You should calculate the money within your possession and pay Zakah upon the passage of a Hawl from the time of this calculation

Q: We are a medical center for treating eye diseases. We have liquid cash both in our safe and in the bank, from which we spend on our needs, including the expansion of the center, and purchase of medical devices. These sums are so variable that it is impossible to determine the passage of a Hawl (one lunar year calculated from the time a property reaches the minimum amount upon which Zakah is due) on them in order for us to pay the Zakah (obligatory charity). For example, the funds for the last year are as follows:The first month was 740,000 SRThe second month was 730,000 SRThe third month was 675,000 SRThe fourth month was 615,000 SRThe fifth month was 605,000 SRThe sixth month was 130,000 SR (Part No. 8; Page No. 230

A: If the reality is as you have just mentioned i.e. you obtain subsequent sums from your work in the center, which makes it difficult for you to determine the Hawl of each sum separately, it is preferable and easier for you to calculate the money within your possession and pay Zakah upon the passage of a Hawl from the time of this calculation. The rate of Zakah that must be paid is one-quarter of one-tenth, i.e., 2.5%.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.