OTDR Distance
Calculate fiber fault distance from OTDR trace timing and the fiber's refractive index.
About OTDR Distance
An OTDR (Optical Time-Domain Reflectometer) sends a light pulse down a fiber and measures the round-trip time until reflected light returns from a fault, connector, or the fiber's end. Because the pulse travels the distance twice (out and back), and travels at the speed of light divided by the fiber's refractive index rather than the vacuum speed of light, distance = (speed_of_light × round_trip_time) / (2 × refractive_index). Using the correct refractive index for the specific fiber type is essential for accurate fault location — an incorrect index directly and proportionally skews the calculated distance.
Formula
Distance from OTDR round-trip time
distance = (speed_of_light × time) / (2 × refractive_index)
- c
- Speed of light in vacuum, 299,792,458 m/s
- t
- Round-trip time in seconds
- n
- Fiber's refractive index
Worked example
With the default values below, OTDR Distance returns Distance to Fault/End of 10,209.52. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Round-Trip Timeminimum 0.001 µs
- 100 µs
- Fiber Refractive Index (n)accepts 1–2
- 1.4682
Results
- Distance to Fault/End
- 10,209.52 m
- Distance to Fault/End
- 10.20952 km
- Light Propagation Speed in Fiber
- 204,190,477 m/s
Step by step
- 1Values used
- 2Distance from OTDR round-trip time
- 3Distance to Fault/End= 10,209.52 m
- 4Distance to Fault/End= 10.20952 km
- 5Light Propagation Speed in Fiber= 204,190,477 m/s
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why divide by 2 in the OTDR distance formula?
The measured time captures the pulse traveling to the reflection point AND back to the OTDR — a full round trip — so the one-way distance to the fault is half of what a straight speed × time calculation over the full round-trip time would give.
Why does the refractive index matter so much?
Light travels slower inside glass fiber than in a vacuum, by a factor equal to the refractive index (typically around 1.4677-1.4682 for standard single-mode fiber). Using the wrong index directly scales the calculated distance by the same proportional error.
Do different fiber types have different refractive indices?
Yes — the exact index varies slightly by fiber manufacturer, core doping, and wavelength used (1310nm vs 1550nm), which is why OTDR equipment lets you input a specific index value rather than assuming one universal constant.
What causes reflections that an OTDR detects?
Any change in refractive index along the fiber path — connectors, mechanical splices, fiber breaks, or the physical end of the fiber — causes some light (Fresnel reflection) to bounce back toward the OTDR, which is what it measures as a distinct event at a given distance.