## Quick Answer

The amount of water a 6 inch pipe can hold depends on the length of the pipe. A 6 inch diameter pipe that is 10 feet long can hold approximately 21.2 gallons of water. Generally, a 6 inch diameter pipe can hold about 2.1 gallons of water per linear foot. So a 100 foot long 6 inch pipe could hold roughly 210 gallons. The exact volume depends on whether the pipe is completely full or not.

## Calculating the Volume of a Cylindrical Pipe

To calculate the volume of water a cylindrical pipe can hold, you need to know the equation for the volume of a cylinder:

Volume = π x r2 x h

Where:

π (pi) = 3.14159…

r = radius of the pipe

h = height or length of the pipe

For a 6 inch diameter pipe:

– The radius is 6 inches / 2 = 3 inches

– π is approximately 3.14

So the equation becomes:

Volume = 3.14 x (3 in)2 x h

Volume = 3.14 x 9 in2 x h

Volume = 28.26 in2 x h

Where h is the length of the pipe in inches.

For example, if the pipe is 10 feet long (120 inches) the equation would be:

Volume = 28.26 in2 x 120 in

Volume = 3,391 in3

To convert to gallons, divide by 231 (the number of cubic inches in a gallon):

3,391 in3 / 231 in3/gal = 14.67 gallons

So a 6 inch diameter pipe that is 10 feet long can hold approximately 14.67 gallons of water when full.

## Volume per Linear Foot

Rather than calculating for every different pipe length, we can derive a factor for how much volume is added per linear foot:

28.26 in2 x 12 in/ft = 339.12 in3 / ft

339.12 in3/ft / 231 in3/gal = 1.467 gal/ft

So for every foot in length, a 6 inch pipe holds about 1.467 gallons of water. Or approximately 1.5 gallons per linear foot.

Therefore, a 100 foot long pipe would hold:

100 ft x 1.5 gal/ft = 150 gallons

A 50 foot pipe:

50 ft x 1.5 gal/ft = 75 gallons

And so on. This provides an easy way to estimate the volume of a 6 inch cylindrical pipe based on the length in feet.

## Factors that Affect Volume

There are a few factors that can affect the actual volume of water a pipe can hold:

### Full or Partial Fill

The calculated volume assumes the pipe is completely full of water. If the pipe is only partially full, the volume would be less. For example, if a 100 foot pipe was half full, the volume might only be around 105 gallons.

### Pipe Material and Wall Thickness

The above calculation uses the full 6 inch inside diameter. However, the wall thickness of the pipe reduces this inside space. A standard steel Schedule 40 pipe has a wall thickness of about 0.154 inches. This decreases the inside diameter by 0.308 inches to 5.692 inches. The volume per linear foot also decreases to around 1.44 gallons per foot.

### Fittings and Connections

Any fittings or connections in the pipe like elbows, tees, valves, adapters, etc. further reduce the available space for water. Their volume must be subtracted from the total.

### Pipe Shape and Orientation

The calculation assumes a straight cylindrical pipe. Any bends or angles in the pipe would decrease the water capacity. Also, if the pipe is oriented vertically, gravity and water pressure affect the volume.

So the actual volume may be 5-10% less than calculated due to these factors. It’s best to use the calculated volume as a maximum capacity for planning purposes.

## Water Pressure and Flow

The volume of water a pipe can hold is different from the water pressure and flow rate.

A 6 inch pipe can potentially deliver over 100 gallons *per minute* if sufficient pressure is applied. But the pipe itself can only hold around 1.5 gallons per linear foot when full.

The flow rate depends on the pressure and needs to be calculated based on the requirements of the plumbing system it is part of.

## Typical Uses for a 6 inch Pipe

Some common uses for a 6 inch diameter pipe:

– Residential plumbing main lines

– Short run irrigation branches

– Drainage lines for downspouts or landscaping

– Branch lines for fire sprinkler systems

– Rainwater harvesting barrels and cisterns

– Agricultural watering and livestock applications

A 6 inch pipe would be too small for a municipal water main line or as a supply for a large building unless it was part of an extensive network of pipes in parallel. The limited volume capacity makes it unsuitable for any application needing high continuous flow.

But for residential or light commercial uses, a 6 inch pipe can provide adequate water volume while still being easy to work with and install.

## Practical Example

Let’s say you wanted to store emergency water supply in a 6 inch PVC pipe storage tank with these dimensions:

– 6 inch diameter pipe

– 50 feet long

– Laid horizontally on the ground

### Calculate Volume

Using 1.5 gallons per foot for 6 inch pipe:

50 ft x 1.5 gal/ft = 75 gallons

So ideally this pipe would hold 75 gallons when full.

### Adjust for Real World Factors

However, to be conservative in our storage estimate we should account for the following:

– 5% reduction for wall thickness

– 10% reduction for fittings and connections

– 5% reduction for imperfect shape when laying horizontally

75 gallons x (1 – 0.05 – 0.10 – 0.05) = 63 gallons

By accounting for these factors, we can estimate the actual reliable water storage volume to be approximately 63 gallons.

This provides a practical real-world example of calculating and adjusting the expected water volume in a 6 inch diameter PVC pipe. For emergency water storage, having a conservative estimate is important.

## Conclusion

The volume of water a 6 inch diameter pipe can hold depends primarily on the length of the pipe. As a general rule of thumb, a 6 inch pipe can hold around 1.5 gallons per linear foot when completely filled.

This can be calculated more precisely using the mathematical equation for the volume of a cylinder based on the radius and length.

Real world factors such as wall thickness, fittings, fill level, and pipe orientation can reduce the actual volume by 5-10% typically.

A 6 inch pipe is useful for residential and light commercial applications that require moderate water volumes. But high flow rates or municipal water supply would require larger diameter piping.

By using the linear feet guideline and adjusting for other factors as needed, you can reasonably estimate how much water would be held by a 6 inch cylindrical pipe installation for planning purposes.

The volume calculation is also distinct from calculating required water pressure and flow rates, which depends on the specifics of the piping system. But knowing the volume capacity is key to proper system design and sizing for any application.

Your article says the formula for how much water a 6” “well” pipe holds is pi X the diameter X 2, which equals 48.95 gallons per foot.

1) it is not reasonable that a 1 foot length of 6 inch pipe can hold nearly 50 gallons. I don’t know what “well” pipe is though.

2) I don’t know why the diameter of the pipe would be used instead of the area (pi X the inner radius squared X length of pipe).

Thank you