What this calculator does
Conduit fill is the one cable-installation check that is pure geometry, which means it is true everywhere on earth. You have a conduit of a given bore and a bundle of cables of given outside diameters; the question is how much of the bore they occupy, whether that is under the limit your wiring rules impose, and how much room is left for the circuit somebody will inevitably ask you to add next year.
This calculator takes the conduit internal diameter and up to three groups of cables — an overall diameter and a quantity for each — and returns the total cable area, the conduit area, the fill percentage, the spare area within your permitted limit, and how many more cables of the largest size entered would still fit. It also reports the jam ratio, which is the check most fill calculators leave out and the one that actually strands pulls on site.
Note the input choice: this tool asks for the internal diameter, not the nominal trade size. Nominal size is a label, not a dimension. “20 mm conduit” has a different bore under BS EN 61386 light-gauge, medium-gauge and heavy-gauge PVC; “3/4 inch” has a different bore in EMT, IMC, rigid steel and Schedule 40 PVC. Any tool that turns a trade size into a bore has quietly picked one country’s product tables and one duty class, and it will be wrong for you roughly half the time. The bore is one line on the manufacturer’s datasheet. Read it once and the answer is exact anywhere.
The formula
Everything follows from the area of a circle.
A = π × d² / 4 area of one cable
A_cable = Σ (n_i × π × d_i² / 4) all cables, summed over groups
A_cond = π × D² / 4 conduit internal area
Fill % = 100 × A_cable / A_cond
= 100 × Σ (n_i × d_i²) / D² π/4 cancels
where D is the conduit internal diameter, d_i is the overall diameter over the outer sheath of one cable in group i, and n_i is how many of them there are. Diameters must all be in the same unit; the tool handles that for you.
The second form is worth remembering. Because the π/4 cancels, fill is just the sum of the cable diameters squared divided by the conduit diameter squared. Three 16 mm cables in a 50 mm bore is 3 × 256 / 2500 = 0.3072, or 30.72 % — a figure you can produce on the back of a delivery note without a calculator at all.
The permitted-fill and spare-capacity outputs then follow directly:
A_perm = A_cond × limit% / 100
Spare = A_perm − A_cable
n_extra = ⌊ Spare / (π × d_max² / 4) ⌋
And the jam check is a ratio, not an area:
Jam ratio = D / d
Reading the result
Fill percentage is the headline, but it is not a physics limit — nothing stops you cramming a conduit to 70 %. The limit exists because of pulling tension, heat dissipation and the ability to draw a cable out again later. Exceed it and the cable jacket takes damage on the pull, the group derating for the circuits gets worse than you assumed when you sized them, and the “spare” conduit becomes a conduit nobody can ever add to.
Spare area and the extra-cable count are the numbers to quote in a design review. “38 % fill” invites an argument; “room for two more 16 mm cables before we hit the limit” ends one. Note that the extra-cable count is measured against your permitted area, not against the full bore — it is spare capacity you are allowed to use, not spare capacity that physically exists.
Jam ratio is the sleeper. When exactly three cables of the same diameter are pulled together, there is a band of conduit-to-cable diameter ratios — roughly 2.8 to 3.2 — in which the three can stop stacking in their stable triangular formation and instead line up side by side across the bore. At a bend they wedge, the pulling tension spikes, and either the pull stops or a jacket tears. Below 2.8 the three physically cannot lie abreast; above 3.2 they have enough clearance to roll past one another. It is a geometric coincidence rather than a code rule, which is exactly why it is universally true and why every cable manufacturer’s pulling guide carries the same warning. The tool raises it only when there are exactly three cables, because jamming is a three-body problem — two cables cannot jam, and four or more cannot line up abreast in the same way.
If you land in the window, the fixes are all easy at design stage and all expensive on site: change conduit size, change cable size, pull the three as a bound triplex so they behave as one body, or install a pull box to shorten the run.
