Calculate pipe diameter or depth of flow in a full pipe using the other value, with results in feet, inches, or meters as needed.
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Depth Of Flow In Pipe Formula
Under the full-pipe assumption used by this calculator, the depth of flow is equal to the inside diameter of the pipe. This applies when the circular pipe is flowing full to the crown.
To solve for pipe diameter when depth of flow is known, the same relationship is rearranged as:
For unit conversion, the calculator uses meters internally and converts the result to your selected unit. Equivalent feet can be found using:
- DF = depth of flow
- PD = pipe diameter
- Value_ft = the entered value converted to feet
- Conversion_Factor = unit conversion factor to convert the selected unit into feet
If you enter the pipe diameter, the calculator returns the depth of flow using DF = PD. If you enter the depth of flow, it returns the pipe diameter using PD = DF. Select the quantity to solve for, enter the known dimension, and choose the result unit.
Common Pipe Diameters and Full-Flow Depths
For a circular pipe flowing full, the full-flow depth is the same as the pipe diameter.
| Pipe Diameter | Equivalent Feet | Depth of Flow When Full |
|---|---|---|
| 6 in | 0.50 ft | 6 in |
| 12 in | 1.00 ft | 12 in |
| 18 in | 1.50 ft | 18 in |
| 24 in | 2.00 ft | 24 in |
| 36 in | 3.00 ft | 36 in |
Length Unit Conversions Used
| Unit | To Feet | From Feet |
|---|---|---|
| Feet | 1 ft = 1 ft | 1 ft = 1 ft |
| Inches | 1 in = 0.083333 ft | 1 ft = 12 in |
| Meters | 1 m = 3.28084 ft | 1 ft = 0.3048 m |
Example Problems
Example 1: Find depth of flow from pipe diameter
You have a circular pipe with a diameter of 24 inches and want the full-flow depth.
The depth of flow is 24 inches, or 2 feet.
Example 2: Find pipe diameter from depth of flow
You know the full-pipe depth of flow is 0.75 meters.
The pipe diameter is 0.75 meters. In feet, this is about 2.4606 feet.
FAQ
Is depth of flow always equal to pipe diameter?
No. Depth of flow is equal to pipe diameter only when a circular pipe is flowing full. If the pipe is partially full, the depth of flow is less than the pipe diameter.
What does depth of flow mean in a pipe?
For a partly filled pipe, depth is measured from the lowest inside point (invert) to the water surface. When the circular cross-section is completely filled, its liquid depth from invert to crown equals the inside diameter. This does not determine pressure head.
Should I use inside diameter or outside diameter?
Use the inside diameter. Flow depth is measured inside the pipe, so the wall thickness should not be included in the diameter value.
