Calculate conduit fill using the cable factor method. Instantly determine the right conduit size for any combination of cables, with bend adjustments and trunking fill mode included.
Calculate fill percentage with BS EN 61386-1 compliance
Fill % = (Total Cable Area ÷ Conduit Area) × 100. Lower fill = easier installation.
Conduit fill refers to the proportion of a conduit's internal space that is occupied by cables. Getting this right is critical for three reasons: heat dissipation, pulling tension during installation, and future capacity for additional cables.
Heat dissipation: When current flows through a cable, the conductor generates heat due to its resistance. This heat must be dissipated through the cable insulation, through the air space inside the conduit, through the conduit wall, and finally to the surrounding environment. If the conduit is packed too tightly with cables, the air space is insufficient, and the cables overheat. Overheating degrades the insulation over time, reducing the cable's lifespan and eventually causing insulation failure, short circuits, and potentially fire. The cable factor tables account for this by limiting the fill to approximately 40% of the conduit's internal cross-sectional area.
Pulling tension: Cables must be drawn through conduit during installation. The friction between the cable sheaths and the conduit wall, and between adjacent cables, determines how much force is needed to pull them through. If the conduit is overfilled, the pulling tension becomes excessive, risking damage to the cable insulation — particularly at bends where the cables are pressed against the conduit wall. Damaged insulation can lead to earth faults, short circuits, and reduced insulation resistance readings during testing.
Future capacity: Good installation practice and BS 7671 encourage leaving spare capacity in conduit systems to accommodate future circuit additions. An installation that is filled to maximum capacity on day one leaves no room for the additional circuits that building alterations, extensions, or technology upgrades may require. Many specifications call for conduit to be no more than 30 to 35% full to allow for future growth.
For these reasons, conduit fill calculations are a fundamental part of electrical design. They should be performed before any conduit is installed, and the results documented as part of the design records. Guessing conduit sizes based on experience alone often leads to under-sized conduit that causes problems during cable pulling, or over-sized conduit that wastes material and installation time. Pair conduit fill checks with the cable derating calculator to account for grouping factors, and the cable sizing calculator to confirm your conductor sizes before pulling.
The cable factor method is the standard approach in the UK for calculating conduit fill. It uses a system of dimensionless "factors" — one for each cable size and one for each conduit size — that make the calculation simple without needing to work with actual cross-sectional areas in square millimetres.
The principle is straightforward: each cable has a cable factor that represents the space it occupies. Each conduit has a conduit factor that represents the space available. If the sum of all cable factors is less than or equal to the conduit factor, the cables will fit with adequate clearance for heat dissipation and pulling.
Sum of cable factors ≤ Conduit factor
If this condition is met, the cables fit. If not, use a larger conduit.
The cable factors are derived from the overall diameter of each cable (including insulation), and the conduit factors are derived from the usable internal cross-sectional area of each conduit size, reduced to approximately 40% to allow for air space and pulling clearance. This means the factor system already has the safety margin built in — you do not need to apply an additional reduction.
The table below shows the cable factors for the most common sizes of single-core PVC insulated cables (to BS 6004) used in conduit installations. These are the "singles" — individual conductors with PVC insulation but no sheath — that are drawn into conduit for wiring circuits.
Note that 1.0 mm² and 1.5 mm² cables have the same cable factor (22) because their overall diameters (including insulation) are very similar. The difference in conductor cross-section is small at these sizes, and the insulation thickness is the dominant dimension.
The conduit factors below are for standard round PVC conduit (heavy gauge, to BS EN 61386). The factor depends on the conduit size and the length and complexity of the run. Straight runs and runs with gentle bends have higher factors (more capacity), while runs with multiple tight bends have lower factors.
The "With Bends" column shows a reduced conduit factor for runs containing two or more bends. For runs with a single bend, use a value between the straight and bends columns. The Elec-Mate calculator adjusts the factor automatically based on the number of bends you specify.
