RVTools
The fastest route to a directional answer, and the one whose single limitation matters most.
What it is
A free utility that produces a multi-tab Excel export of a vCenter environment. Not a vendor platform, not a subscription, and not something you need to procure. Most VMware administrators already have it, and AWS tooling accepts the export directly.
How we use it
We use it to scope, not to conclude. It tells us the shape and size of the estate quickly enough to have a sensible first conversation, and it identifies what utilisation data we still need before anything is priced as a recommendation.
One thing to watch: RVTools reports what is provisioned, not what is used. A business case built on an export alone sizes the target environment to specification, which is precisely the over-provisioning that right-sizing exists to remove. The gap is not theoretical. On one of our own engagements a fibre network operator showed 332 TB provisioned against 184 TB actually used. It is also VMware-only, so anything on Hyper-V, on bare metal or already in a cloud is invisible to it.
Worth knowing: RVTools is now published by Dell, and Dell name downloads.dell.com as the only authorised download location.
Official resources
We would rather point you at the vendor than paraphrase them. What we add is what happens when you run it against a real estate.
An export is enough to start.
Send us an RVTools file and we will come back with the shape of the estate, what it suggests, and what we would want to measure before putting a number on it.