Fluid Gas Handling

Three Key Words in Hydraulic Hose Sizing: I.D., Dash Size and Nomogram

Three Key Words in Hydraulic Hose Sizing: I.D., Dash Size and Nomogram - Hydraulic Hose Sizing - Parker Hose Products Division - Inner and Outer DiameterSizing is the most basic and essential factor in hydraulic hose selection. Sizing specifications for hydraulic hose include inner diameter, outer diameter, and length.

  • Outer diameter (O.D.) measures the entire span of the hose cross-section, including the tube, reinforcement, and cover. The difference between the O.D. and I.D. is the thickness of the hose wall. Outer diameter is important for sizing industrial hose to appropriate fittings and system components.
  • Inner diameter (I.D.) is the span of the empty space in a circular hose cross-section that affects the flow velocity of the fluid within the hose. A hose with an oversized I.D. will result in sluggish system performance, while an undersized I.D. can cause excessive pressure drops, leaks, and system damage.
  • The length of the hydraulic hose defines its maximum reach between end-points. Increasing length can also increase the pressure drop in the system because hose length is a function of head loss due to friction.

What is the dash size?

People new to the hydraulic industry often ask what hose dash size means. Simply put, a dash size refers to an I.D. of a hydraulic hose or fitting in 1/16" increments. For example, -6 hose would indicate a hose with I.D. of 6/16" - or 3/8".

Hoses are measured by inside diameter, unlike tubes which are measured by outside diameter. As a matter of fact, that is how the hoses and tubes are differentiated.

Hydraulic Hose Sizing - Parker Hose Products Division - Hose ID Chart

Dash numbers, the industry standard, are typically used to describe hose and coupling size. View Parker’s extensive hydraulic hose and fitting offering.

How to determined dash size for your specific hydraulic application

By using a standard nomogram, we can find the relationship between three variable quantities using simple geometric construction. In this instance, our variables are Dash Size (Hose I.D.), Desired Flow Rate, and Recommended Flow Velocity. If any two of these factors are known, you can easily determine the third factor.

How to use the nomograph shown below:

  1. Pick the two known values. 

  2. Lay a straightedge to intersect the two values. 

  3. Intersection on the third vertical line gives the value of that factor.

For example, if we know the Desired Flow Rate is 16 gallons per minute and our Recommended Flow Velocity is 20 feet per second, we can mark these numbers in the corresponding left- and right-hand columns. When we lay a straightedge between the columns and draw a line to connect the values, we see where the line crosses the center column graph; where it intersects determines the Hose I.D. / Dash Size. In this case, we would require a  0.625" (5/8") inch (or larger) hose. The nomogram is an easy and effective tool for helping size your hydraulic hoses.

Hydraulic Hose Sizing - Parker Hose Products Division - Nomogram for Dash Size


Kyri McDonough, Hose Products Division, Parker HannifinArticle contributed by Kyri McDonough, marketing services manager at Hose Products Division, Parker Hannifin.





Other related topics on hydraulic hoses, hose application, and selection criteria:

Successful Hydraulic Hose Assembly Starts Here
Measure and Specify Hydraulic Hose Length With Ease
Decoding a Hydraulic Hose Layline
Hydraulic Hose 101: Fast Facts
Top 8 Reasons Hydraulic Hoses Fail


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Comments for Three Key Words in Hydraulic Hose Sizing: I.D., Dash Size and Nomogram

William Haddad
What is defined as "Transportation and Refrigerant hoses", and what is "All Except..." in your Dash No table ?
Thanks, William

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