Fiber Optic Cable Size Chart: Core and Cladding Sizes Explained (9/125, 50/125, 62.5/125)

A fiber optic cable is described by two numbers — its core and cladding diameter in microns (µm), written core/cladding. The three standard sizes are 9/125 (single-mode), 50/125 (laser-optimized multimode, OM2–OM5) and 62.5/125 (legacy multimode, OM1). The cladding is always 125 µm, so in practice you are only reading the core size. The finished cable diameter is a separate spec — from Ø1.8 mm for a 1-fiber indoor cable to Ø15.8 mm for a 144-fiber armored outdoor cable.

Key takeaways

• Cladding is always 125 µm. Only the core changes: 9 µm single-mode, 50 or 62.5 µm multimode.
• Build-up: 9–62.5 µm core → 125 µm cladding → 250 µm coating → 900 µm tight buffer.
• Cable OD is not fiber size. It runs Ø1.8 mm (1-fiber indoor) to Ø15.8 mm (144-fiber armored), set by fiber count and construction.
• Never mix 62.5/125 and 50/125 in one link — the core mismatch causes high loss.

Standard fiber optic sizes at a glance

The core / cladding / coating notation (e.g. 9/125/250) describes the glass and its primary coating in microns. An optical fiber is built up in concentric layers, and every layer except the core is standardized:

The proportions are easy to underestimate. For a physical reference: the 125 µm cladding is about as thick as a human hair (a hair runs ~70–100 µm), and the 9 µm single-mode core inside it is roughly a tenth of that hair's width — about 1/14th of the glass around it, and 1/100th of a 900 µm tight-buffered fiber. The core is a small fraction of the glass, and the glass is a small fraction of the finished cable.

Core vs. cladding: how to read the numbers

A glass optical fiber has two concentric regions of glass, plus protective layers on top:

• Core — the central region that actually carries the light. Its refractive index is slightly higher than the cladding. Core diameter is what changes between fiber types (9, 50, or 62.5 µm) and it determines how light propagates.
• Cladding — the surrounding glass layer with a slightly lower refractive index. That index step traps light in the core by total internal reflection. The cladding is standardized at 125 µm across virtually all telecom and datacom fiber.

That 125 µm figure is why the second number rarely changes: standardizing the outer glass diameter means one family of ferrules, splice sleeves, and alignment V-grooves fits every fiber type. A smaller core forces light down a single path (single-mode); a larger core lets many light paths, or modes, travel at once (multimode).

All three standard fibers share the same outer glass and differ only in the core:

Single-mode sizes: 9/125 (OS1 & OS2)

Single-mode fiber (SMF) has a core of roughly 8–9 µm, universally labelled 9/125. The core is narrow enough that light travels essentially one path, eliminating modal dispersion and enabling very long reach and very high bandwidth. It is the choice for long-haul, metro, FTTH, and any link beyond a few hundred metres.

Single-mode has two ISO/IEC performance grades, both 9/125:

• OS1 — tight-buffered, indoor-focused cable, attenuation up to about 1.0 dB/km at 1310/1550 nm. Typical reach on the order of 2 km for 10G.
• OS2 — loose-tube / blown, outdoor and long-haul cable, attenuation as low as 0.4 dB/km. Supports 10G to 10 km and beyond, and is the mainstream choice for new outside-plant builds.

On the glass itself, the relevant ITU-T standards are G.652 (standard SMF, e.g. G.652.D) and G.657 (bend-insensitive SMF, grades A1/A2/B2/B3) used where tight bends matter, such as FTTH drops and in-building routing.

One nuance worth knowing: "9 µm core" is a nominal label, not the spec that is actually measured. Single-mode fiber is specified by its mode field diameter (MFD) — the width of the light-carrying field, which is slightly wider than the physical core because the mode extends into the cladding. For G.652.D that is typically ~9.2 µm at 1310 nm and ~10.4 µm at 1550 nm (MFD grows with wavelength). This matters in practice: splice and connector loss in single-mode links is driven by MFD mismatch between the two fibers, not by the "9/125" label they share. Two fibers can both be 9/125 and still splice poorly if their MFDs differ.

Multimode sizes: 50/125 (OM2–OM5) and 62.5/125 (OM1)

Multimode fiber (MMF) uses a much larger core so many modes propagate together. That makes it cheap to couple light into with low-cost transceivers (VCSELs/LEDs), but modal dispersion limits distance. It dominates inside data centers and enterprise buildings, where links are short and transceiver cost matters.

