How we calculate the cooking time

In short: The app calculates how long the heat takes to travel from the water to the edge of the yolk, for the egg you’ve chosen, whether you put it into boiling water or start it in cold water. All times are marked “Calculated”: they’re calculated with a physical model of how heat moves into the egg, and we haven’t cooked and measured them ourselves yet.

Five things decide the time

1. How you like your egg. Soft-boiled, jammy or hard-boiled. Each has a temperature the edge of the yolk should reach.
2. The size. A bigger egg needs longer, because the heat has further to go. We use the classes on the carton: S (under 53 g), M (53–63 g), L (63–73 g) and XL (73 g or more). If the carton says M/L, the eggs are mixed: choose M, or L for the biggest ones. If it says M/L/XL, choose L, or M for the smallest ones. US sizes are different: a US large egg is about an M.
3. The temperature of the egg. An egg straight from the fridge (about 5 °C, 41 °F) needs about one minute more than an egg at room temperature (about 20 °C, 68 °F), for a soft-boiled M egg. With cold water the difference is a little smaller, because the egg first takes on the temperature of the water.
4. The water you start in. Boiling water or cold tap water. With cold water, the clock starts when you turn on the stove, so the time includes heating the water. See “Cold water” below.
5. Altitude. Higher up, water boils at a lower temperature, so eggs need longer.

The calculation

An egg heats from the outside in. The calculation treats the egg as a sphere of the same weight and works out when the edge of the yolk reaches the target temperature for the egg you’ve chosen. According to the physicist Charles Williams at the University of Exeter, the white starts to set at about 62–65 °C (144–149 °F) and the yolk at 65–70 °C (149–158 °F).

Williams also made a well-known formula for the cooking time. It uses only the first term of the solution, which gives times that are too short when the cooking time is short. For a soft-boiled M egg, the time is 26 seconds too short from the fridge and 38 seconds too short at room temperature. So we use the exact solution of the heat equation for a sphere, with the first 200 terms.

The values for the egg are the ones Williams used: specific heat 3700 J/(kg·K), density 1038 kg/m³ and thermal conductivity 0.54 W/(m·K). We take the boiling point of water as 100 °C minus the altitude in meters divided by 285. At 900 m, water boils at about 96.8 °C.

Each type of egg has one target temperature for the edge of the yolk: 63 °C (145 °F) for soft-boiled, 73 °C (163 °F) for jammy and 83 °C (181 °F) for hard-boiled. 63 °C is Williams’ target for soft-boiled. 73 °C gives roughly the times that Norwegian and international recipes use for a jammy egg, about 7 minutes for an egg straight from the fridge: the yolk is firm at the edge and thick like jam in the middle. 83 °C is chosen so the middle of the yolk sets too: in the calculation it reaches at least 70 °C (158 °F), counting the heat that keeps moving inward after the egg comes out of the water. For an M egg at sea level, that gives 7:00 for jammy and 10:00 for hard-boiled straight from the fridge, and 6:00 and 9:00 at room temperature. We haven’t adjusted any of the targets to our own measurements yet, so the times may change. Physics does the rest: other sizes, eggs from the fridge or at room temperature, and altitude.

The time is shown in steps of 15 seconds. The calculation gives the time to the second, but it isn’t that accurate: eggs in the same size class weigh different amounts, and that alone can move the time by more than 15 seconds. The timer uses exactly the time you see.

If the time would be shorter than 1 minute 30 seconds or longer than 30 minutes, or if the water can’t get hot enough at the altitude you’ve chosen, the app shows a message instead of a time.

At altitude

The additions apply to M eggs straight from the fridge in boiling water, compared with the time at sea level. They’re calculated before the time is rounded to 15 seconds, so the time the app shows may differ by a few seconds. A soft-boiled M egg, for example, is 5:15 at 300 m, compared with 5:00 at sea level.

With cold water, the water boils a little sooner at altitude on the same stove, but at a lower temperature. The egg still needs more time, and the app takes that into account.

Cold water

You can also put the eggs in cold tap water and turn on the stove. The clock then starts when you turn on the stove, not when the water boils. That’s one tap, and you don’t have to judge when the water boils. The time includes heating the water. The egg heats the whole time while the water gets warmer, and the edge of the yolk should reach the same target temperatures as in boiling water.

