Interactive formula for poached eggs

A poached egg is a boiled egg without its shell: the same physics, but the heat goes straight into the white, the yolk sits closer to the surface, and the water must not boil. The calculator above works out the time for your egg, the yolk you want and the water temperature you poach at – and it allows for the little trick that makes all the difference: scalding the egg with boiling water before it goes into the pan. If you'd rather keep the egg in its shell, use the formula for boiled eggs.

How to do it

Fill a pan with at least 6–7 cm of water and heat it to 85–90 °C: small bubbles on the bottom, no rolling boil. A thermometer is easiest; otherwise bring it to the boil and turn down until the surface goes still.
Set the egg's weight – it's on the carton, or weigh the egg. The calculator deducts the shell itself; if you weigh the cracked egg instead, switch Weighed with shell off. Crack the egg into a cup or small bowl. Use the freshest egg you have – the thick white holds together around the yolk better.
The trick: add a tablespoon of vinegar to the cup and pour boiling water from the kettle over the egg until it is covered. Wait about 15 seconds. The thin, watery part of the white turns milky and sets as a skin around the egg, while the vinegar helps the proteins firm up faster. The result is an egg that already has its shape before it even touches the pan – no wisps, no foam, and you can't taste the vinegar.
Carefully pour most of the water out of the cup, hold the cup right down at the water's surface and let the egg slide into the pan. Press Start timer.
Lift the egg out with a slotted spoon when the timer rings and let it drain on kitchen paper. If it has to wait, it can sit in cold water and be reheated for 30–40 seconds in hot water just before serving.

What happens in the pan

The outermost white meets the water directly and sets within seconds, while the yolk in the middle warms up more slowly. The white starts to firm at 62 °C and is set at 65 °C; the yolk thickens from 65 °C, is jammy at 67–68 °C and fully firm at around 76–80 °C. Poached at 85–90 °C rather than in boiling water, the white turns soft and silky instead of rubbery, and the bubbles don't tear the egg apart.

A poached egg is lower than it is wide, and the yolk sits just under the surface. That is why it cooks faster than a boiled egg in its shell even though the water is cooler: less mass to heat, no shell, and a shorter path to the yolk.

The formula

The starting point is the same model as for boiled eggs – Charles D. H. Williams' formula for a sphere heated from the surface – with three adjustments:

t = k \cdot 0.451 \cdot {(0.88 \cdot M)}^{\frac{2}{3}} \cdot \ln [0.76 \cdot \frac{T_{egg} - T_{water}}{T_{yolk} - T_{water}}] - t_{cup}

0.88 · M is the egg's weight without the shell. The shell is about 12 % of a hen's egg, and it doesn't go in the pan.
k = 0.60 is a shape factor. Williams' formula describes a sphere; a poached egg is a flat cushion with the yolk close to the surface, so the heat gets there faster. The factor is tuned to practice – a medium egg from the fridge at 88 °C gives about 3½ minutes for a runny yolk and 4 minutes for jammy, which matches what cooks hit with a stopwatch.
T_water is now a parameter instead of the boiling point. The cooler the water, the longer it takes – at 80 °C about 20 % longer than at 90 °C.
t_cup is the 15 seconds the egg spends in boiling water in the cup. They count as cooking, so the calculator subtracts them when the trick is switched on.

The weight is on the carton – the EU classes are weight classes – or read off kitchen scales. If you weigh the egg in its shell, the calculator deducts the 12 % itself; if you weigh the cracked egg in the cup, switch the toggle off and get the exact mass directly.

Size (EU)	Weight with shell	Typical	Without shell
S	under 53 g	48 g	≈ 42 g
M	53–63 g	56 g	≈ 49 g
L	63–73 g	68 g	≈ 60 g
XL	over 73 g	78 g	≈ 69 g

Altitude plays no part here as long as you stay below the boiling point: 88 °C can be reached up to about 3,500 metres.

Vinegar, vortex and freshness

Vinegar lowers the pH, so the proteins in the white set faster and at a lower temperature – which is why the cup trick works so well. A tablespoon in the cup is plenty; in the pan itself it isn't needed, and too much gives a sour taste and a dull surface.

The vortex – stirring the water and dropping the egg into the middle – gathers the white around the yolk in the pan. It's redundant once the egg has been scalded in the cup, and it only works for one egg at a time.

Freshness matters more than anything else. The thick white closest to the yolk gets thinner and more watery with age, so a two-week-old egg spreads out like a fan in the pan. An egg under a week old holds its shape almost by itself. If your eggs are older, crack them into a fine sieve and let the thin white drain off before scalding.

Several eggs at once: give each egg its own space and add them a couple of seconds apart, so you can take them out in the same order. Four eggs drop the water temperature by a couple of degrees – turn up a little, or add 10–15 seconds.

How accurate is it?

The physical core is Williams' model, but the shape factor is empirical, and a poached egg is less uniform than a boiled one: the white may lie thicker on one side, and the egg may sit askew on the bottom. Allow ±20 seconds and feel it with the slotted spoon: the yolk should give like a small water balloon. Too loose? Add 20 seconds next time.

Sources

Charles D. H. Williams, The Science of Boiling an Egg, University of Exeter – the model behind the formula.
Harold McGee, On Food and Cooking (Scribner, 2004) – coagulation, acid and the age of the egg.
Cook's Illustrated, Perfect Poached Eggs – tests of vinegar, water temperature and the sieve method.