How do you quantify autonomy?

When in a debate about abortion or murder, I'll generally use autonomy as a value. I will generally assume that, objectively, an adult has more autonomy than a child, a child has more autonomy than an unborn, and that humans have greater autonomy than animals. When talking about autonomy, how do you "quantify" it? Not necessarily in a metric sense, but how you'd scale it.

Acer: Since Autonomy is generally synonymous with “Independence”, I’d generally agree with your rankings. However context may subtly change the perspective.
For example, in certain contexts, Autonomy could be seen as a RIGHT to independence, a possibly as yet unrealised potential for Autonomy that therefore must be respected as though it were a realised thing today.
If that were the case, then your rankings may not make sense anymore.

It should also be noted that Autonomy is not necessarily inherent. It can be given and taken away.
So it’s not as simple as Adults have more autonomy than children simply because they’re adults. The fact that they’re adults really doesn’t have anything to do with it. It’s more that they’ve earned, or assumed or been given their Autonomy.
Case in point, consider an adult prisoner, or an elderly person no longer able to drive. Contrast their Autonomy with that of a wild 30 foot saltwater crocodile. Who has more autonomy there? I’d argue the saltie has more than either.

Finally, I would be careful not to confuse “has more” autonomy with “their Autonomy should be prioritised over”.
When it comes to Humans v Animals, it seems pretty clear that by a dictionary definition, both animals AND humans have the same capacity for autonomy. I suspect what YOU’RE saying is that where those two Autonomies come into conflict, the Human’s is/should be prioritised over the animal’s.
I wouldn’t necessarily disagree, but you need to be careful to acknowledge that that’s just an opinion of how things should be, not necessarily an objective truth.

Acer

A prime and necessary indicator or autonomy is a free will.

Without the ability to exercise ones free will, there can be no autonomy. Right?

Regards
DL