The formula to calculate the volume of a digester tank with a 15% freeboard is:

\[ V_{dt} = w_m \cdot N_a \cdot T_r \cdot MR \]

Where:

• \(V_{dt}\) is the volume of the digester tank in cubic meters (m³).
• \(w_m\) is the daily manure production in kilograms per day per animal (kg/day-animal).
• \(N_a\) is the number of animals.
• \(T_r\) is the retention time in days.
• \(MR\) is the mixing ratio (1 for 1:1 and 2 for 1:2).