The Black model volatility expresses the size of percentage changes in a rate, in %, while normal volatility expresses the size of the change in the rate itself. With forward rate FF and Black model volatility σB\sigma_B, the two are linked by the approximation σN≈FσB\sigma_N \approx F \sigma_B. Consider a hypothetical example in which the forward rate is 1.00% and the Black model volatility is 30%. Normal volatility is about 30bp.

This link is only an approximation, and the gap widens as the strike moves away from the forward. Also, because the level of the forward enters the conversion, normal volatility can fall when rates are falling, even while the Black model volatility is rising.