Conduit fill limits exist so cable can be pulled in without damaging its insulation, and the permitted percentage depends on how many conductors share the conduit: 53% for one, 31% for two, and 40% for three or more. Enter the conduit and conductor diameters and the number of conductors to get the fill percentage and the maximum number that will fit.
Calculator
Units:
mm
Internal bore, not the trade size or outside diameter
mm
Including insulation. Roughly 4.5 mm for a 4 mm² single
All conductors present, including neutral and protective conductors
Calculation Result
Press Calculate for the conduit area, the total conductor area, the fill percentage and the maximum number of conductors of this size that the conduit will accept. Passing the fill check does not by itself mean the conductors can carry their rated current.
Step-by-Step Solution
Preliminary design aid. Results follow the published formulas cited
below and are intended for estimating, study and early design. Final design must be
verified by a licensed Professional Engineer against the code in force for your project.
Key Benefits
✓Applies the correct NEC allowance for the number of conductors present
✓Reports the maximum conductor count for the conduit, not just a pass or fail
✓Warns about the ampacity derating that grouping triggers
✓Sensitivity chart shows how fill scales with conduit diameter
✓Works from actual overall diameters rather than a fixed cable table
✓Shareable links and CSV export for installation records
What Is Conduit Fill?
Conduit fill is the fraction of a conduit's internal cross-section taken up by the conductors inside it. The limits exist for one reason above all others: cable has to be pulled through, sometimes over long runs with several bends, and a conduit packed too tightly generates enough friction to strip or tear the insulation. The limits also leave room for heat to escape and for a future conductor to be drawn in.
Why two conductors get the tightest allowance
One conductor sits centrally and can rotate freely, so 53% is workable. Three or more form a bundle that behaves like a single larger, roughly circular mass, and 40% suits that. Two round conductors are the awkward case: they can align side by side and wedge against opposite walls, jamming rather than sliding. The 31% limit is calibrated to that failure mode, which is why it is tighter than either of its neighbours.
Fill is about pulling, not current
A conduit that passes the fill check may still be unusable at the conductors' rated current, because grouping conductors together means each one has less opportunity to shed heat. Derating factors apply from four current-carrying conductors upward — typically 80% for four to six and 70% for seven to nine. The two checks answer entirely different questions and both have to be satisfied.
Formula
Fill% = (n · π·d²/4) / (π·D²/4) × 100
Fill percentage from n conductors of overall diameter d in a conduit of internal diameter D
Related Formulas
Fill% = n · (d/D)²
n_max = ⌊(A_conduit · limit) / A_conductor⌋
limit = 53% (1), 31% (2), 40% (3 or more)
Variable Definitions
Symbol
Variable
Unit
Description
D
Conduit Internal Diameter
mm
Internal bore of the conduit, not its trade size or outside diameter.
d
Conductor Overall Diameter
mm
Including insulation and any sheath — not the conductor size in mm².
n
Number of Conductors
—
All conductors in the conduit, including neutrals and protective conductors.
Fill%
Fill Percentage
%
Proportion of the conduit cross-section occupied.
n_max
Maximum Conductors
—
The most conductors of this size the conduit will accept.
How to Use This Calculator
Use the internal diameter, not the trade sizeConduit is labelled by nominal or trade size, which is neither the internal nor the external diameter. A 25 mm conduit may have an internal bore anywhere from about 21 to 25 mm depending on material and wall thickness, and fill depends on the square of it.
Use the conductor's overall diameterThat means the outside of the insulation, not the copper. A 4 mm² single is roughly 4.5 mm overall against a 2.26 mm conductor diameter — the insulation roughly quadruples the area that has to be accommodated.
Count every conductor in the conduitNeutrals and protective conductors occupy space just as line conductors do. Undercounting is common and pushes the real fill above the calculated one.
Note which allowance appliesThe limit changes with the count: 53% for one, 31% for two, 40% for three or more. Two conductors are held tighter than three, so adding a third conductor can move a marginal design from failing to passing.
Check the ampacity derating separatelyFrom four current-carrying conductors upward, each one must be derated because grouping restricts heat loss. Fill and ampacity are independent checks — a conduit can pass one and fail the other.
