Free conduit fill calculator
Add the cables going in, pick the conduit, and see how full it is, how many more will fit and the smallest size that works. Flat TPS, single-core, double-insulated, data and your own cables.
Check a conduit
No sign-up. Works offline, nothing is uploaded, and every conduit size is shown at once.
Open the calculatorOpens in a new tab. Works on a computer, tablet or phone.
What it does
- How full the conduit is: the cables’ total cross-section against the inside of the conduit, with the usual 53% / 31% / 40% limits or your own.
- Every size at once: the fill in every size of the conduit type you picked, so you can see the smallest that fits.
- How many more fit: for each cable, how many more you could pull in.
- Common cables built in: flat TPS, single-core building wire, single-core double insulated, circular multicore, Cat5e, Cat6, Cat6A and RG6, with typical sizes you can change. Or type your own.
- PVC and corrugated conduit: medium-duty grey and heavy-duty orange rigid conduit from 16 to 100 mm, and corrugated flexible conduit.
- Jamming and oversized cables: it warns when three cables could wedge in a bend, or a cable is nearly as wide as the conduit.
- A one-page report for the job file: your logo, business and licence number, the job, conduit run and customer, the fill against the limit, a schedule of the cables, every size of the conduit, the method and a sign-off line. Or copy a short summary into a quote or email.
Conduit fill limits
| Cables in the conduit | Usual fill limit |
|---|---|
| One | 53% of the inside area |
| Two | 31% |
| Three or more | 40% |
Leave more room than this for long runs, several bends or cables that may be added later.
How to use it
- Pick the conduit. Type and size. Change the inside diameter if you know your brand’s.
- Add the cables. Use Quick add or pick the cable and size, and set how many.
- Read the fill. Green fits, amber is tight, red is too full. The table shows every size, and the checks tell you the smallest that fits.
- Print the report or copy the summary. Name the conduit run so the report says which one it is.
Common questions
How full can a conduit be?
The usual rule is 53% of the inside area for one cable, 31% for two and 40% for three or more. Two cables get less because they sit side by side across the conduit. For long runs or several bends, a lower limit makes pulling in easier.
How many 2.5 mm² TPS cables fit in a 25 mm conduit?
Two, by the usual rule, in typical 25 mm medium-duty conduit. The calculator counts flat TPS by its full width × height, about 53 mm² for 2.5 mm² 2C+E, which is on the safe side, and a third cable takes it just over 40%. It works it out for your brand’s sizes too.
Why are the sizes only “typical”?
Inside diameters of conduit and outside sizes of cable differ between brands and duty ratings. The calculator fills in typical figures and lets you type the ones from the conduit or the cable’s data sheet.
What is cable jamming?
Three round cables of the same size can wedge against each other in a bend when the conduit is about 2.8 to 3.2 times their diameter. The calculator warns you so you can go up a size or keep the run straight.
Does it check the current rating of the cables?
No. Cables grouped in a conduit carry less current, so allow for derating under AS/NZS 3008.1.1 and follow AS/NZS 3000 and the conduit maker’s instructions.
Can I print the check for the job file?
Yes. Print report gives a one-page A4 report with your logo, business and licence number, the job and conduit run, the fill against the limit, a schedule of the cables, every size of the conduit type and a sign-off line.
Is anything uploaded?
No. It all runs in your browser, works offline once open, and keeps your last calculation on your device.
More free tools
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