Can the target be hotter than my hot water?
No. A two-water mixture cannot finish outside its starting temperature range without another heat source or sink.
Why is my measured result slightly different?
The vessel and room also exchange heat, while the calculator models only the water.