When designing ideal reactors it is possible to use experimental rate data to calculate the volume required. Based on conversion the reactor volume can be given by,
| CSTR | PFR |
| $$V=\frac{F_{A,0}}{-r_A}X$$ | $$V=\int_0^X\frac{F_{A,0}}{-r_A}\text{d}X$$ |
This means that if $F_{A,0}/-r_A$ is plotted against the conversion $X$ then the volume of a CSTR is given by the rectangle from the exit conversion to the inlet conversion, whereas the volume of a PFR is given by the area under the curve.
The graph below shows the volume of 2 reactors in series. For a PFR the total volume is always the same, but sensible selection of the intermediate conversion for the CSTRs can reduce the total reactor volume.