Hong Life CalculatorsUnderstanding the egg boiling time calculator — concept and principle
This calculator works out how long to boil an egg for a soft, medium or hard yolk, based on the egg's weight, its starting temperature and your altitude. It uses a physics model of heat flowing into the egg, so it adjusts properly for large eggs, cold eggs from the fridge and lower boiling points in the mountains.
How it is calculated
Time = M^(2/3) × c × ρ^(1/3) ÷ (K × π² × (4π/3)^(2/3)) × ln[0.76 × (T_egg − T_water) ÷ (T_yolk − T_water)]
- M = egg mass in g; c = 3.7 J/(g·K); ρ = 1.038 g/cm³; K = 5.4 × 10⁻³ W/(cm·K). Together this is about 0.451 × M^(2/3) × ln[…] minutes.
- T_yolk is the temperature reached at the yolk's edge: 63 °C soft, 66 °C medium, 72 °C hard.
- T_water is the boiling point at your altitude, from the standard-atmosphere pressure and the Clausius–Clapeyron equation.
- Time starts when the egg goes into water that is already boiling.
The principle
Heat moves into an egg by conduction from the shell inwards. Physicist Charles Williams solved this for a sphere: the time to bring the centre region to a target temperature grows with the square of the radius, and since mass grows with the cube of the radius, time grows with mass to the power 2/3. The logarithm compares how far the egg starts from the water temperature with how close the yolk edge must get. Yolk proteins begin to thicken at about 65 °C and set at about 70 °C, which is why a few degrees separate soft from hard. At altitude water boils below 100 °C, so eggs take longer.
Worked example
Default inputs: 60 g egg from the fridge (4 °C), soft yolk (63 °C), altitude 0 m.
- Water boils at 100 °C; target 63 °C.
- Constant × M^(2/3) = 0.4508 × 60^(2/3) = 0.4508 × 15.33 = 6.91.
- ln[0.76 × (4 − 100) ÷ (63 − 100)] = ln(1.972) = 0.679.
- Time = 6.91 × 0.679 = 4.7 minutes, shown as 0:04:42.
How to read the result
Use the time as a starting point and adjust to taste: eggs differ in shape and the water may cool when you add several eggs. Bring the water back to the boil quickly or use a large pan. Stop the cooking by moving the egg into cold water if you want the yolk to stay exactly as timed. At high altitude a hard yolk may be impossible if the water boils below the target temperature; the calculator warns you.
Common mistakes
- Starting the timer when the egg goes into cold water rather than boiling water.
- Using the fridge setting for eggs kept at room temperature.
- Cooking many eggs in a small pan, which cools the water.
- Ignoring altitude in mountain areas.
Frequently asked questions
How long do I boil an egg for a runny yolk?
About 4.5–5 minutes for a 60 g egg straight from the fridge at sea level.
Why do eggs take longer at altitude?
Water boils at a lower temperature, so heat flows into the egg more slowly.
Does egg size matter?
Yes. Time grows with mass to the power 2/3, so a large egg needs noticeably longer.
Sources: C. D. H. Williams, "The Science of Boiling an Egg", University of Exeter, School of Physics · H. McGee, On Food and Cooking (Scribner, 2004)