Typical limits
Limits belong to the rules in force where you are working, not to this tool. As a guide to the figures you will meet:
- The most widely used convention, in the style of NEC Chapter 9, Table 1, is 53 % for one cable, 31 % for two, and 40 % for three or more. The 31 % figure for two looks anomalous but is not: two circles in a circle is the geometrically worst-packing case, and the lower number keeps a sensible clearance.
- Practice descended from IEC 60364-5-52 and the British tradition works instead to a space factor of about 45 %, applied through a cable-factor/conduit-factor table rather than a straight percentage, with the factor falling for longer runs and more bends.
- AS/NZS 3000 and others land in the same territory. Short straight runs are sometimes allowed more; runs with several bends are allowed less.
- For sleeves, nipples under 600 mm (24 in), and short protective sleeves, most regimes allow 60 %.
Whatever your figure, it is the derated one that matters: a run with three 90° bends and 30 m between pull points is not the same duty as a 2 m sleeve through a wall, even at identical fill. Check the standard applicable in your jurisdiction — for example NEC Chapter 9, IEC 60364-5-52, BS 7671 Appendix 5, or AS/NZS 3000.
Worked example
A 50 mm bore conduit is to carry three power cables of 16 mm overall diameter and two control cables of 8 mm overall diameter. The applicable limit for three or more cables is 40 %.
Conduit internal area:
A_cond = π × 50² / 4 = π × 625 = 1963.50 mm²
Cable areas:
16 mm: π × 16² / 4 = π × 64 = 201.06 mm² × 3 = 603.19 mm²
8 mm: π × 8² / 4 = π × 16 = 50.27 mm² × 2 = 100.53 mm²
total = 703.72 mm²
Fill percentage, by the shortcut:
Fill = 100 × (3 × 256 + 2 × 64) / 50² = 100 × 896 / 2500 = 35.84 %
That passes the 40 % limit. The permitted area is 0.40 × 1963.50 = 785.40 mm², so the spare area is 785.40 − 703.72 = 81.68 mm². One more 16 mm cable needs 201.06 mm², so the answer to “can we add another power cable” is no — you are 119 mm² short, and the next conduit size up is the honest answer.
The jam ratio is 50 / 16 = 3.125, which is inside the 2.8–3.2 window. Here it does not apply, because there are five cables in the conduit rather than three. Pull the three 16 mm cables on their own first, though, and the jam window is live — worth knowing before the crew starts.
FAQ
Do I use the cable’s overall diameter or its conductor size? Overall diameter, over the outer sheath, from the manufacturer’s datasheet. Conductor cross-section (mm² or AWG) says nothing about how much room the cable takes up — insulation and sheath thickness dominate, and two cables with the same 4 mm² conductor can differ by several millimetres of OD.
Why is the limit for two cables lower than for three? Because two round cables pack badly. Three arrange into a triangle that nests inside the bore efficiently; two sit side by side and sweep a much less efficient envelope, so the same percentage would leave far less usable clearance for pulling.
Does a full conduit affect the cable rating? Yes, but through a different check. Grouping several current-carrying circuits in one enclosure reduces the current each may carry, by a derating factor in your wiring rules. Passing the fill check does not mean the cables are still rated for the load — size the cables first, then check they fit.
What about the bends? Fill percentage takes no account of them, and it should not. Bends govern pulling tension and sidewall pressure, which is a separate calculation, and most regimes also cap the total bend angle between pull points at around 360°. A run at 40 % fill with four bends can be unpullable while the same fill in a straight run is trivial.
Can I mix the three groups freely? Yes. The groups are only a convenience for entering different cable sizes; the arithmetic simply sums areas. If you have more than three sizes, combine the smaller ones conservatively by entering the largest of them at the total quantity — that overstates the fill slightly, which is the safe direction.
This tool provides indicative figures for preliminary design and checking. Permitted fill, derating and pulling limits must be verified against the wiring rules and product standards applicable in your jurisdiction.