How long can you use your black tank without needing to dump it? It’s a common question for RV families embarking on their first dry camping trip. Nobody wants the surprise of the dreaded poop pyramid. 

By the end of this article, you will know exactly how many days you can go without dumping your black tank based on A) the size of your family, B) the size of your black tank, and C) how often you eat at Taco Bell. 

TL;DR: For most average-sized RVs with 2-3 occupants, you can go about a week before dumping your black tank. 

What’s the Comparable Gallons Per Flush of a Residential Toilet? 

Let’s begin with a reference point: the toilet in your house.

In ye olden days, house toilets used from 3.5-7.0 gallons per flush! In modern times, however, the current federal standard is 1.6 gallons per flush, while certified WaterSense toilets use 1.28 gallons per flush or less. (RVs are exempt from these requirements.)

Unfortunately, there is no direct parallel to RV toilets. As you probably know, most RV toilets are simple gravity-fed and gravity-drain models, not siphons. For most toilets, you hold the handle down with your foot or finger; you decide how much water gets flushed. We’ll talk about that more in a minute.

Why Do We Need Water in an RV Sewer System?

Let’s set some ground rules. There are three basic reasons why you need water in a sewage system:

1. Water is a liquid carrier that dissolves and carries away solid waste. Without water, solids turn into plugs, bricks, and sludge in your black tank.
2. Water is a cleaning solvent that scours the toilet bowl and the drain pipes. 
3. Water collects in the bottom of the toilet bowl, forming an odor seal (similar to a P-trap).

Don’t over-conserve water! Not to be indelicate, but you’ll get sludge instead of soup. Sewage will NOT completely drain out of the black tank, even if you have a black tank flush/rinse system. It will stick to the sides of your tank and turn crusty, decreasing your storage capacity and tricking your black tank sensors into thinking your tank is full.

Even worse, you’ll get the dreaded poop pyramid, where solids pile up directly beneath your toilet instead of dispersing throughout the tank. This is difficult (and unsavory) to remedy.

How Much Water Does an RV Toilet Use Per Flush?

> Gravity Toilets

For gravity-fed toilets: Depending on how long you hold down the handle, you’ll typically use:

• 1/10th-1/3rd (0.10-0.33) of a gallon per flush for liquid waste
• 1/3rd-3/4ths (0.33-0.75) of a gallon per flush for solid waste

Obviously, the more solids and the more toilet paper in the bowl, the longer you’ll need to hold down the handle. If you’re female, you’ll typically require more water per flush after urination, for instance, because of toilet paper. Some RVers choose to adopt the European strategy of putting toilet paper in a wastebasket rather than down the drain.

> Electric Macerator Toilets

There are also electric flush and macerator toilets, like the Dometic 8700 electric toilet. This toilet has two flush modes: Normal (0.85 gallons) and Dry Bowl (0.45 gallons). The Thetford Silence Plus 2G electric macerator toilet also has two flush buttons, with total flush volume ranging from 0.1-0.7 gallons per flush. These are just two examples.

How Long Can I Go Without Dumping My Black Tank?

Now that we’re armed with some background information, let’s run a few numbers. You’ll have to tailor these numbers to fit your family. Unfortunately, “bathroom habits” vary widely between us homo sapiens. Pooping anywhere from three times a day to three times a week is considered normal; urinating anywhere from 3-7 times a day is average.

You’ll have to run the research on your clan, but for our purposes, we’ll make some assumptions. We will also assume that you are conserving water, but you’re not measuring with a dropper. 

• 5x No. 1 (liquids) daily
• 1.5x No. 2 (solids) daily
• 0.20 gallons per flush for liquids
• 0.65 gallons per flush for solids

With the help of a little arithmetic, the average person will “contribute” at least 2.3 gallons of sewage per day. This is a fairly conservative number; you could easily use twice as much if you’re not careful. 

Hot Hint: If you’re using the toilet flush cycle to scour the toilet bowl clean, you can save water by using a handheld sprayer and a toilet scrubber (optional) instead!

After a little more math: If you have a 4-person family with a 50-gallon black tank, you could make it about 4-5 days. A couple could a little more than a week.

You should dump your black tank when it’s between 50 and 90% full – and try not to drive around with a full tank! (they’ve been known to fall out). Dump your black tank first and your gray tank second. Always add at least 2-3 gallons into your black tank after dumping and before its first use.

When Water Conservation Becomes Too Much

Some campers follow the old rule: If it’s yellow – let it mellow; if it’s brown – flush it down.

If you strictly follow this rule, you can probably cut your water usage in half – but it’s risky, and I don’t recommend this practice for longer than a boondocking weekend. Your sewage system depends on sufficient fluidity to function properly.

Plus, as water evaporates from urine at the bottom of the toilet bowl, it increases the ammonia concentration, which makes it smell like cat pee. Eww.

If you need more time than your black tank allows, either leave a spare kid at home, or invest in a portable RV waste tank, also known as a “honey wagon” or “blue boy.” You can buy as many of these as you need – just remember they are heavy and need to be transported to an approved dump station or sewer hookup. NEVER dump raw sewage on the ground, no matter where you are!

If you’re really, truly desperate for long-term water-free sewer, spend $1,800 on the latest incineration toilet, and you’ll never have to worry about water again. Or yes, it is possible to travel in a camper without a bathroom!