Free tool
Conduit and trunking fill calculator
Check a pull before you cut the wall. Pick an EU or EMT conduit, enter how many Cat6, coax, fibre, speaker, and mains cables are going in, and get the fill percent against the 53, 31, and 40 percent limits, plus how many spare cables still fit. Free, and nothing leaves your browser.
Set a count for every cable you are pulling. Leave the rest at zero.
Cable cross section
190 mm²
Conduit area
360 mm²
Fill, limit 40%
52.8 %
Spare Speaker 2x2.5 mm²
0
6 cables, so the 40% limit applies. Over the limit. Go one size up, or split across two runs.
Fill percentages follow NEC Chapter 9 Table 1. Inner diameters are typical values, so check the manufacturer sheet for the exact product. Estimate only, not a substitute for a qualified electrician.
Overfilled conduit is found the hard way
Nobody discovers a conduit is too full at the design stage. It gets discovered halfway down a 25 metre run with a rope, a helper, and a ceiling already closed. The check itself is trivial: add up the cross section of every cable, compare it to the bore of the conduit, and see whether the ratio clears the allowance for that number of cables.
The maths
- Area of a round cable is π × Ø² ÷ 4, using the outer jacket diameter, not the conductor size.
- Conduit area uses the inner bore, which is a good deal smaller than the size on the label.
- Fill percent is total cable area divided by conduit area.
- Allowances: 53% for one cable, 31% for two, 40% for three or more.
Cable outer diameters vary between manufacturers, so override any row with the figure from the actual datasheet when the run is tight. Planning the whole job? Avitly routes cabling off the floor plan with validated connectors, so the bundles you size here match the ones on the drawing.
Common questions
Why is the limit 53% for one cable but only 31% for two?
It is geometry, not caution. A single round cable slides down a round conduit happily, so NEC Chapter 9 Table 1 allows 53%. Put two in and they wedge against each other and against the wall, which is the worst packing case there is, so the allowance drops to 31%. From three cables upward they nest and roll over one another again, and the limit settles at 40%. The calculator picks the right one from the total cable count for you.
Do I use the conduit's inner or outer diameter?
Inner, always. Conduit is sold by a nominal size that usually refers to the outside, so a 25 mm EU conduit only has about 21.4 mm of usable bore, and the wall thickness eats around 27% of the area you might have assumed. This calculator already stores inner diameters for the listed sizes. For flexible conduit, oval trunking, or anything unusual, measure the bore and use the custom option.
Does fill percentage account for bends and pulling force?
No. Fill is a static area check, and it says nothing about whether you can physically get the bundle through. A run with four 90 degree bends and a 30 metre length can be impossible at 35% fill, while a straight 5 metre riser is fine at the limit. Rules of thumb worth keeping: no more than 360 degrees of total bend between pull points, and add a pull box if you go past that.
Can I run mains and data cable in the same conduit?
In most jurisdictions, no, or only with a segregating barrier. Mains and low voltage data are usually required to be separated, both for safety and to keep induced noise off unshielded twisted pair. The mains row in this calculator is there for runs that are mains only, or for trunking with a divider where you want to check each compartment. Check your local wiring regulations before mixing anything.
Should I fill a conduit right up to the limit?
Only if you never plan to touch it again. Practical experience says leave one spare pull of the largest cable in every conduit that reaches a rack, a display position, or a ceiling void, because there is always one more device six months later. The spare capacity tile shows exactly how many more of your biggest cable would still fit, so you can size up while the wall is still open rather than rerouting later.
Do these numbers apply to trunking and cable tray too?
Partly. Rectangular trunking is normally worked to a fill of about 45%, and open cable tray uses depth and width rules rather than a percentage, so a round conduit calculation is conservative for both. If you enter a trunking compartment, use the custom option and pick a diameter that gives the same cross section, then read the result as a rough guide rather than a compliance check.
This is built into Avitly. Same maths, running on your real project instead of numbers you retype.
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