Generally breast cancer is divided into four stages, from stage 1, which is small and localised, to stage 4, where the disease has spread to other parts of the body. If the cancer has spread to distant parts of the body, this is known as secondary or metastatic cancer.

A commonly used staging system is described below:

Stage 1 The tumour measures less than 2cm (¾in) across. The lymph glands in the armpit (axilla) are not affected and there are no signs that the cancer has spread to somewhere else in the body.
Stage 2 The tumour measures 2–5cm (¾–2in) across, or the lymph glands in the armpit are affected, or both. However, there are no signs that the cancer has spread further.
Stage 3 The tumour is larger than 5cm (2in) across, and may be attached to surrounding structures such as the muscle or skin. The lymph glands are usually affected, but there are no signs that the cancer has spread beyond the breast or the lymph glands in the armpit.
Stage 4 The tumour can be of any size, but the lymph glands are usually affected and the cancer has spread to other parts of the body, such as the bones or lungs. This is secondary or metastatic breast cancer.