A conduit run carries two lighting circuits. Each circuit requires one 1.5 mm² line conductor and one 1.5 mm² neutral conductor, plus a shared 1.5 mm² CPC. Total conductors: 5 singles of 1.5 mm².
Total cable factor = 5 x 22 = 110
Conduit factor for 20 mm (straight run) = 460
Result: 110 ≤ 460 — PASS. Plenty of room. You could fit up to 20 conductors of 1.5 mm² in a 20 mm straight run (20 x 22 = 440 ≤ 460).
A conduit run with two 90-degree bends carries one 2.5 mm² radial circuit (line, neutral, CPC = three 2.5 mm² singles) and two 1.5 mm² lighting circuits (line, neutral each = four 1.5 mm² singles, plus one shared 1.5 mm² CPC = five 1.5 mm² singles).
Cable factor for 2.5 mm² singles: 3 x 30 = 90
Cable factor for 1.5 mm² singles: 5 x 22 = 110
Total cable factor = 90 + 110 = 200
Conduit factor for 25 mm (with bends) = 560
Result: 200 ≤ 560 — PASS. A 25 mm conduit is adequate. A 20 mm conduit with bends (factor 320) would also work: 200 ≤ 320.
When planning to mix circuits in the same conduit or trunking, check whether any circuit is Band I (extra-low voltage, e.g. SELV, data, fire, security) alongside Band II (low voltage, 230 V). OSG Reg 7.4.1 prohibits Band I and Band II circuits in the same wiring system part unless one of these five conditions is satisfied:
Source: IET On-Site Guide 9th Ed (A4:2026), Reg 7.4.1; BS 7671 Reg 528.1.
An electrician wants to run three 2.5 mm² radial circuits and one 4 mm² cooker circuit through a single 20 mm conduit with three bends. Each 2.5 mm² circuit has 3 conductors; the 4 mm² circuit has 3 conductors.
Cable factor for 2.5 mm² singles: 9 x 30 = 270
Cable factor for 4.0 mm² singles: 3 x 43 = 129
Total cable factor = 270 + 129 = 399
Conduit factor for 20 mm with bends = 320
Result: 399 > 320 — FAIL. The 20 mm conduit is too small. Upgrading to 25 mm conduit with bends (factor 560) solves the problem: 399 ≤ 560. Alternatively, install a draw-in box to split the run and use the straight-run factor for 20 mm (460), which also works: 399 ≤ 460.
The number of bends in a conduit run has a significant impact on cable pulling difficulty. Each bend increases friction between the cables and the conduit wall, and the cables press against each other at bend points, increasing the pulling tension required. Excessive pulling tension can stretch conductors, damage insulation, and even pull cables off their terminations.
As a general rule, a conduit run should not contain more than two 90-degree bends (or the equivalent) between draw-in points. If the run requires more bends, a draw-in box or inspection tee should be installed to break the run into manageable sections. Each section is then treated as an independent run for the purposes of conduit fill calculations.
Long straight runs also present challenges. Friction accumulates over length, and a 15-metre straight run requires more pulling force than a 5-metre run. For runs exceeding 10 metres without a bend, some specifications recommend reducing the conduit factor by 10% to account for the additional friction. The Elec-Mate calculator allows you to input the run length and automatically flags long runs that may benefit from intermediate draw-in points.
Cable lubricant (sometimes called "cable wax" or "pulling compound") can significantly reduce friction during installation and is recommended for any run that is near the conduit factor limit or contains multiple bends. This does not change the conduit factor calculation, but it makes the practical installation easier and reduces the risk of cable damage.
While conduit fill uses the cable factor method, trunking fill uses a direct cross-sectional area calculation. The IET On-Site Guide and Guidance Note 1 recommend that the total cross-sectional area of all cables installed in trunking must not exceed 45% of the internal cross-sectional area of the trunking. BS 7671 does not state this percentage limit in normative regulation text, but the guidance is universally applied in UK practice.
