Stop guessing. This calculator applies the Williams physics equation — derived by Exeter physicist Charles D. H. Williams in The Science of Boiling an Egg — to your exact egg: weight, starting temperature, target doneness, altitude, and cold-start vs boiling-start. Then run the built-in countdown timer and ice-bath timer.
Move the eggs straight into ice water to stop carryover cooking.
| Doneness | Yolk-boundary target | Time |
|---|---|---|
| Soft (runny) | 63 °C | 4:36 |
| Jammy (custardy) | 66 °C | 5:11 |
| Medium (just set) | 70 °C | 6:02 |
| Hard (fully set) | 75 °C | 7:16 |
| Egg weight | Time |
|---|---|
| 43 g | 4:56 |
| 50 g | 5:28 |
| 57 g | 5:58 |
| 64 g | 6:26 |
| 71 g | 6:54 |
| Altitude | Water boiling point | Time |
|---|---|---|
| 0 m | 99.8 °C | 7:16 |
| 1,000 m | 96.5 °C | 8:01 |
| 2,000 m | 93.1 °C | 8:55 |
| 3,000 m | 89.7 °C | 10:04 |
The engine implements Williams' result verbatim: the egg is treated as a homogeneous sphere of mass M, and the cooking time is when the white-yolk boundary reaches the target temperature Tyolk, with water at Twater and the egg starting at Tegg. Constants are Williams': density 1.038 g/cm³, specific heat 3.7 J/g/K, thermal conductivity 5.4×10⁻³ W/cm/K.
It depends on the egg's weight, its starting temperature, how set you want the yolk, your altitude, and whether you start with boiling or cold water. As one example: a large 58 g egg straight from the fridge (4 °C), lowered into already-boiling water at sea level, needs 6:02 for a medium-set yolk. Use the calculator above for your exact combination — it applies the Williams physics equation rather than a one-size table.
The calculator uses the equation derived by Charles D. H. Williams (University of Exeter) in 'The Science of Boiling an Egg': t = [M^(2/3) · c · ρ^(1/3)] / [K · π² · (4π/3)^(2/3)] · ln[0.76 · (T_egg − T_water) / (T_yolk − T_water)], with ρ = 1.038 g/cm³, c = 3.7 J/g/K and K = 5.4×10⁻³ W/cm/K. It models the egg as a homogeneous sphere and solves for when the white-yolk boundary reaches the target temperature. Williams verified it against real eggs: a 57 g fridge egg takes about 4.5 minutes to soft-boil, exactly what the formula predicts.
For a runny yolk with a just-set white (yolk boundary 63 °C): a large 58 g egg from the fridge needs 4:36 in boiling water at sea level. The same egg at room temperature (20 °C) needs only 3:22. Smaller eggs scale down — a 43 g small egg from the fridge is done in about 3:46.
A jammy egg — set white, thick custardy yolk that still flows slightly — targets a yolk-boundary temperature of 66 °C. A large 58 g fridge egg needs 5:11 in boiling water at sea level. Jammy is the narrowest target of the four, so use the timer and move the eggs to an ice bath the moment it rings.
For a fully set yolk (boundary 75 °C): a large 58 g fridge egg needs 7:16 in boiling water at sea level. The traditional rule of thumb — twice the soft-boil time — gives 9:13, a little longer and a little firmer. If you want a completely chalky, dry yolk, add about 2 minutes past the calculator's hard time.
Yes. Water boils at a lower temperature as air pressure drops, so heat transfers into the egg more slowly. At 3,000 m water boils at only 89.7 °C, and a large fridge egg needs 10:04 for hard-boiled versus 7:16 at sea level. One honest caveat from Williams himself: at altitude the equation slightly under-predicts real times, because protein coagulation is a rate process, not an instant switch at one temperature — so add a small margin when cooking on a mountain.
Boiling-start (lowering eggs into already-boiling water) is more predictable and is what the Williams equation models directly. Cold-start — eggs in cold water, then bring to a boil — adds the water heat-up time: for a large fridge egg to hard-boiled with 1.5 L of water on a 2,000 W burner, the total from the moment you turn on the heat is about 11:09, of which roughly 5:58 is just heating the water (the egg does absorb some heat during the ramp, which the calculator credits at 35%).
Yes — a lot. For a soft-boiled large 58 g egg at sea level, fridge-cold (4 °C) needs 4:36 while room-temperature (20 °C) needs only 3:22: a 74-second difference. This is the single biggest variable after size, and it is why fixed recipe times fail — always note your eggs' starting temperature.
No. Each egg heats independently, so the time per egg is unchanged whether you boil one or six. What changes is logistics: use a bigger pot with enough water that the temperature doesn't crash when the cold eggs go in, and keep the water at a steady boil.
Carryover cooking: the hot shell and outer white keep driving heat into the yolk after you drain the pot, so a perfect jammy egg can turn hard in minutes. An ice bath (ice + water, not just cold tap water) halts cooking immediately and also makes peeling easier. Three minutes in the ice bath is enough for most eggs — the calculator includes an ice-bath timer.
That grey-green film is ferrous sulfide: above roughly 70 °C, hydrogen sulfide released from the white reacts with iron in the yolk. It is harmless but looks unappetizing and tastes sulfurous. It forms when eggs are overcooked or cooled slowly — which is exactly why the calculator's hard target stops at a 75 °C yolk boundary and why the ice bath matters.
Two reasons. First, the Williams equation solves for the white-yolk boundary temperature; the yolk center lags slightly behind, so the calculator's 'hard' is firmly set but still moist — add ~2 minutes for a fully chalky yolk. Second, many recipes assume cold-start or older, larger eggs, or simply pad the time. The honest trade-off: the equation is validated physics (within seconds on Williams' own test eggs), but coagulation is gradual, not instant — treat the result as a precise starting point and adjust once to your taste.
Physics: C. D. H. Williams, "The Science of Boiling an Egg", University of Exeter. Doneness gradient and cold-start credit include documented estimates as labeled. Not affiliated with the University of Exeter.