We assume that tap water is about 10 °C and that it boils after about 4 minutes. That fits a small pot, 14–16 cm across the bottom, with the eggs in one layer and water just over them, on the highest normal setting of an induction stove, with no lid. With a weaker stove or a bigger pot, the water may take longer. Both are estimates, and neither has been measured.

The time is longer than in many recipes because the heating is included. Norwegian recipes with cold water count from when the water boils, and none we found count from when the eggs go in. So the app’s time is usually 2–3 minutes longer than in those recipes. For an M egg straight from the fridge at sea level, you get 7:00 for soft-boiled, 9:00 for jammy and 12:00 for hard-boiled, compared with 5:00, 7:00 and 10:00 in boiling water. That’s about 2 minutes more, and according to the calculation, the egg turns out the same.

According to the calculation, the number of eggs matters little for the heating when the water stands just over the eggs. That hasn’t been measured. Several Norwegian sources say eggs crack less often when they start in cold water. We haven’t checked that ourselves.

If you need a fully cooked egg, boiling water is more predictable than cold, because the result with cold water depends more on the pot and the stove. Check in any case that the yolk is firm all the way to the middle.

Why our times may differ from other advice

Many recipes give a range, such as 5–6 minutes for soft-boiled, without saying anything about the size, the temperature of the egg, or whether the water should be at a rolling boil or a simmer. A range of one minute is a lot for a soft-boiled egg. Some recipes let the water simmer, while the app, with boiling water, assumes a rolling boil. Simmering water is cooler, so the egg needs longer. Our times for jammy and hard-boiled eggs are within the range of many Norwegian recipes, or a little below. Soft-boiled is often shorter than in recipes, so a soft-boiled egg may turn out a little softer than with many of them. We give one time for the egg you have, and we tell you it’s calculated, not measured.

Many eggs in the pot

In boiling water, the number of eggs, the pot and the stove matter little for the cooking time itself, as long as the water returns to a rolling boil quickly after the eggs go in. By our calculations, twelve eggs in 2 liters of water change the time by less than 10 seconds when the stove is on full heat. With full heat, water that covers the eggs is enough, for up to 12 eggs. If your stove is weak, cook no more than 6 eggs at a time.

What “Calculated” means

Every time in the app is calculated with the model above. We haven’t cooked and measured them ourselves yet, so we don’t know how well they match your kitchen. If your egg is too soft or too firm, answer “How did your egg turn out?” after the alarm, and the app adjusts the time for your next egg of the same type. If you need a fully cooked egg, check that the yolk is firm all the way to the middle.

With cold water, two more assumptions come in, and they haven’t been measured either: that tap water is about 10 °C, and that the water boils after about 4 minutes.

What the time assumes

• With boiling water, the water is at a rolling boil when the egg goes in, and the whole time after. Don’t turn down the heat. If the water simmers at 98 °C, a soft-boiled M egg needs about 12 seconds more, and at 96 °C about 25 seconds more.
• With boiling water, the egg is lowered into water that covers it. An egg that sticks out of the water cooks more slowly at the top. If the egg starts in cold water, choose Cold in the app, because the time is different.
• With cold water, the water boils after about 4 minutes. The stove stays on the same setting the whole time, also when the water boils. If the water takes 1 minute longer, the egg needs about half a minute more, and with the app’s time the egg turns out a little softer. If it takes 6 minutes, for example with a bigger pot or a weak stove, the egg is missing a little over a minute, and a hard-boiled egg can be soft in the middle.
• The egg goes straight into cold water when the time is up, soft-boiled too. Otherwise it keeps cooking inside.
• Straight from the fridge means about 5 °C. Room temperature means the egg has been out of the fridge for at least 2 hours. An egg that has been out for half an hour is still cold inside, about 11 °C.

“How did your egg turn out?”

After the alarm, you can answer Too soft, Perfect or Too firm. Too soft gives your next egg of the same type 15 seconds more, and Too firm gives it 15 seconds less, up to 45 seconds either way. Kitchens differ, and this is how the app adapts to yours. Your answer is stored only on your phone and never changes the times for anyone else. Boiling and cold water each have their own adjustment, so an answer after cold water doesn’t change the time for boiling water. You can reset the adjustment next to the time.