Introduction

Cable glands use three major thread systems worldwide — Metric (DIN 63486 / EN 50462), PG (Panzergewinde, DIN 40430), and NPT (National Pipe Taper, ASME B1.20.1). Buyers often ask whether a PG11 gland can replace an M20 gland, or whether an NPT fitting will thread into a metric hole. The short answer is no — the three systems are mechanically incompatible, and forcing them together destroys the seal and risks equipment damage.

This guide explains the dimensional differences, shows practical thread-to-thread conversion by cable outer diameter (OD) band, and lists the adapter solutions that let you bridge legacy PG enclosures to modern metric or NPT hardware without re-machining.

The Three Thread Standards at a Glance

Before converting anything, you need to know what each standard actually is:

Two facts decide compatibility:

• Metric and PG are both straight (parallel) threads , but their pitch and flank angle differ — an M20×1.5 has 1.5 mm pitch at 60°, while PG11 has ~1.014 mm pitch at 80°. They will not mesh.
• NPT is tapered , so it wedges tighter as it screws in. Mixing a taper into a straight hole (or vice-versa) leaves only 1–2 threads engaged and no reliable mechanical lock.

Why You Cannot Mix Threads

Engineers sometimes assume that if the major diameter is “close enough,” the gland will work. It will not, for three reasons:

1. Pitch mismatch. Even when two threads share a similar OD, the distance between crests differs. Metric coarse pitch (e.g. M20×2.5) versus the fine gland thread means the second turn binds and cross-threads.
2. Flank angle. A 60° metric crest against an 80° PG crest contacts at a knife edge, not a flanking face. Sealing torque drops and the gland loosens under vibration.
3. Sealing strategy. Metric and PG glands seal on an O-ring or elastomer gasket against the enclosure face (parallel thread + sealing ring). NPT seals on the thread flanks themselves with thread sealant or PTFE tape. You cannot get an NPT-style metal-to-metal seal from a parallel metric thread, and vice-versa.

The result of forcing a mismatch is a gland that may look installed but has no IP rating, low pull-out resistance, and a thread that galls — frequently the cause of water ingress failures found in field returns.

Conversion by Cable Outer Diameter

When a customer needs a “PG11 equivalent” or “M20 to NPT,” what they really need is a gland that fits the same cable OD band and a thread that mates the enclosure hole. The conversion is therefore done in two steps: match the sealing range to the cable, then match (or adapt) the entry thread.

The table below maps the most common PG sizes to their metric “equivalents” — defined as the metric gland whose cable sealing range overlaps the PG gland’s range.

Note: “Equivalent” here means same cable OD coverage, not thread interchangeability. The metric thread has a different OD than the PG thread, so the enclosure entry hole must still match.

NPT Conversion

NPT sizes are nominal in inches and tapered, so they don’t map cleanly to metric ODs. Field conversions rely on adapted cable OD ranges instead of nominal sizes:

Because NPT is tapered and metric/PG are parallel, an NPT gland will not seal in a metric tapped hole without a dedicated sealing ring or adapter. Always verify the enclosure entry thread standard before specifying the gland.

Adapter Solutions for Legacy Enclosures

When the enclosure hole cannot be re-tapped — for example a sealed stainless-steel cabinet or a moulded plastic housing — the practical fix is a thread adapter (reducing bushing). These convert the entry thread on one side and provide the new standard on the other:

• PG → Metric: A PG11 female / M20 male adapter lets you fit a modern M20 gland into a legacy PG11 tapped hole.
• Metric → PG: The reverse — M20 female / PG11 male — for new enclosures that must accept older PG glands.
• NPT → Metric: A 1/2″ NPT female / M20 male adapter bridges US-spec fittings to metric glands.
• Locknuts and sealing rings are still required on the parallel side; NPT-side sealing uses thread sealant or PTFE tape.

Adapters add roughly 8–15 mm of stack height and one more sealing interface, so for new builds the cleaner solution is always to specify the correct thread standard from the start. Adapters are a retrofit tool, not a design shortcut.

How to Identify an Unknown Thread

If you’re replacing a gland and the thread standard isn’t marked, identify it before ordering a replacement:

1. Measure the major (outer) diameter of the entry thread with a caliper. Cross-reference to the PG OD table (12.5, 15.2, 18.6, 20.4…) and the metric table (12, 16, 20, 25, 32…). A reading of ~20.0 mm is almost certainly M20; ~20.4 mm is PG13.5.
2. Measure the pitch with a thread gauge (comb). Metric coarse pitches are 1.0–2.5 mm; PG pitches run finer (~1.0 mm at PG11). NPT pitches are given in threads-per-inch (TPI), e.g. 1/2″ NPT = 14 TPI.
3. Check for taper. Roll the fitting on a flat surface — if one end lifts, it’s tapered (NPT). If it lies flat, it’s parallel (metric or PG).
4. Confirm the cable OD range marked on the gland body, and match the replacement’s sealing range to it.

This four-step check prevents the most common ordering error: buying a gland by “size” without confirming thread standard, then discovering the new gland won’t thread into the enclosure.

Choosing the Right Brass Cable Gland

For most industrial and outdoor enclosures, brass cable glands offer the best balance of mechanical strength, EMC shielding (when earthed), and long-term sealing. A standard brass cable gland from a qualified manufacturer is available in metric, PG, and NPT entry threads, with IP68 sealing and a full cable-OD range. Specifying the thread standard up front — and using adapters only for retrofit — keeps the sealing system reliable and the installation certifiable.

Conclusion

Metric, PG, and NPT are not interchangeable thread systems; they differ in pitch, flank angle, and taper. Conversion is practical only through cable-OD matching plus thread adapters — never by forcing mismatched threads. For new builds, specify the thread standard explicitly; for retrofits, use a certified adapter and keep the sealing interface intact. When in doubt, measure the major diameter and pitch before ordering, and source glands from a manufacturer that lists all three standards.

Frequently Asked Questions

Can I screw an M20 gland into a PG11 hole?

No. The M20 thread has a 20.0 mm OD at 60° flank angle with 1.5 mm pitch; the PG11 hole is cut for an 18.6 mm OD at 80° with ~1.0 mm pitch. They will bind after the first turn and cannot seal. Use an M20-female / PG11-male adapter, or re-tap the hole.

Is NPT the same as metric thread?

No. NPT is a tapered thread (3/4″ per foot) that seals on the thread flanks with sealant; metric is a straight thread that seals on an O-ring or gasket against the enclosure face. The two cannot be interchanged without an adapter.

What does “PG11 to M20” actually mean?

It refers to a conversion where a cable gland sized for the same cable OD band (roughly 5–10 mm) is changed from a PG11 entry thread to an M20 entry thread. The cable fit is equivalent; the entry thread is not, so an adapter or re-tapped hole is required.

Are PG threads still manufactured?

Yes, but DIN 40430 (the PG standard) has been formally withdrawn in favour of metric threads. Many legacy enclosures still carry PG taps, so PG glands and PG-to-metric adapters remain in production for retrofit and maintenance — not for new designs.

How do I convert NPT to metric for a cable gland?

Match the cable OD range first, then use an NPT-to-metric adapter bushing (e.g. 1/2″ NPT female to M20 male). Apply thread sealant on the NPT side and a sealing ring on the metric side. Never thread an NPT gland directly into a metric tapped hole.