A wide, shallow cup cools faster than a narrow, deep one because more surface is exposed to the air. Blowing, stirring or pouring between two cups speeds cooling further. Hot drinks above about 65 °C (149 °F) can scald.
How to use the Water Cooling Calculator
- Method
- Temperature unit
- Starting temperature
- Desired temperature
- Room temperature
- Cold water temperature
- Volume in the cup
- Cup diameter (top)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose waiting or mixing, enter the temperatures, the volume and the cup diameter.
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Let it cool in the cup, Add cold water |
| Temperature unit | Choose one: °C, °F |
| Starting temperature | A number |
| Desired temperature | A number |
| Room temperature | A number |
| Cold water temperature | A number |
| Volume in the cup | Units: ml, cups, fl oz, L, tbsp |
| Cup diameter (top) | Units: cm, in |
Example values
How the main result changes when Starting temperature changes and everything else stays at the example values above:
| Starting temperature | Time to cool (min) |
|---|---|
| 71.25 | 3.6 |
| 95 | 9.2 |
| 118.75 | 13.1 |
| 142.5 | 16.3 |
| 190 | 21 |
Formula
Waiting: t = −ln((T − T_room) ÷ (T₀ − T_room)) ÷ k, k = h × A ÷ (m × c), A = π × (d ÷ 2)² (open top), h ≈ 284 W/(m²·K)
Mixing: cold water = V × (T₀ − T) ÷ (T − T_cold)
Worked example
Inputs — Method: Let it cool in the cup; Temperature unit: °C; Starting temperature: 95; Desired temperature: 60; Room temperature: 21; Volume in the cup: 300 ml; Cup diameter (top): 8 cm.
Results — Time to cool = 9.2 min; Cold water to add = 0 ml; Final volume (mixing) = 300 ml.
Frequently asked questions
How long does boiling water take to cool to drinking temperature?
About 10 minutes in a wide mug to reach 60 °C, longer in a narrow or insulated one.
What is the best temperature for hot drinks?
Around 57–60 °C (135–140 °F) for comfortable drinking.
How can I cool a hot drink quickly?
Add a little cold water or milk, or pour it between two cups.