Round when it simplifies the calculation without materially changing the business conclusion. Keep more precision when the result is close to an important threshold or comparison.

Case math is usually about making a sound business decision, not producing unnecessary decimal precision. Carrying too many digits can increase error risk and distract from the insight.

If a market estimate is roughly $502 million, using $500 million may be perfectly adequate. If the client needs a margin above 20% and your result is 20.2%, rounding too aggressively could change whether the target is met.

Before rounding, ask how precise the decision needs to be. State the approximation when it matters, keep units visible, and sanity-check whether the rounded result still supports the same conclusion. If useful, this guide on case interview mental math gives more detail.

Rounding should follow the decision. If 20% is a cutoff, keep more precision.