The right time for the egg you want

Rules of thumb, three minutes for soft-boiled and nine for hard-boiled, only work for an average egg at room temperature and at sea level. A large egg straight from the fridge, or a kitchen in the mountains, changes the result by minutes. This calculator uses Charles Williams's physical formula, which describes how heat travels into an egg, to work out the time from weight, starting temperature, yolk consistency and altitude.

How the calculation works

The formula treats the egg as a small sphere that heats from the outside in. The time grows with the weight raised to 2/3, that is with the surface area, and with the logarithm of the ratio between temperature differences:

• Weight: a bigger egg takes longer to heat through to the centre.
• Starting temperature: an egg cold from the fridge starts further away.
• Yolk temperature: it decides the consistency; the higher it is, the longer you cook.
• Altitude: in the mountains water boils at a lower temperature, about 1 °C less every 300 m, and cooks more slowly.

The time applies to an egg lowered into water that is already boiling, not to an egg started in cold water.

Which yolk temperature to choose

• 60-63 °C: runny yolk, for soft-boiled eggs or dipping toast.
• 64-66 °C: jammy yolk, thick but not set.
• 67-71 °C: soft but set yolk, a medium-boiled egg.
• 72-80 °C: firm yolk, for hard-boiled eggs, salads and fillings.

These are indicative values: the white sets before the yolk, and consistency also depends on freshness and personal taste.

Times by size and doneness

Eggs from the fridge, 4 °C, at sea level:

The same eggs at room temperature, 20 °C:

Starting from the fridge adds from about one minute to a minute and a half, depending on size and doneness: the most common reason the same recipe turns out differently from one time to the next.

Boiling eggs in the mountains

With a 57 g egg from the fridge and a 70 °C yolk:

The closer the boiling water is to the temperature you want in the yolk, the slower the cooking: at high altitude a hard-boiled egg takes much longer.

Tips for precise cooking

• Lower the eggs into water that is already boiling with a spoon, then start the timer.
• Use enough water to cover them and keep boiling when you add them.
• For hard-boiled eggs, cool them straight away in iced water: they peel more easily and the yolk does not turn greenish.
• Weigh one egg from the box: the stated size covers a range of several grams.

Limits of the calculator

The formula treats the egg as a uniform sphere and does not consider shape, shell thickness, the amount of water or the temperature drop when the eggs go in. The boiling point at altitude is estimated with a simple rule. Use the result as a starting point and adjust it by a few seconds for your stove and taste.

Frequently asked questions

How long should a soft-boiled egg cook?

A 57 g egg from the fridge, at sea level, has a runny yolk after about 4 min 30 s in water that is already boiling. At room temperature about 3 min 20 s is enough.

How long should a hard-boiled egg cook?

A 60 g egg from the fridge reaches about 77 °C at the centre in about 8 minutes. For a firm but not dry yolk, 6-7 minutes is enough.

Does it matter if the egg is cold from the fridge?

Yes: it starts further away and needs from about one minute to a minute and a half more than an egg at room temperature, depending on size and doneness.

Why do eggs cook more slowly in the mountains?

Because water boils at a lower temperature, about 1 °C less every 300 m. At 2,000 m a medium-boiled egg takes over a minute longer.

What is the Williams formula?

It is a physical model of heat conduction proposed by Charles Williams: it links the time to the egg's weight and to the temperatures of the egg, the water and the yolk.

Should I start the egg in cold or boiling water?

The calculator assumes water that is already boiling: it is the most repeatable method. Starting in cold water makes the time depend too much on the stove and pan.

How do I know how much my egg weighs?

Weigh one on a kitchen scale. In the European Union medium eggs weigh 53-63 g and large ones 63-73 g; other countries use different sizes.

Why does a hard-boiled yolk turn greenish?

It is a reaction between iron in the yolk and sulphur in the white caused by overcooking. Stick to the time and cool straight away in iced water.

Are very fresh eggs hard to peel?

Often yes: eggs a few days old peel more easily. Rapid cooling helps in any case.

Does it work for quail eggs?

No, the calculator accepts eggs from 30 to 100 g. Quail eggs are much smaller and cook in 2-3 minutes.

Does the time apply to several eggs at once?

Yes, if the pan holds enough water to keep boiling when you add them. With too many eggs the water cools and a few more seconds are needed.

Can I use it for poached eggs?

No, poached eggs cook without the shell in water below boiling. The calculator applies to eggs in their shell.