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RJ11 vs RJ12: Understanding the Difference
If you've ever picked up a telephone cable and wondered whether it's an "RJ11" or "RJ12" connector, you're not alone — the two look almost identical and, confusingly, often use the exact same plastic housing. In this guide, you'll learn what actually separates RJ11 from RJ12, why one will plug straight into the other's socket, and which cables, crimp plugs and couplers from our range are right for your job — whether that's a broadband router, a phone extension, or a PBX/alarm system that needs all six wires.
Key Differences: RJ11 vs RJ12
| Feature | RJ11 | RJ12 |
|---|---|---|
| Housing size | 6-position (6P) modular plug | 6-position (6P) modular plug — identical shell |
| Contacts wired | 2 or 4 contacts (6P2C / 6P4C) | All 6 contacts (6P6C) |
| Typical use | Telephone line, ADSL/broadband modem | PBX extensions, alarm panels, some control/automation equipment |
| Cross-compatibility | Plugs into an RJ12 socket, but only outer/middle pins connect | Plugs into an RJ11 socket, but middle pins may be unused |
| Wiring needed | Line pair only (voice/data signal) | Line pair plus extra control/signalling pairs |
| Best-selling Kenable product | ADSL Modem Cable RJ11 to RJ11, 0.5m | Flat RJ12 6P6C Cable, 2m |
Pro tip: Because RJ11 and RJ12 share the same 6-position housing, an RJ11 plug will physically fit into an RJ12 socket and vice versa. The difference is purely in how many of the six contacts are actually wired — so a cable that looks right may not carry the signals your equipment needs.
RJ11 Cables: What They're For
RJ11 is the connector you'll find on almost every UK telephone cable and ADSL broadband modem lead. It uses only the two centre pins (sometimes four, for a second line) inside the 6-position housing, which is all a standard voice or broadband line needs.
ADSL Broadband Modem Cable RJ11 to RJ11, Black, 0.5m
£0.48Available in 9 lengths and 2 colours from £0.48 — See all variations
A short, tidy RJ11-to-RJ11 lead for connecting a phone socket directly to a router or ADSL modem. Ideal where the master socket sits right next to your router.
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BT to Modem RJ11 Cable, Dialup/Sky, 2-wire, 5m
£1.25Available in 3 lengths from £0.68 — See all variations
A BT-plug-to-RJ11 lead for wiring a modem, Sky box or fax machine into a standard BT telephone socket — handy where the router is further from the wall than a short RJ11 cable can reach.
View ProductRJ11 pros and cons
- Pro: Fits every UK phone socket, router and ADSL modem out of the box
- Pro: Cheap, widely stocked, and available in short lengths for neat installs
- Pro: The BT-plug version connects straight into a wall master socket without an adapter
- Con: Only 2 or 4 contacts wired — no use for equipment needing extra control wires
- Con: Easy to mistake for RJ12 when buying loose crimp plugs
RJ12 Cables: What They're For
RJ12 uses the same 6-position housing as RJ11, but all six contacts are wired. This is essential for equipment that needs more than a simple voice pair — such as some PBX phone extensions, intercoms, alarm panels and control/automation gear that pass extra signalling or power over the extra pairs.
Flat RJ12 6P6C to RJ12 6P6C Cable, 2m
£1.30Available in 5 lengths from £0.82 — See all variations
A fully-wired 6-core flat cable with RJ12 plugs at both ends. Fits the same socket as RJ11 but carries all six wires, making it suitable for PBX extensions, alarm panels and control equipment that need the extra pairs.
View ProductRJ12 pros and cons
- Pro: All 6 wires connected, so it works anywhere a simple RJ11 phone cable won't
- Pro: Flat cable design is easy to run under carpets or along skirting
- Pro: Same housing as RJ11, so it fits standard phone-style sockets
- Con: Overkill (and unnecessary cost) for a simple broadband or phone connection
- Con: Less commonly stocked in shops, so it's worth keeping spares on hand
Crimp Plugs: RJ11 6P4C vs RJ12 6P6C
If you're making up your own cables rather than buying pre-terminated leads, the plug you crimp on determines whether the finished cable behaves as RJ11 or RJ12 — the cable itself may be the same flat 6-core stock either way.
RJ11 6P4C Crimp Ends – 100 Pack
£5.11Bulk 6P4C crimp plugs for making up your own ADSL and telephone cables. The 4-contact wiring covers standard voice and broadband lines, and a 100-pack is ideal for installers or workshops getting through plenty of leads.
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RJ12 6P6C Modular Crimp Plugs – 10 Pack
£0.96Fully-populated 6-conductor crimp plugs, perfect for terminating 6-core cable where you need every pin connected — PBX extensions, alarm loops or control wiring. Supplied as a smaller 10-pack for occasional jobs and repairs.
View ProductNote: Both plug types use the same 6-position shell, so it's easy to grab the wrong pack. Check whether your job needs just the line pair (choose RJ11 6P4C) or all six conductors (choose RJ12 6P6C) before crimping.
Joining Cables: The RJ11/RJ12 6P6C Coupler
Because RJ11 and RJ12 plugs share the same housing, one coupler can join either type — as long as the wiring on both sides matches what your equipment needs.
RJ11/RJ12 6P6C Female Coupler Adapter
£0.84A straight-through female-to-female coupler that joins two RJ11 or RJ12 cables to extend their reach, or connects an RJ11 lead to an RJ12 lead where only the shared pins are needed. A handy stock item for any phone or PBX installer.
View ProductWhich Should You Choose?
- Connecting a router or modem to a phone socket: go for the RJ11-to-RJ11 ADSL cable — it's designed exactly for this job.
- Wiring a modem, fax or Sky box straight from a BT master socket: the BT-to-Modem RJ11 cable gives you the longer 5m reach.
- Wiring a PBX extension, alarm panel or control device that specifies 6 wires: choose the RJ12 6P6C flat cable instead of a standard phone lead.
- Making your own cables in bulk for broadband/telephone jobs: the RJ11 6P4C crimp plug 100-pack is the most cost-effective option.
- Terminating a handful of 6-core cables for PBX/alarm work: the RJ12 6P6C crimp plug 10-pack is sized right for smaller jobs.
- Extending or joining an existing cable run: the RJ11/RJ12 coupler works with either connector type.
Pro tip: Most of these cables and crimp packs are also available in other lengths and pack sizes on Kenable — if 0.5m, 2m, 5m or a 10/100-pack doesn't match your job, check the product page for alternative options.
Frequently Asked Questions
Will an RJ11 plug fit into an RJ12 socket?
Yes. RJ11 and RJ12 both use the same 6-position modular housing, so the plugs are physically interchangeable. The difference is in how many of the six contacts are wired — an RJ11 plug only connects the middle two (or four) pins, so it won't carry the extra signals an RJ12 device needs.
Can I use an RJ12 cable instead of RJ11 for my broadband?
Yes, because the centre pins used for voice/broadband on an RJ11 line are also present and wired on an RJ12 cable. It'll work fine, though it's unnecessary cost since a standard RJ11 ADSL cable does the same job for less.
What is RJ12 typically used for?
RJ12 is used wherever equipment needs all six wires in the connector rather than just the line pair — common examples include some PBX telephone extensions, alarm system loops, and control or automation devices that pass extra signalling over the spare pairs.
Which crimp plugs do I need to make my own RJ11 or RJ12 cables?
For standard telephone or broadband cables, use RJ11 6P4C crimp plugs, which wire the middle four pins. For cables that need every conductor connected, use RJ12 6P6C crimp plugs, which wire all six.