What is the difference between simmering and boiling?

What each looks like in the pan, the temperature ranges, and why using the wrong one toughens meat and clouds stock.

Difficulty: beginner
Time: 5 min
Read: 3 min

Short answer

A simmer is around 85 to 95°C with small bubbles rising occasionally; a boil is 100°C with the whole surface in constant motion. Simmering is for anything delicate or long-cooked; boiling is for pasta, green vegetables and reducing hard.

Recipes use the words interchangeably and they should not. The physical difference is agitation as much as temperature, and it is why the same stew is silky one week and stringy the next.

Step by step

Recognise a bare simmer.Around 85°C. Occasional small bubbles breaking the surface, a slight tremble. This is where stocks, poached eggs, custards and delicate fish belong.
Recognise a proper simmer.Around 95°C. A steady stream of small bubbles from the base, the surface moving gently but not rolling. Stews, braises, pulses and rice.
Recognise a rolling boil.100°C, the whole surface in vigorous motion, large bubbles bursting continuously. Pasta, green vegetables, potatoes and reducing a sauce quickly.
Simmer anything with meat in it.Boiling agitates and contracts the muscle fibres, squeezing water out. It is the reason boiled stews turn out stringy and dry despite hours of cooking.
Simmer stock, never boil it.Boiling emulsifies fat and breaks up the protein scum, both of which cloud the liquid permanently and make it taste greasy.
Boil pasta and green vegetables.They need the temperature and the movement. A simmer cooks pasta unevenly and lets green vegetables sit long enough to lose their colour.
Boil to reduce.Evaporation is what concentrates a sauce, and it happens far faster at a rolling boil in a wide pan. There is no delicate protein at risk in most reductions.
Use the lid and the ring size to control it.Getting a stable simmer on a fierce burner is difficult. Move to a smaller ring, use a heat diffuser, or crack the lid to let heat escape.
Adjust after adding cold ingredients.Anything cold going into the pan drops the temperature. Bring it back and then reduce again rather than leaving it on high.

Tips

If the surface is rolling, it is boiling — whatever the dial says.
Boiling a stew is the fastest way to make cheap meat tough. Keep it at a tremble.
Getting a stable simmer is easier on a small ring than on a large one turned down.

Common mistakes

Boiling a stew or braise to speed it up — The temperature barely rises but the agitation increases sharply, contracting the muscle fibres and forcing moisture out. Collagen conversion depends on time, not on violence, so you get stringy meat and no time saved.
Boiling stock — It emulsifies the fat into the liquid and breaks the protein scum into particles too fine to skim, which makes the stock permanently cloudy and gives it a greasy taste.

If it doesn't work

Stew meat stringy after hours

Cause: Boiled rather than simmered — Fix: Keep it at a bare simmer — surface trembling only — and give it more time.

Stock cloudy and greasy

Cause: Boiled — Fix: Simmer gently and skim the scum in the first ten minutes. Cloudiness cannot be reversed.

Cannot keep a stable simmer

Cause: Ring too powerful — Fix: Move to a smaller ring or use a heat diffuser, and crack the lid.

Pasta cooking unevenly

Cause: Water not at a rolling boil — Fix: Bring it back to a full boil before the pasta goes in and keep it there.

Poached egg shredded

Cause: Water boiling — Fix: Bare simmer only — the surface should barely move.

Questions people ask

What temperature is a simmer?

Roughly 85 to 95°C depending on how gentle it is. A bare simmer with occasional bubbles is around 85; a steady simmer with a continuous stream of small bubbles is around 95. Boiling is 100°C at sea level.

Does food cook faster at a rolling boil?

Barely. Water cannot exceed 100°C, so a rolling boil is the same temperature as a gentle one — it just agitates more and evaporates faster. For anything containing meat that agitation is actively harmful.