A siphon is a tube that moves liquid from a higher container to a lower one, even though the liquid has to travel uphill through part of the tube to get there. All you need is a length of tubing, two containers at different heights, and a way to get the flow started. Once running, gravity does all the work with no pump required.
Why a Siphon Works
A common explanation is that atmospheric pressure pushes liquid through the tube, but that’s not quite right. Siphons have been demonstrated even in a vacuum. The real driver is gravity: liquid falling down the exit side of the tube pulls the liquid behind it along for the ride, creating a continuous chain of flow. As long as the outlet end sits lower than the surface of the liquid you’re draining, the siphon keeps running until the source container is empty or the tube breaks contact with the liquid.
There is a height limit, though. At sea level, a water siphon can lift liquid roughly 10 meters (about 33 feet) above the source before the column breaks apart. A 2015 experiment published in Nature managed to push a siphon past 15 meters under carefully controlled conditions, but for any practical DIY setup, keeping the high point of your tube well under 10 meters is the safe rule.
What You Need
The setup is simple. You need a flexible tube (garden hose, vinyl tubing, or aquarium tubing all work), a source container positioned higher up, and a destination container or drain positioned lower. The greater the height difference between the water surface in your source container and the outlet end of the tube, the faster the flow. A larger tube diameter also increases flow rate, since more liquid can move through the cross-section at once. For draining a fish tank or rain barrel, standard 1/2-inch or 5/8-inch tubing works well. For smaller jobs like transferring wine between carboys, 3/8-inch tubing gives you more control.
How to Set It Up
Place your source container on a table, shelf, or any elevated surface. Set the destination container (or just an open drain) on the ground below it. Submerge one end of the tube in the source liquid, making sure it reaches well below the water line. Route the tube up and over the rim of the container, then down to the lower destination. The outlet end must sit below the surface level of the liquid in the source container, or the siphon won’t flow.
Before anything moves, you need to prime the siphon, meaning you fill the tube completely with liquid so there are no air gaps. This is the step that trips most people up. An unprimed tube is just an empty hose draped between two containers. The liquid has no reason to climb uphill on its own until the tube is already full and gravity can act on the descending column.
Three Ways to Prime Without Using Your Mouth
Sucking on the end of the tube is the oldest priming method, but it’s unpleasant with dirty water and genuinely dangerous with chemicals like gasoline. Swallowing even a small amount of gasoline can burn your throat and esophagus, and inhaling it into your lungs can cause serious, potentially permanent damage. Several poisoning cases each year result from people trying to mouth-siphon fuel. Use one of these safer methods instead.
The Submersion Method
This is the most reliable approach. Push the entire length of tubing underwater in the source container, letting it fill completely. You’ll see air bubbles escaping as water displaces the air inside. Once the bubbles stop, cap or pinch the outlet end with your thumb, pull it out of the container, and lower it to the destination below. Release your thumb, and the siphon starts immediately. This works perfectly for aquariums, buckets, and any container large enough to submerge the full tube.
The Squeeze Bottle Method
Attach a flexible plastic bottle (like an empty water bottle) to the outlet end of the tube. Submerge the other end in the source liquid. Give the bottle a firm squeeze to push air out through the submerged end of the tube. When you release the squeeze, the bottle springs back to its original shape and creates suction, pulling liquid up and over the high point. Once liquid starts flowing into the bottle, disconnect it and let gravity take over. A variation: poke a small hole in the bottom of the bottle, squeeze it, cover the hole with your finger, then release. Once liquid flows, uncover the hole so it drains freely.
The Pre-Fill Method
Fill the tube from a faucet or pitcher before you put either end in place. Pinch both ends shut, position the intake end in the source container and the outlet end at the lower destination, then release both ends at the same time. This works well when your source container is hard to reach or too small to submerge the full tube.
How Fast Will It Flow
Flow speed depends on two things: the height difference between the water surface and the outlet, and the diameter of your tube. The velocity of liquid exiting the siphon follows a simple relationship: it equals the square root of twice the gravitational acceleration multiplied by the height difference. In practical terms, doubling the height difference increases the exit speed by about 41%, not double. A bigger tube doesn’t make the liquid move faster, but it lets more volume pass through per second because of the larger opening.
For a rough sense of scale, a 5/8-inch garden hose siphoning from a rain barrel elevated 3 feet above the outlet will drain about 3 to 5 gallons per minute. Raising the barrel higher or using a wider tube speeds things up. Narrower tubing, like the 1/4-inch variety used for aquarium work, gives a gentler trickle that’s easier to manage when you’re vacuuming gravel or doing partial water changes.
Common Problems and Fixes
The number one reason a siphon stops working is air getting into the tube. Even a small air bubble can break the liquid column and stall the flow. This usually happens when the intake end isn’t submerged deeply enough and starts sucking air as the water level drops, or when there’s a loose connection where the tube meets a fitting. Keep the intake end well below the surface and check all joints for a snug seal.
If the siphon slows to a trickle and then stops, look for a kink in the tube. Flexible vinyl tubing kinks easily at sharp bends, especially over the rim of a container. A short piece of rigid pipe or a commercially available siphon clip at the bend point prevents this. You can also loop the tubing in a gentle curve rather than forcing it into a tight angle.
Temperature matters in one specific way: warm water releases dissolved gas more readily than cold water. If you’re siphoning warm water and notice tiny bubbles collecting at the highest point of the tube, that gas is coming out of solution and will eventually break the siphon. Keeping the peak of the tube as low as possible (just high enough to clear the rim of the source container) minimizes this problem by reducing the pressure drop at the top of the arc.
Practical Uses
Aquarium owners use siphons constantly for water changes and gravel cleaning. You push a wide-mouthed tube into the gravel, let the siphon pull debris-laden water into a bucket on the floor, and stop by lifting the tube out of the tank. Rain barrel owners siphon water to garden beds without needing a spigot. Homebrewers transfer beer and wine between fermentation vessels without disturbing sediment, since the intake end can be positioned just above the layer of settled yeast.
For draining a water heater, emptying a flooded basement with a long hose run to a lower yard, or pulling water from a pond for irrigation, the same principles apply. Position the outlet lower than the source, prime the tube, and let gravity handle the rest. The only limit is that roughly 33-foot ceiling on how high the tube can rise above the source water before the column breaks.