Bag of Holding Fish Tank

How long can a sahuagin live in a water-filled Bag of Holding?

In my last session, my players took a sahuagin captive. Intent on returning their captor to Saltmarsh for inquisition, they bound him, filled their Bag of Holding with water, and deposited the fish man inside. When they poured out the contents back in town, their captive unceremoniously flopped out dead. It had been 10 hours.

The Argument

The 5th Edition Dungeon Master’s Guide helpfully instructs that there’s 10 minutes of air in a Bag of Holding. This is an oversimplification, but in the case of one air-breathing creature, they can last 10 minutes.

To me, it didn’t matter whether the bag was filled with air or water; any amphibious creature inside had 10 minutes to breathe. My players protested this rationale, drawing an analogy to a fish tank. Convincing, but I figured this precise situation as more of a goldfish in a bag.

Since the players didn’t drive their new pet home fast enough, we agreed the sahuagin was most certainly dead after 10 hours. But, it got me thinking…

How much air is in a water-filled Bag of Holding?

For simplicity’s sake, we’re going to make several assumptions which might not directly apply to our fantasy scenario. Specifically, we’re assuming fresh water at sea level at room temperature. As you can see from this chart, altitude and temperature have an impact on the oxygen saturation of water.

I did some research to answer the following questions:

• How much water can the Bag of Holding hold?
• How much oxygen is in water?
• How much oxygen does a fish breathe per hour?
• How much oxygen do fish need to breathe?

How much water can the Bag of Holding hold?

This question is easy because we rip the answer from the D&D rules. A Bag of Holding can hold no more than 64 cubic feet of items. That’s roughly 1812 liters, which will be important because the rest of this stuff is measured in metric.

Edit: Thanks to Teulisch for pointing out Bag of Holding’s 500 pound weight limit. Water weighs a lot, which greatly limits the volume of your fish tank.

How much oxygen is in water?

Oxygen saturation in water is measured in parts per million (ppm). At standard temperature elevation, fresh water is generally 9.1 ppm of oxygen.

Thankfully, ppm is just a fancy way of saying milligrams per liter (mg/l). Generally, lower temperatures allow the water to hold more oxygen. This has a doubling effect, as creatures also consume more oxygen in warmer water.

How much oxygen does a fish breathe per hour?

Fish farming website Aquapona suggests that trout consume around 200-250 milligrams of oxygen per hour for each kilogram the fish weighs (mg/kg/hr).

How much oxygen do fish need to breathe?

The U.S. Fish and Wildlife Service indicates that most fish begin to suffer at less than 5 ppm, and suffocate at less than 2 ppm.

The Monster Math

We need to find out how long it’s going to take the Sahuagin to breathe the water in the Bag of Holding down from 9 ppm to 2 ppm. So, let’s just treat the zero on the oxygen meter as 2 ppm and the starting oxygen as 7 ppm.

Assuming No Weight Restriction

A Bag of Holding holds 1812 liters of water. At 7 effective ppm, those 1812 liters of oxygen have 12,684 mg of oxygen inside. We’re being generous here since we’re not accounting for the fact that the sahuagin takes up space inside the bag.

Assuming sahuagin are a normal humanoid size, the Player’s Handbook places their average weight at 165 pounds. That’s roughly 75 kilograms. So a sahuagin will consume at least 15,000 to 18,750 mg of oxygen per hour.

So, under normal circumstances, that sahuagin will be dead in under an hour. Specifically, they’ll last just over 50 minutes at low oxygen consumption before dropping into suffocation (see Player’s Handbook p. 182).

Accounting for Weight

The Bag of Holding capacity is 500 pounds. Water weighs a lot: 62.4 pounds per cubic foot. Sahuagin is humanoid size so let’s take human weight which is 165 average. You can fit 5.4 cubic feet of water in the remaining 335 pounds. That’s 153 liters, which is 1377 mg of dissolved oxygen. A 75 kilogram sahuagin will consume at least 15,000 mg of oxygen per hour, so the oxygen only lasts 9% of an hour, which is 6.5 minutes.

This is notably less than the breathable air available in a Bag of Holding. But, it doesn’t really make sense that an amphibious creature would die quicker because you added water to the bag. We can only conclude that the extradimensional space is hyperoxygenated.

If you’re not satisfied to pick a time between 10 minutes and 1 hour, you can calculate the ratio of actual breathing time:10 minutes::water breathing time:5.4 minutes. ThinkDM does not recommend calculating actual breathing time by putting your players in a 64 cubic foot ziploc.

Oxygen Consumption

Sahuagin are no 20-pound lake trout. Realistically, it’s my understanding that the energy required for larger systems is not linear, but either geometric or exponential.

Let’s round the sahuagin down to 160 for simplicity’s sake. If you double the size of the trout (40), then double the size of the trout again (80), then double the size of the trout again (160), you’re roughly at sahuagin size.

If you square the oxygen consumption for each time you double the weight, you wind up with an astronomical oxygen consumption range. Do you have any other way to calculate the time to breathe? Post your answers in the comments below!