How this calculator works
An egg cooks from the outside in. Heat crosses the shell and the white before it reaches the yolk, so the times that work are set by how far heat has to travel and how large the temperature difference is. A larger egg has more distance to the centre and takes longer, and the time rises a little less than in proportion to mass because the surface area also grows. An egg from the fridge starts about 16 C colder than one from the counter, so it needs longer in the same pot. Altitude matters because water boils at a lower temperature as air pressure falls: at 1500 m water boils at about 95 C, and at 3000 m at about 90 C, which slows the heat flow and adds time. Soft, medium and hard refer to the yolk. The white sets at about 63 C, while the yolk firms up later, so a soft egg is one whose white has set and whose yolk has not.
Formula
time = reference time x (egg mass / 57 g)^(2/3) x altitude and temperature factor. The factor is ln((boiling point - starting temperature) / (boiling point - 70 C)) divided by the same expression for a 4 C egg at sea level. Boiling point is found from the standard atmosphere pressure at the altitude and the Clausius-Clapeyron relation for water. Reference times for a 57 g egg from 4 C at sea level are 5 minutes soft, 7 minutes medium and 11 minutes hard.
Worked example
A Canadian large egg weighs about 59.5 g. From the fridge at 4 C at sea level the mass factor is (59.5 / 57)^(2/3) = 1.029, so the soft time is 5 x 1.029 = 5.1 minutes and the hard time is 11 x 1.029 = 11.3 minutes. At 1500 m, water boils at about 94.9 C and the temperature factor becomes ln(90.9 / 24.9) / ln(96 / 30) = 1.113, giving 5.7 minutes for soft and 12.6 minutes for hard. A room temperature egg at sea level needs 5 x 1.029 x 0.843 = 4.3 minutes for soft.
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Frequently asked questions
Do I start the timer before or after the water boils?
These times assume the egg goes into water that is already at a rolling boil and the pot returns to a boil quickly. If you start eggs in cold water, use a recipe written for that method, because the come-up time adds a large and variable delay.
Why do eggs at high altitude take longer?
Water cannot get hotter than its boiling point, and that point falls as air pressure falls. At 3000 m the pot is about 10 C cooler than at sea level, so the egg heats more slowly and the yolk sets later. Recipes written for sea level therefore need extra time in the mountains.
How do I peel them easily?
Plunge the eggs into ice water as soon as the timer ends. Cooling stops the cooking at the doneness you wanted, and the sudden chill helps the white pull away from the shell. Very fresh eggs are harder to peel because the white holds more tightly to the inner membrane.
Are soft boiled eggs safe for everyone?
No. Runny eggs are not recommended for young children, pregnant people, older adults or anyone with a weakened immune system. Health Canada lists 74 C (165 F) for egg dishes, and the USDA advises cooking eggs until both white and yolk are firm for anyone at higher risk.