So exactly how much dragon blood do you need for the half-dragon ritual?

[Greywander]

So I decided to look into what might be involved in becoming a half-dragon by way of the ritual of bathing in dragon blood.

This Reddit thread (https://www.reddit.com/r/UnearthedArcana/comments/64gdkb/homebrew_halfdragon_blood_ritual_concept_5e/) gives us a starting point by postulating what such a ritual might look like. While the details listed there differ between different types of dragons, the common factor is an expensive bathtub filled with dragon blood that you nap in for an hour. So the question then would be, just how much dragon blood do we actually need, and can a dragon safely provide enough blood without major side effects from blood loss?

According to one search result, the average bathtub holds anywhere from 80 to 110 gallons, so we're probably safe to say that we need an even 100 gallons of blood.

The human body is about 7 percent blood. Another search result says elephants have 10 percent of their body weight in blood. Since dragons aren't real, we're going to have to do some guesswork here.

According to another Reddit thread, referencing the Draconimicon, a Huge dragon weighs roughly 20,000 lbs., and larger dragons can push over a million pounds.

Another search says that people can lose about 14% of their blood without serious side effects.

A gallon of water is about 8.35 pounds.

I think this is all the pieces we need, so let's do some math.

An adult dragon weighing 20,000 lbs. would have about 2000 lbs. of blood, or about 240 gallons of blood. 14% of this is 33.5 gallons. Ergo, you'd need three adult dragons contributing to your bathtub if you didn't want to harm them. Killing the dragon would let you turn two people into half-dragons.

An ancient dragon weighing one million lbs. would have about 100,000 lbs. of blood, or about 11,976 gallons of blood. 14% of this is 1676.6 gallons. Ergo, you could turn up to 16 people into half-dragons without harming the ancient dragon. Killing the dragon would get you close to enough blood for 120 people to become half-dragons.

I think the take away from this is that if you want to become a half-dragon via ritualistically bathing in dragon blood, you're going to need to either kill an adult or larger dragon, or get an ancient dragon to willingly give you blood. Anything smaller requires using multiple dragons, which is a problem if the blood loses potency after too long (unless you can get the dragons in one place).

[Mongobear]

How much do you need?

As much as the DM says you need.

A full tub, enough to actually submerge yourself would work, but so would a 5 gallon bucket you pour over yourself while performing a ritual. I've had a DM mention n that he would allow as little as ~1 gallon if given willingly and the ritual is performed perfectly.

It all depends on the DM.

[Greywander]

True, as far as I'm aware, there's no official ritual, so any ritual to become half-dragon will need to be homebrew or DM fiat.

Still, it seems like something that shouldn't be super easy to do. This could either be because dragons aren't too keen on creating half-dragons (at least, via a blood ritual), or it just requires a lot of blood (more than a typical dragon can safely provide). Of course, you could slap other restrictions on it, too, like only being able to do in at certain times (e.g. a new moon, specific holidays, a blood moon, etc.) or requiring rare and elaborate artifacts.

In a way, it reminds me of the ritual to become a lich, except I think liches are a lot stronger so the ritual is necessarily more difficult. But you don't want it to be so easy that all the PCs end up becoming half-dragons (at least, for optimization reasons alone; if it's intentionally part of the narrative, that's fine).

[Sigreid]

I would require a willing dragon participating in the ritual and accepting you as its child.