$\%\,\text{yield} = \dfrac{m_a}{m_t} \times 100$ — what you got, over what the paper promised, times one hundred. Keep the flask's number on TOP. Both entries are masses of the same product, so the grams cancel completely: a percent yield is a pure number, and it travels without units, like the coefficients of friction do in physics.

The classic slip is flipping the ratio — and it diagnoses itself: a yield past 100 %. Stoichiometry never pays out more than it predicts, so a 125 % yield is not a triumph — it is your fraction upside down (or, at the bench, a product still wet with solvent, quietly padding the balance). Above 100, suspect the ratio first and the drying oven second.