Conduit Fill Calculator
Estimate conduit fill percentage from conductor outside diameter, quantity and conduit inside diameter.
About the Conduit Fill Calculator
Use the Conduit Fill Calculator below to work through conductor sizing, cable selection and voltage-drop planning in seconds, right in your browser. Estimate conduit fill percentage from conductor outside diameter, quantity and conduit inside diameter. Cable and conduit decisions made on paper are far cheaper to correct than after the wall, trench or conduit run is already closed up.
It’s built for electricians, panel installers, solar and marine installers, and homeowners running new circuits, and every field updates instantly so you can try a few scenarios before committing to a real installation.
How the calculation works
Here is the relationship the calculator solves for you:
Where each input maps to a field in the calculator above:
- Wire Outside Diameter in mm.
- Number of Conductors.
- Conduit Inside Diameter in mm.
Every one of these values can be edited above, and the calculator updates the result the moment you press Calculate.
A worked example
For example, start with 4 mm for wire outside diameter, 6 for number of conductors and 20 mm for conduit inside diameter. Press Calculate and the Conduit Fill Calculator returns the result instantly — then change one field at a time to see how sensitive the answer is to that particular input.
When to use the Conduit Fill Calculator
Typical reasons people reach for the Conduit Fill Calculator:
- Sizing a new circuit before it is pulled, so the conductor is neither wasteful nor undersized.
- Checking whether an existing run is long enough to cause a noticeable voltage drop at the load.
- Comparing copper against aluminum conductors for the same run.
- Verifying a cable choice against a target voltage-drop percentage before ordering material.
Good practice: Ambient temperature, conduit fill and how many current-carrying conductors share a raceway all reduce a cable's safe ampacity.
Also worth knowing: Size a conductor for the full expected load current, not just the nameplate rating of one appliance on the circuit.
Frequently asked questions
What does the Conduit Fill Calculator calculate?
Estimate conduit fill percentage from conductor outside diameter, quantity and conduit inside diameter.
Is the result exact enough to build from?
The formula above is applied exactly to the values you enter, so the math itself is precise. Real-world results can still differ due to component tolerances, temperature, installation method, equipment efficiency and local code requirements, so treat the output as a strong planning estimate and verify final designs against manufacturer data and the electrical code that applies to your installation.
Does this calculator store or send my data anywhere?
No. The Conduit Fill Calculator runs entirely in your browser, and nothing you type is transmitted or stored anywhere.
Why not just estimate this instead of calculating it?
An undersized conductor runs hotter than it should, wastes energy as resistive loss, and can become a fire risk long before a breaker ever trips.
Can I use this on a phone, or does it need a desktop?
The Conduit Fill Calculator works on any modern browser, phone or desktop, and recalculates as soon as you press Calculate. If a result looks off, double-check that every field is filled in with the unit shown next to it (mixing up units, such as W with kW, is the most common cause of an unexpected answer) and re-enter the values.