acid base - Calculating Buffer Capacity

I completed a titration of Ammonium Acetate buffer solution and to it I added $\pu{2M}$ Hydrochloric Acid.

I measured the initial $\ce{pH}$ of the buffer solution before any acid was added and I used methyl orange as an indicator which has a $\mathrm{p}K_\mathrm{a}$ value of 3. My independent variable was the temperature of the buffer.

I was just wondering how I would calculate the buffer capacity. Is it $\Delta\ce{pH}$/Volume?