• 62.5/125 — OM1: the legacy grade (orange jacket). Limited to ~33 m at 10G; kept alive mostly for patching into existing OM1.
• 50/125 — OM2: 50 µm core, not laser-optimized. Roughly 82 m at 10G.
• 50/125 — OM3: laser-optimized (aqua jacket), 2000 MHz·km EMB at 850 nm. 300 m at 10G, 100 m at 40G/100G.
• 50/125 — OM4: higher-bandwidth laser-optimized (aqua), 4700 MHz·km EMB. 400–550 m at 10G, ~150 m at 40G/100G.
• 50/125 — OM5: wideband multimode (lime-green jacket), adds shortwave WDM (SWDM) across 850–950 nm on top of OM4-class reach.

The critical field rule: never mix 62.5/125 and 50/125 in one link. OM3, OM4, and OM5 share the same 50 µm core and can be patched (design to the lowest grade), but the 62.5 µm OM1 core is a hard boundary.

The complete fiber optic cable size chart

Core/cladding size mapped to fiber type, OS/OM grade, standard jacket color (TIA-598-C), and typical application:

Distances are typical maximums and vary by transceiver, connector loss budget and installation quality — always confirm against the transceiver and cable datasheets.

Fiber count to cable diameter chart (real production specs)

The size chart above describes the glass. The finished cable outer diameter (OD) is driven by fiber count, construction and jacket — not by the core size. Below are measured production dimensions from DYS cable, with tolerances, so you can spec conduit fill, reel size and bend radius accurately.

Indoor & FTTH drop cable

Outdoor loose-tube cable (2–144 fibers)

Read the band that belongs to your construction — the fiber-count bands are not the same across cable types, and lining them up in one column is how spec errors happen. An armored GYTS covers 2–60F in a single Ø10.5 mm build; a non-metallic GYFTY starts at 12F and is slimmer (Ø8.4 mm) through 48F.

Tolerances are per row as shown. Shared mechanicals across the three outdoor constructions: crush 1000 N / 100 mm short-term, 300 N long-term; bend radius 20× OD dynamic, 15× OD static; operating −40 to +70 °C. Data-center MPO/MTP trunks ship at 12F/24F/48F/72F/96F/144F in OM3, OM4 and OS2.

Which standard defines each size

Fiber and cable sizes are governed by specific standards — useful when a spec or RFQ cites them:

Modern fiber: 200 µm, rollable ribbon & high-count cable

The mainstream size chart assumes 250 µm coated fiber, but two changes now affect the count-to-diameter math:

• 200 µm reduced-coating fiber keeps the 125 µm cladding but thins the coating from 250 to 200 µm, packing roughly 1.4× more fibers in the same cable OD — the enabler behind today’s hyperscale high-count cables.
• Rollable (intermittently-bonded) ribbon lets fibers flex like a loose bundle for splicing yet mass-fusion-splice like a ribbon — the basis of 1728, 3456 and 6912-fiber data-center and long-haul cables that a conventional 250 µm loose-tube build could never reach.

Core size is not cable diameter

To avoid the most common mix-up, the build-up from light path to installed cable is:

• Core: 9, 50, or 62.5 µm — the light path.
• Cladding: 125 µm — the bare glass fiber.
• Coating: ~250 µm — the acrylate protective coating.
• Tight buffer: ~900 µm — where used, for handling and connectorization.
• Cable OD: ~2 mm (flat drop) to 15+ mm (armored 144F) — set by fiber count, strength members, water blocking, armoring and jacket.

Bend radius, pulling tension & handling by size

Cable OD drives the two limits that most often get violated on site. The DYS rule of thumb, consistent across the cable lines above:

• Minimum bend radius: 15× the cable OD static (installed), 20× OD dynamic (during pulling). A Ø8.4 mm 48F cable therefore needs ≥126 mm static / ≥168 mm dynamic.
• Pulling tension: stay within the long-term rating, and note it differs per cable — 600 N for outdoor loose-tube (GYFTY / GYTS / GYDXTW), 100 N for flat drop and duplex indoor (GJXFH / GJFJBV), but only 60 N for 1-fiber simplex (GJFJV). Exceeding it stresses the glass and raises attenuation.
• Crush: outdoor loose-tube withstands ~1000 N / 100 mm short-term, 300 N long-term — mind conduit and cleat pressure.

Fiber grades across the DYS product line

As a cable manufacturer since 2009, DYS builds to every grade in this chart:

• Single-mode 9/125 — supplied in G.652D and bend-insensitive G.657A1/A2/B3; the self-supporting FTTH drop cable is built on G.657A for tight in-premise routing.
• Multimode — 50/125 in OM2/OM3/OM4/OM5 and 62.5/125 OM1; data-center MPO/MTP trunks ship in OM3 (12F), OM4 (24F) and single-mode (72F/96F/144F).
• Fiber sourcing — glass from YOFC, Corning, Sumitomo and Fujikura by grade and customer specification.

