Let us express the process of calculating approximate percentage price change for a given change in yield and a given duration using the following formula:
Approximate percentage price change = - duration × Δy ×100 ...Eq. (2)
As an inverse relationship exists between the price change and yield change, duration is preceded by a minus sign.
For example, let us take a 20-year bond with 8.5% coupon rate, currently trading at Rs.115.89 whose duration is 10.22. The approximate percentage price change for a 20 basis point increase in yield (i.e., Δy = +0.002) is
Approximate percentage price change = -10.22 × (+0.002) ×100 = -2.044%