To perform a trunking fill calculation, you need the overall diameter of each cable (including insulation and sheath for multicore cables, or insulation only for singles). The cross-sectional area of each cable is calculated as pi times the radius squared. Sum the areas of all cables, then compare against 45% of the trunking's internal cross-sectional area (width x height for rectangular trunking).
For example, consider a 100 mm x 50 mm trunking. The internal cross-sectional area is 100 x 50 = 5000 mm². 45% of 5000 = 2250 mm². If the total cross-sectional area of all cables is 2000 mm², the trunking is adequate: 2000 ≤ 2250.
The 45% limit is more generous than the approximately 40% limit for conduit because trunking has a removable lid — cables are laid in from the top rather than pulled through, so the friction and pulling tension constraints are less severe. However, the heat dissipation requirement still applies, which is why the fill is limited to less than half.
The Elec-Mate calculator includes a trunking fill mode alongside the conduit fill mode. Select the trunking dimensions, add the cables, and the calculator shows the fill percentage with a pass/fail indication against the 45% limit.
Note also that BS 7671 Reg 530.4.3 requires any trunking system used to carry electrical equipment (such as switches or socket-outlets) to comply with the BS EN 50085 series. Always verify that the trunking product is BS EN 50085-certified before mounting equipment within it.
List every cable that will be drawn into the conduit. For each circuit, count the individual conductors — for example, a single-phase circuit in singles requires a line conductor and a neutral conductor (and a CPC if required). Note the cross-sectional area of each conductor (1.0 mm², 1.5 mm², 2.5 mm², 4 mm², etc.).
From the cable factor table, find the factor for each conductor size. Common values are: 1.0 mm² = 22, 1.5 mm² = 22, 2.5 mm² = 30, 4 mm² = 43, 6 mm² = 58, 10 mm² = 105, 16 mm² = 145. These factors apply to single-core PVC insulated cables (BS 6004 singles).
Multiply the cable factor for each size by the number of conductors of that size, then sum all the results. For example, if you have 4 conductors of 2.5 mm² and 2 conductors of 1.5 mm²: total = (4 x 30) + (2 x 22) = 120 + 44 = 164.
Look up the conduit factor for the conduit size you plan to use. Standard conduit factors for straight runs are: 16 mm = 290, 20 mm = 460, 25 mm = 800, 32 mm = 1400, 40 mm = 2100, 50 mm = 3500. If the conduit run includes bends, reduce the factor according to the difficulty — typically 10% per bend or use the reduced factors from the tables.
If the total cable factor is less than or equal to the conduit factor, the cables will fit. If not, try the next larger conduit size. For the example above (total factor 164), a 16 mm conduit (factor 290) is sufficient. Always leave some spare capacity for future cables if the installation design permits it.
Purpose-built for UK electricians working to BS 7671. Eliminates guesswork and ensures your conduit is correctly sized every time.
Enter the number and size of cables, and the calculator recommends the minimum conduit size. Supports 16 mm to 50 mm conduit and all standard cable sizes.
Handles any combination of cable sizes in the same conduit. Add 1.0 mm² through to 16 mm² singles in any quantity…
Specify the number of bends in the conduit run, and the calculator reduces the conduit factor accordingly.
Shows the fill percentage as a visual gauge. Colour-coded: green for comfortable fill, amber for approaching the limit…
Switch between conduit fill (cable factor method) and trunking fill (45% area method). Calculate trunking fill for all standard trunking sizes.
All cable factors and conduit factors verified against BS 7671:2018+A4:2026 and the IET On-Site Guide. Values match the published tables.
Free conduit fill calculator: check how many cables fit your conduit using the space-factor method. Sizes for PVC and steel conduit to BS 7671.
Try Elec-Mate free
Verified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Join 1,000+ UK electricians using Elec-Mate for on-site calculations. 7-day free trial, cancel anytime.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer, DP Electrical
From £6.99/mo after trial — less than a coffee a week
Manage your privacy settings
We use cookies to enhance your experience and analyse platform usage. Cookie Policy