Conduit Fill Calculator
Calculate geometric fill from your actual dimensions, not a trade-size guess.
Calculated area fill
- Occupied area
- Conduit area
- Remaining geometric area
Auditable area trail
A fill percentage alone does not show whether round cables can pack, bend, pull safely, satisfy heat requirements, or meet an adopted code. Confirm the actual product dimensions and applicable rules before installation.
Your dimensions, results, and text export stay in this browser tab. After the page loads, new calculations and exports work offline.
What this calculator measures
Enter the actual inside diameter of the conduit and the overall outside diameter, including insulation or jacket, of each cable or conductor group. The calculator uses circular cross-sectional areas to estimate what fraction of the conduit area those entered circles occupy. It does not choose a raceway, conductor, code edition, jurisdiction, or limit for you.
Formula and units
Conduit area = π × inside diameter² ÷ 4. Each group area = quantity × π × outside diameter² ÷ 4. Area fill = 100 × total group area ÷ conduit area. Remaining geometric area is conduit area minus total group area when nonnegative. The trail keeps its numerator, denominator and rows in the same selected area unit; NIST defines 1 inch as exactly 25.4 millimeters. The displayed numbers carry extra digits for recomputation, not a claim about measurement accuracy.
How to use the optional limit
Leave the maximum blank if you have not established a limit applicable to your project. If you enter one, the result says only whether the calculated area is above or below your entered number. The page does not contain NEC or manufacturer dimension tables and cannot identify which code edition or amendments govern the installation. As checked September 28, 2026, NFPA lists the 2026 NEC as its current edition; your state or local authority may have adopted a different edition or amendments. The NFPA links below are places to check official editions, not evidence of approval for this calculation. Verify dimensions against current product documents and requirements with the responsible authority or a qualified professional before installation.
Geometry is not pullability
Circle-area fill alone does not establish that cables can be packed, bent, pulled, cooled, or safely installed. If the summed circular areas exceed the conduit cross-section, the calculator shows an over-area diagnostic and does not report a usable remaining area.
Frequently asked questions
Where do I find the actual conduit inside diameter?
Use current documentation for the exact conduit product or an appropriate measurement of the clear opening. A nominal trade-size label is not the measured inside diameter.
Can I enter mixed cable sizes?
Yes. Add a group for each distinct overall outside diameter and enter its quantity. The calculator adds the circular area of every group before dividing by the conduit area.
Why isn't a 3/4-inch trade size a .750-inch inside diameter?
A trade size is a product designation, not a measurement of its clear inside opening. Enter the actual inside diameter from the product you will use.
Is 40% always the allowed maximum fill?
No universal maximum is assumed here. If you have verified an applicable limit for your installation, enter it yourself. The comparison is arithmetic only and does not determine which rule applies.
Why might this differ from a manufacturer calculator?
Manufacturer tools may use their own product dimensions, conductor data, code selections, and other installation assumptions. This page calculates circular areas only from the actual diameters and quantities you enter.
Does being under my entered limit mean my installation is code compliant?
No. It only compares calculated area fill with the number you entered. It does not establish an adopted code edition, permitted fill, physical packability, pulling safety, heat derating, or installation approval.
Can I use this for CAT6 cable?
You can estimate circular area from its actual overall outside diameter, but this geometry alone does not approve a communications-cable installation or determine its applicable rules.