A buffer works within one pH unit of the pKa of its weak acid, where pH = pKa + log([salt]/[acid]): a 0.1 mol/L ethanoate buffer sits at pH 4.76, and 10 mmol of hydrochloric acid moves it only to 4.58, against a fall from 7 to 2.00 in water.
The weak acid and its salt are both there to soak up added base or acid, so the ratio changes only a little. The capacity is greatest when they are equal, at pH = pKa, and is 2.3 times the concentration divided by 4: about 58 mmol per litre per pH unit for 0.1 mol/L. At the half-way point of a titration of a weak acid the pH equals the pKa for the same reason.
The equation is for a buffer much stronger than the acid and base it must soak up: add more than about its own concentration and the pH runs away. It also assumes ideal solutions, and the pKa shifts with temperature and with the salt content, which is why a buffer is made up and checked with a meter.
Estimate with the rule, then check it against the calculator that models it properly.
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