Fill Ratio for Jetting Installations
January 28, 2026
In our Explainer Series, we break a critical concept in conduit networks into handy pieces of knowledge that pay dividends time after time. Here, it’s fill ratio for jetting installations.
What is Fill Ratio?
In simple terms, fill ratio is a measure of the space taken up by one thing inside another. In fiber networks, it typically refers to the space a micro cable occupies in a MicroDuct, expressed as a percentage. For optimal jetting performance, Dura-Line recommends a fill ratio of 50 to 75%, based upon years of experience and thousands of successful installations.
Why is it Important?
Fill ratio is the most important factor in choosing the right combination of cable and MicroDuct. If the pathway is already in place, it gives a clear range of cable sizes that will fit well. If there’s a specific number of fibers required along a route, it sets a network owner’s expectation on the size of MicroDuct they’ll need and, therefore, the size of the trench, MicroTrench, or bore needed to install it.
How Does It Impact Jetting Performance?
Micro cables have exploded in popularity because they can be installed up to 6,000 ft. in a single step at speeds of up to 300 ft. per minute via air-jetting. But fill ratio can make or break a jetting project. Too high, and there may be limited airflow, increasing friction between the cable and pathway and limiting jetting distance. Too low, and there may be too much space around the cable, potentially leading to kinking or buckling.
How is it Calculated?
Divide the outer diameter (OD) of the cable by the inner diameter (ID) of the MicroDuct and multiply by 100:
Is There Anything Else to It?
While fill ratio is the most important factor in choosing the right combination of cable and MicroDuct, it is not the only factor in a successful jetting installation. Other variables that should be considered when planning an installation include:
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Condition of the MicroDuct
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Installation route, including any bends and elevation changes
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Combination of jetting equipment being used
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Cable design, including any preferential bend
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Ambient temperature and humidity on the day of the installation