Worked Examples
Example 1
Three conductors of 4.5 mm overall diameter in a 25 mm internal diameter conduit.
Step-by-Step Solution
Conduit area: π × 25²/4 = 490.9 mm²
One conductor: π × 4.5²/4 = 15.90 mm²
Three conductors: 3 × 15.90 = 47.71 mm²
Fill: 47.71 / 490.9 = 9.72%
The allowance for three or more conductors is 40%, so this passes comfortably
Maximum conductors of this size: 490.9 × 0.40 / 15.90 = 12.35, so 12
Interpretation: at under 10% fill this conduit is barely used. Twelve conductors of this size would still be within the limit, which suggests either a smaller conduit or scope to add circuits later.
Example 2
The same three conductors in a 16 mm conduit instead — and then what happens when a fourth, fifth and sixth are added.
Step-by-Step Solution
Conduit area: π × 16²/4 = 201.1 mm², well under half the 25 mm conduit's 490.9 mm²
Three conductors: 47.71 / 201.1 = 23.73% — still within the 40% allowance
Five conductors: 79.52 / 201.1 = 39.55%, just inside the limit
Six conductors: 95.43 / 201.1 = 47.46% — over, so the maximum is five
Note how the count collapsed: the 25 mm conduit takes twelve of these conductors and the 16 mm takes five.
That is not a coincidence but the square law at work: (25/16)² = 2.44, and 5 × 2.44 = 12.2, which rounds down to exactly the 12 the calculator reports.
The practical consequence is that stepping up one conduit size is a far larger change than the labels suggest. Going from 16 mm to 25 mm is a 56% increase in diameter and a 144% increase in capacity.
It also means the six-conductor case has a second problem the fill check does not show. Six current-carrying conductors in one conduit require an ampacity derating of about 80%, so even in a conduit large enough to hold them, each conductor may no longer carry its tabulated current.
Conduit Size Sensitivity
Fill falls with the square of conduit diameter, so a modest increase in size buys a large reduction. The maximum conductor count rises with the square for the same reason — one trade size up typically adds more capacity than expected. The marker shows your current conduit.
Fill Percentage vs Conduit Internal Diameter
Recomputed live from your inputs. The marker shows your current value.
Line chart of Fill Percentage against Conduit Internal Diameter. The same
values are listed in the data table below.
Values plotted above, sampled across the conduit internal diameter range.
How to Interpret Your Results
The fill percentage is checked against the allowance for the number of conductors present. A low figure is not wasted capacity — it eases the pull and leaves room for future circuits.
Fill Percentage: < 20Lightly filled
At your result% the conduit is well within any limit and the pull will be easy. Consider whether a smaller conduit would suit, unless spare capacity for future circuits is wanted — which is often a reasonable reason to leave it as it is.
Fill Percentage: 20 – 40Well used, within limits
At your result% the conduit is efficiently used and still within the 40% allowance for three or more conductors. Check the bend count as well — most standards limit a pull to 360° of total bend between draw points.
Fill Percentage: 40 – 53Over the limit for three or more
At your result% this exceeds the 40% allowed for three or more conductors, and the 31% allowed for two. It is only permissible for a single conductor. Use a larger conduit or fewer conductors.
Fill Percentage: ≥ 53Exceeds every allowance
At your result% the conduit is over the limit for any number of conductors. Pulling cable in will be difficult and risks damaging the insulation, which is precisely the failure the limits exist to prevent.
Maximum Conductors: < 1Only one conductor fits
This conduit will accept just one conductor of this size. Check the two diameters — a conductor whose overall diameter approaches the conduit bore is usually a sign that the conduit size or the cable specification needs revisiting.
Common Mistakes to Avoid
Using the conduit trade size as the internal diameter
Why it matters:Trade size is a label, not a dimension. Internal bore varies with material and wall thickness, and because fill depends on the square of diameter, a 10% error in bore becomes a 21% error in fill.
✓How to avoid it:Look up the actual internal diameter for the specific conduit type and size. Steel, PVC and flexible conduits of the same nominal size differ.
Using conductor cross-section instead of overall diameter
Why it matters:A 4 mm² conductor is 2.26 mm across in copper but around 4.5 mm overall with insulation, so its true footprint is nearly four times the copper area. Using the copper figure understates the fill drastically.
