esxi memory resource usage

I have 3 esxi host and host Each esxi host has 85% memory usage. I can't be sure I should get physical memory. how do I know?  these values are the value used or the value assigned to the virtual machines. I have vrealize operation manager in the infrastructure. is it right to look at the use of memory from there? some virtual machines are given 30 GB, but when I look at the vrealize it uses 20 gb. This value is where I should look at memory usage. According to it. I must  to buy memory  or I don't need

thanks

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