A composting toilet is a self-contained system that breaks down human waste into compost instead of flushing it away

You build a composting toilet by stacking a waterproof basin, a seat, a carbon layer (usually sawdust or coconut coir), and a ventilation pipe. The waste sits in the basin, you cover it with carbon material after each use, and air circulation breaks it down into usable compost over time. No plumbing, no septic tank, and no water required—which is why people install them in cabins, RVs, off-grid homes, and places where a traditional toilet isn't practical.

The basic design works the same way whether you buy a kit or build one from scratch. The difference is cost and how much time you spend sourcing parts. A DIY build typically costs $150 to $400 in materials. A commercial unit runs $800 to $3,000. Both do the same job if you follow the core principles: separate solids from liquids, add carbon after each use, and vent odors outside.

Key Takeaways

  • A composting toilet needs a waterproof container, a seat with a lid, a carbon material like sawdust, and a vent pipe that runs outside to prevent odor buildup.
  • You must separate urine from solid waste—urine goes into a separate jug or container, while solids stay in the main basin with carbon cover material.
  • After each use, add a handful of sawdust, coconut coir, or peat moss to cover waste and speed decomposition.
  • The vent pipe must extend at least 12 inches above your roof line or highest point on the structure to create proper air flow and eliminate smell.
  • A finished composting toilet takes 6 to 12 months to produce usable compost, and you should never use it on edible plants until it has aged for at least a year.

Choose your container and seat design

Start with a waterproof basin that will hold waste without leaking. A 5-gallon food-grade bucket works for a temporary or portable toilet. A plastic storage container (18 to 20 gallons) is better for a permanent installation because it holds more and lasts longer. Some people use a wooden box lined with heavy plastic sheeting, but plastic containers are simpler and more durable.

Cut a hole in the lid or top of the container large enough to fit a toilet seat. A standard toilet seat is 14 to 15 inches wide. You can buy a basic plastic seat at any hardware store for $20 to $50, or use a wooden seat you build yourself. The seat should fit snugly so it doesn't rock, and it should have a lid that closes completely to contain odor between uses.

If you want to separate urine from solids (which many people do to speed up composting), you'll need a urine diverter—a funnel-shaped insert that sits under the seat. Urine runs down into a separate container, while solids drop into the main basin below. You can buy a diverter online for $30 to $80, or build one from PVC pipe and a plastic funnel if you're comfortable with basic plumbing.

Build the ventilation system

Odor is the biggest complaint about composting toilets, and ventilation is what stops it. You need a pipe that runs from the bottom of your container to the outside air, at least 12 inches above the highest point of your roof or structure. A 4-inch PVC or ABS pipe works well. The pipe should slope slightly upward as it leaves the container so air can flow freely.

At the top of the pipe, install a vent cap or roof flashing to keep rain out. You can buy a standard roof flashing kit at any hardware store for $15 to $30. The vent cap should have a screen to keep insects out, but it should not have a damper or one-way valve—those block air flow and defeat the purpose.

For extra odor control, add a small inline fan (a 4-inch bathroom exhaust fan, around $40 to $100) inside the vent pipe. The fan pulls air through the toilet and pushes it outside, which creates negative pressure and keeps smells from escaping into your living space. If you add a fan, run it on a timer or switch so it runs for 15 to 30 minutes after each use, or leave it on continuously if you're using the toilet regularly.

Assemble the container and install the seat

Place your basin or container in the location where your toilet will sit. Make sure it's level and stable—a rocking toilet is uncomfortable and can spill. If you're building a permanent installation, you can build a wooden frame around the container to hide it and make it look more like a traditional toilet.

Drill or cut the hole in the lid for your seat. The hole should be centered and large enough that the seat rim sits flat on the container without gaps. Attach the seat to the container using bolts, screws, or a seat hinge kit (which lets you lift the seat like a regular toilet). Make sure the lid closes tightly so odors don't escape.

If you're using a urine diverter, install it inside the container according to the manufacturer's instructions. The diverter sits just below the seat opening and directs urine into a separate tube or container. Attach a jug or bottle to collect the urine—a 1-gallon or 2-gallon container works well. Empty it every few days or when it's full.