✓How to avoid it:Use the overall diameter from the cable datasheet, including insulation and any sheath.
Forgetting the two-conductor limit is tighter
Why it matters:It reads as an anomaly and is easily missed. Two conductors are limited to 31% while three or more get 40%, because two round conductors can wedge against each other and the wall in a way a bundle cannot.
✓How to avoid it:Apply the correct allowance for the actual count. A design at 35% fill passes with three conductors and fails with two.
Treating the fill check as sufficient
Why it matters:Fill governs whether cable can be pulled in without damage. It says nothing about whether the conductors can carry their rated current once installed, and grouping restricts heat loss.
✓How to avoid it:Apply the ampacity derating factors as well — typically 80% for four to six current-carrying conductors and 70% for seven to nine.
Ignoring bends
Why it matters:Pulling tension rises steeply with each bend, roughly exponentially with the total angle. A conduit at an acceptable fill can still be impossible to pull if it contains too many changes of direction.
✓How to avoid it:Limit the total bend between draw points, conventionally to 360°, and add pull boxes where more is unavoidable. Fill and bend count constrain the same operation from different directions.
Omitting neutrals and protective conductors from the count
Why it matters:They occupy exactly as much space as line conductors. A three-phase circuit with neutral and earth is five conductors, not three, and the real fill is 67% higher than a three-conductor calculation suggests.
✓How to avoid it:Count every conductor physically in the conduit. For the ampacity derating, count only the current-carrying ones — the two counts legitimately differ.
Practical Applications
▸Sizing conduit for a given set of conductors
▸Checking whether an existing conduit can take an additional circuit
▸Verifying an installation against fill requirements
▸Planning spare capacity for future circuits
▸Comparing conduit sizes during design
▸Estimating how many conductors a run will accept
Industry Use Cases
Commercial installation
Conduit is routinely sized above the minimum so that circuits can be added later without rewiring the containment. The square-law relationship makes this cheap: one trade size up more than doubles the capacity for a modest increase in material cost.
Industrial retrofit
Adding a circuit to an existing conduit requires checking both the fill and the derating consequences for the conductors already there. The second check often governs, because adding a fourth current-carrying conductor triggers a derating that the existing circuits were not designed for.
Cable installation
Pulling tension, not fill percentage, is what actually damages cable, and it depends on fill, bend count, run length and lubrication together. Fill limits are a proxy that keeps most pulls within tolerance without requiring a tension calculation for every run.
Expert Tips
💡Fill depends on the square of the diameter ratio — one size up more than doubles capacity.
💡Two conductors are limited to 31%, tighter than the 40% for three or more.
💡Use overall diameter including insulation, not the conductor cross-section.
💡Count neutrals and protective conductors — they take up the same space.
💡Passing the fill check does not mean the conductors can carry rated current.
💡Most standards limit a pull to 360° of total bend between draw points.
Advantages & Limitations
Advantages
✓Applies the correct allowance automatically for the conductor count
✓Gives the maximum count as a property of the conduit, not of the entry
✓Works from actual diameters rather than a fixed cable table
✓Flags the ampacity derating that fill alone does not address
✓Simple enough to check on site before starting a pull
Limitations
!Assumes all conductors are the same diameter
!Uses the NEC allowances; other standards express the requirement differently
!Does not calculate pulling tension, which also depends on bends, length and lubrication
!Does not apply ampacity derating, only warns that it is required
!Assumes round conductors; flat and armoured cables need a different treatment
!Takes no account of conduit fittings, which reduce the effective bore locally
!Does not address bend radius limits for the cable itself
The Same Three Conductors in Different Conduits
Three conductors of 4.5 mm overall diameter. Fill falls with the square of the conduit diameter, and the maximum count rises with it — which is why one size up is a bigger change than the labels suggest.
4.5 mm conductors, 40% allowance for three or more. From 16 mm to 40 mm the bore rises by a factor of 2.5 and the capacity by a factor of 6.2 — the square law. Every count in the last column matches 5 × (D/16)² rounded down.
53% for a single conductor, 31% for two, and 40% for three or more. The two-conductor limit is deliberately the tightest, because two round conductors can wedge against each other and the conduit wall.
