Simmer vs Boil
Boiling is liquid at its boiling point, 100 °C (212 °F) for water at sea level, with large, vigorous bubbles. Simmering is just below that, roughly 85–96 °C (185–205 °F), with small bubbles that rise steadily and gently. The few degrees between them make a big difference to texture.
Reading the pot
| Stage | Temperature | What you see | Used for |
|---|---|---|---|
| Poach | 70–82 °C / 160–180 °F | Steam, shimmer, almost no bubbles | Eggs, fish, chicken |
| Slow simmer | 82–88 °C / 180–190 °F | An occasional lazy bubble | Stocks, consommé |
| Simmer | 85–96 °C / 185–205 °F | Small, steady bubbles | Stews, braises, sauces, soups |
| Full boil | 100 °C / 212 °F | Big bubbles, churning surface | Pasta, blanching, reducing |
| Rolling boil | 100 °C / 212 °F, high heat | Boil that keeps going when stirred | Pasta, sterilizing jars |
Note that a rolling boil is not hotter than a gentle boil. Once water reaches its boiling point, extra heat just turns more water into steam faster. Salt and sugar raise the boiling point slightly; 1 tbsp salt in 4 L of water raises it by well under 1 °C.
Why simmer
Tender meat. Muscle proteins tighten and expel moisture as they heat. At a hard boil, the outside of a stewing cut overcooks long before collagen in the centre breaks down. At a simmer, collagen has time to dissolve into gelatin while the fibres stay relatively juicy.
Clear stock. Boiling agitates fat and protein particles into tiny droplets that stay suspended, making stock cloudy and greasy. A slow simmer lets impurities rise for skimming.
Intact food. Beans, potatoes, dumplings, and fish break apart when tumbled by violent bubbles.
Less scorching. Thick sauces and soups burn on the bottom at a hard boil.
Why boil
Pasta. A large pot at a full boil recovers its temperature quickly after the pasta goes in, so the surface starch sets fast instead of turning gluey.
Blanching green vegetables. Speed matters: the faster the vegetables heat, the brighter their colour and crisper their texture.
Reducing. Boiling evaporates water fastest when you want to concentrate a sauce or syrup.
Altitude
Water boils about 1 °C lower for every 300 m of elevation. At 1,500 m it boils at about 95 °C / 203 °F, which is a simmer at sea level. Food cooks more slowly, so add time to pasta, beans, and boiled eggs, and use a lid to help hold heat.
Holding a simmer
Bring the pot to a boil first, then lower the heat. A heavy pot and a partial lid make the temperature far more stable. A thermometer is the fastest way to calibrate your eye.
Questions
What does a simmer look like?
Small bubbles rise steadily from the bottom and break gently at the surface, with wisps of steam. A rolling boil, by contrast, has large bubbles that churn the whole pot and do not stop when you stir.
What heat setting is a simmer?
There is no fixed dial number. Bring the pot to a boil, then turn the burner down to low or medium-low until the bubbles slow to a gentle, steady stream; adjust as the pot settles.
Why simmer stew instead of boiling it?
Hard boiling squeezes moisture out of meat fibres, making them dry and stringy, and churns fat into the liquid so it turns cloudy and greasy. A gentle simmer gives collagen time to melt into gelatin while the meat stays moist.
Does water boil at a lower temperature at high altitude?
Yes. It drops about 1 °C for every 300 m (roughly 2 °F per 1,000 ft). At 1,500 m water boils at about 95 °C / 203 °F, so simmered and boiled foods take longer to cook.