Connect the vent pipe and test for leaks

Drill a hole near the bottom of your container (or on the side, close to the bottom) for the vent pipe. The hole should be just large enough for the pipe to fit snugly. Seal around the pipe with silicone caulk or plumber's putty so air doesn't leak around the edges—you want all the air to flow up through the pipe, not around it.

Run the vent pipe up through your wall, roof, or outside the structure. If you're venting through a roof, use a roof flashing kit to seal the hole and prevent leaks. The pipe should rise at least 12 inches above the roof line. If you're venting through a wall, the pipe can run straight out, but it should still extend a few inches beyond the wall surface.

Before you use the toilet, test the system by pouring water into the container and checking for leaks around the base and around the vent pipe connection. If water leaks, drain it, dry the area, and reseal with caulk. Once it's dry, the toilet is ready to use.

Stock your carbon material and set up maintenance

Keep a supply of carbon material next to the toilet. Sawdust is the most common choice—it's cheap, absorbs moisture, and speeds decomposition. You can get it free or cheap from woodworking shops, lumber yards, or online. Coconut coir (coconut fiber) is a good alternative if you don't have sawdust; it's more expensive but holds moisture better. Peat moss works too, though it's less sustainable. A 5-gallon bucket of sawdust costs $5 to $15 and lasts several weeks depending on use.

After each use, add a handful (roughly 1 to 2 cups) of carbon material to the container. This covers the waste, absorbs moisture, and feeds the microbes that break it down. The more carbon you add, the faster decomposition happens and the less odor you'll have. Don't skip this step—it's the most important part of keeping the toilet working well.

Empty the urine container every few days or when it's full. You can dilute urine with water (1 part urine to 8 parts water) and use it as a fertilizer for non-edible plants like trees, shrubs, or ornamental gardens. Never use it on vegetables or edible plants until it has been stored for at least 6 months.

Harvest and cure your compost

When the container is nearly full (usually after 6 to 12 months of use), stop using it and let it sit undisturbed for at least 6 months. This resting period allows the waste to fully decompose and kills pathogens. During this time, move to a second toilet or use a temporary solution.

After the resting period, carefully remove the compost from the container. It should look like dark, crumbly soil with no visible waste. Spread it in a pile in a sunny spot and let it age for another 6 to 12 months. This second curing period is important for safety—never use fresh compost from a human waste toilet on edible plants.

Once fully cured, you can use the compost on ornamental plants, trees, shrubs, and non-edible landscaping. Some people choose never to use it on food crops out of caution, and that's a reasonable choice. At minimum, follow local health department guidelines for your area—rules vary by state and county.

Frequently Asked Questions

Do I need a permit to build a composting toilet?

Permit requirements vary by location. Some counties and municipalities allow composting toilets without a permit if they're for a cabin or off-grid property. Others require inspection or approval. Contact your local health department or building inspector before you build to find out what's required in your area.

Can I use a composting toilet in a house with regular plumbing?

Yes. Some people install a composting toilet as a backup or secondary toilet, or in a guest house or cabin on their property. You don't have to choose between one or the other. Just make sure you have proper ventilation and follow the maintenance routine.

What if the toilet starts to smell bad?

Smell usually means you're not adding enough carbon material or the vent pipe is blocked. Add more sawdust after each use, check that the vent pipe is clear and not kinked, and make sure the lid closes tightly. If you have a fan, make sure it's running. If the smell persists, the container may be too full and needs to be emptied.

How much does it cost to build a composting toilet?

A basic DIY build costs $150 to $400 depending on what materials you already have and whether you add a fan. The biggest expenses are the container ($30 to $80), the seat ($20 to $50), the vent pipe and fittings ($40 to $100), and a fan if you choose to add one ($40 to $100). Sawdust and other supplies are cheap and ongoing.

Can I use a composting toilet in winter?

Yes, but decomposition slows down in cold weather. Keep adding carbon material as usual, and make sure the vent pipe doesn't freeze. If you live in a very cold climate, insulating the container and keeping the toilet indoors or in a heated space helps. Some people close their composting toilet for winter and use a traditional toilet instead.