Why is the limit for two conductors lower than for three?
Geometry. A single conductor can rotate freely and a bundle of three behaves like one roughly circular mass, but two conductors can align side by side and jam against opposite walls. The 31% figure is calibrated to that failure mode.
How do I calculate conduit fill?
Divide the total conductor area by the conduit's internal area. Three 4.5 mm conductors in a 25 mm conduit give 47.71/490.9 = 9.72%.
Do I count the earth conductor in the fill?
Yes. Every conductor physically present occupies space, including neutrals and protective conductors. For the ampacity derating, however, count only the current-carrying ones — the two counts differ legitimately.
Does passing the fill check mean the cables are adequately rated?
No. Fill governs whether the cable can be pulled in without damage. Grouping conductors restricts heat loss, so ampacity derating applies from four current-carrying conductors upward — typically 80% for four to six.
How much more does one conduit size up hold?
Capacity scales with the square of the bore. Stepping from 16 mm to 25 mm is a 56% increase in diameter and a 144% increase in cross-sectional area — five conductors of 4.5 mm becomes twelve.
Should I use the conductor size in mm² for the diameter?
No. Use the overall diameter including insulation. A 4 mm² conductor is 2.26 mm of copper but about 4.5 mm overall, so its real footprint is nearly four times the copper area.
How many bends can a conduit run have?
Conventionally 360° of total bend between draw points. Pulling tension rises steeply with bend angle, so a run within the fill limit can still be impossible to pull if it changes direction too often.
Is conduit trade size the same as internal diameter?
No. It is a nominal label, and the actual bore depends on material and wall thickness. Because fill goes with the square of diameter, using the wrong figure introduces a substantial error.
Can I fill a conduit completely if I am pulling only one cable?
Not completely — the limit for a single conductor is 53%. Even one cable needs clearance to be pulled without abrading its insulation against the conduit wall.
Glossary
Conduit fill
The proportion of a conduit's internal cross-section occupied by conductors.
Trade size
The nominal label for a conduit, which matches neither its internal nor its external diameter.
Overall diameter
A conductor's diameter including insulation and sheath — the figure fill calculations use.
Ampacity derating
Reduction of rated current when conductors are grouped and cannot shed heat freely.
Current-carrying conductor
A conductor carrying load current, counted for derating; a protective conductor is not one.
Pulling tension
The force needed to draw cable through a conduit, rising steeply with bends and fill.
Draw point
An access position from which cable can be pulled, limiting the bend total in any one section.
Raceway
Any enclosure designed to hold conductors, of which conduit is one type.
Jam ratio
The ratio of conduit bore to conductor diameter at which conductors can wedge rather than slide.
Bend radius
The minimum radius a cable may be bent to without damage, separate from the conduit's own limits.
Scientific & Standards References
NFPA 70 (National Electrical Code), Chapter 9 Table 1 — Percent of Cross Section of Conduit and Tubing for Conductors — National Fire Protection Association
NFPA 70, Annex C — Conduit and Tubing Fill Tables for Conductors of the Same Size — National Fire Protection Association
IEC 61386 — Conduit systems for cable management — International Electrotechnical Commission
BS 7671 — Requirements for Electrical Installations, Appendix 4: Current-carrying capacity and grouping factors — Institution of Engineering and Technology
NECA/NEIS 101 — Standard for Installing Steel Conduits — National Electrical Contractors Association
Conclusion
Conduit fill limits exist to protect insulation during the pull, and the allowance depends on how many conductors are present: 53% for one, 31% for two, 40% for three or more. The two-conductor case looks anomalous but is not — two round conductors wedge in a way that one or three do not. The relationship worth internalising is the square law. Fill depends on the square of the diameter ratio, so the same three conductors occupy 23.7% of a 16 mm conduit and 9.7% of a 25 mm one, and the maximum count goes from five to twelve. Stepping up one size is a much larger change than the labels imply. Finally, the check is about pulling, not current. A conduit that passes at 40% fill may still hold conductors that cannot carry their tabulated ampacity, because grouping four or more of them triggers a derating that the fill calculation knows nothing about.
Enter your conduit bore and conductor diameter above to check the fill and the maximum count.
