Simmer and boil both describe hot liquid, but the difference is intensity. Boiling is heating a liquid until it bubbles rapidly and turns to vapour, while simmering keeps it just below that point, with gentle, slower bubbles. A good memory hook is the kettle boils, but the sauce simmers.
The core difference
Boiling is vigorous bubbling at full heat, while simmering is gentler cooking just below boiling point.
- simmer — to cook gently in liquid just below boiling point: Let the sauce simmer for twenty minutes, stirring occasionally.
- boil — to heat a liquid until it bubbles and turns to vapour: Boil the water before adding the pasta.
- simmer — the state of cooking gently just below boiling: Bring the soup to a simmer, then reduce the heat.
How to tell them apart
Recipes often say to bring something to the boil, then reduce the heat to a simmer, which captures the relationship neatly: boiling is the more intense, faster state, and simmering is the gentler one you settle into for slow cooking. Watching the bubbles is the simplest test, since a rolling boil breaks the surface constantly, while a simmer shows smaller, slower bubbles.
Simmer has a figurative use too, describing anger or excitement that is barely controlled, as in resentment simmering in an office for weeks before it boils over, which shows the same intensity contrast carried into everyday language.