More references in this series: Fiber Basics hub · Cable selection & application guide · Data center cabling.

Frequently asked questions

What is the standard size of a fiber optic cable?

The glass fiber is standardized at a 125 µm cladding, with a core of 9 µm (single-mode) or 50/62.5 µm (multimode), a 250 µm coating and an optional 900 µm tight buffer. The finished cable outer diameter is not fixed — across the DYS range it runs from Ø1.8 mm for a 1-fiber indoor simplex (and 2.0 × 3.0 mm for a flat FTTH drop) up to Ø15.8 mm for a 144-fiber armored outdoor cable, set by fiber count, construction, strength members and jacket.

What does 9/125 mean in fiber optic cable?

9/125 describes the fiber geometry in microns (µm): a 9 µm light-carrying core surrounded by a 125 µm glass cladding. It denotes standard single-mode fiber. The 125 µm cladding diameter is shared by almost every telecom and datacom fiber, so only the first number — the core size — changes between fiber types.

Is the single-mode core really 9 µm?

9 µm is a nominal label. The value actually specified and measured is the mode field diameter (MFD) — the width of the propagating light field, which is a little wider than the physical core. For ITU-T G.652.D fiber, MFD is typically ~9.2 µm at 1310 nm and ~10.4 µm at 1550 nm. Splice loss in single-mode links comes from MFD mismatch between the two fibers, not from the shared "9/125" label.

What is the standard cladding diameter for optical fiber?

125 µm. Single-mode (9/125), OM1 62.5/125 multimode, and OM2–OM5 50/125 multimode all use a 125 µm cladding. Standardizing the outer glass diameter lets connectors, ferrules (typically 2.5 mm or 1.25 mm bores), splice sleeves, and alignment hardware work across fiber types.

What is the diameter of a 12-, 24-, 96- or 144-fiber cable?

Outer diameter depends on the construction, not just the count — and the count bands differ per construction. For DYS loose-tube outdoor cable: a non-metallic GYFTY runs Ø8.4 mm (±0.2) at 12–48F, Ø12.0 mm at 74–96F and Ø15.0 mm at 122–144F; an armored GYTS runs Ø10.5 mm across 2–60F, then Ø11.5 mm at 62–72F, Ø13.0 mm at 74–96F and Ø15.8 mm at 98–144F (±0.5 unless noted). Indoor and drop cables are far smaller — a 1-fiber simplex is Ø1.8 mm and a flat drop is 2.0 × 3.0 mm.

Is 50/125 better than 62.5/125?

For new installations, yes. 50/125 laser-optimized multimode (OM3/OM4/OM5) supports far higher bandwidth and longer 10G/40G/100G reach than legacy 62.5/125 (OM1). 62.5/125 is a legacy grade kept mainly for patching into existing OM1 plant. Never mix 62.5/125 and 50/125 in the same link — the core mismatch causes high loss.

What is the difference between 250 µm and 900 µm fiber?

250 µm is the primary-coated bare fiber (125 µm glass + acrylate coating) used inside loose-tube and ribbon cables. 900 µm is a tight-buffered fiber — the same 250 µm fiber with an extra buffer layer for direct handling, connectorization and patch cords. Same glass and same 125 µm cladding; the difference is the outer protective layer.

What is the difference between core and cladding?

The core is the central glass region that carries the light signal; the cladding is the surrounding glass layer with a slightly lower refractive index that traps light in the core by total internal reflection. Core size sets the fiber type (single-mode vs multimode); cladding is standardized at 125 µm.

Does the core size equal the cable diameter?

No. Core size (9 µm, 50 µm, 62.5 µm) is the glass light path only. The bare fiber with cladding is 125 µm, coated to 250 µm, and buffered to 900 µm. The finished cable outer diameter (OD) — 2 mm drop cable up to 15+ mm armored outdoor cable — depends on fiber count, strength members, and jacket, not on the core size.

What jacket color identifies single-mode vs multimode fiber?

Under TIA-598-C, single-mode (OS1/OS2) uses a yellow jacket; OM1 and OM2 multimode use orange; OM3 and OM4 use aqua; and OM5 wideband multimode uses lime green. Color coding is a convention, not a guarantee — always confirm the print legend on the cable jacket.

Sourcing a specific size or fiber count? DYS Fiber Optic manufactures single-mode and multimode cable across FTTH, outside-plant, indoor and data-center lines — built to your fiber count, grade, jacket and length, with the production tolerances shown above.