The physics behind the timer
How long an egg needs depends on how fast heat travels from the boiling water into the yolk. In 1998, physicist Charles Williams derived a formula from heat-conduction theory that cooks still use:
t = 0.451 × M^(2/3) × ln[0.76 × (T_egg − T_water) ÷ (T_yolk − T_water)]
- t: time in minutes
- M: egg weight in grams
- T_egg: starting temperature of the egg
- T_water: temperature of the boiling water
- T_yolk: target temperature where the yolk meets the white
The yolk is soft and runny around 64 °C (147 °F), jammy around 70 °C (158 °F) and fully set around 80 °C (176 °F).
Worked example
A large egg (about 57 g) straight from the fridge (39 °F), for a jammy yolk, at sea level:
- Water boils at 212 °F (100 °C).
- t = 0.451 × 57^(2/3) × ln[0.76 × (4 − 100) ÷ (70 − 100)] ≈ 5.9 minutes.
- That is about 356 seconds: lower the egg into boiling water, start the timer, then move it to ice water.
Typical times from the fridge
| Egg size | Runny | Jammy | Hard-boiled |
|---|---|---|---|
| Medium (50 g) | 4.3 min | 5.4 min | 7.9 min |
| Large (57 g) | 4.7 min | 5.9 min | 8.6 min |
| Extra large (64 g) | 5.1 min | 6.4 min | 9.3 min |
| Jumbo (71 g) | 5.5 min | 6.9 min | 10.0 min |
Room-temperature eggs need about 1 to 1.5 minutes less.
Altitude
Water boils about 1 °F lower for every 500 feet of elevation. At 5,000 feet it boils near 203 °F, and a large jammy egg needs about 6.8 minutes instead of 5.9.
Tips
- Start in boiling water, not cold water, so the timing is predictable. Lower the eggs gently with a spoon.
- Keep the water boiling. Use enough water to cover the eggs by an inch so adding them does not stop the boil for long.
- Ice bath: 3 to 5 minutes stops the cooking and makes peeling easier.
- Older eggs peel more easily than very fresh ones, because the white sticks less to the membrane.
- The formula assumes a single layer of eggs; a crowded pot cools the water and adds time.