OTDR Distance Calculator
Calculate fiber fault distance from OTDR round-trip time and the fiber's refractive index.
Inputs
Time for the light pulse to travel to the fault/end and reflect back
Typical single-mode fiber: 1.4677-1.4682 at 1310/1550nm
Distance to Fault/End
10,209.52m
Distance to Fault/End
10.20952km
Light Propagation Speed in Fiber
204,190,477m/s
Step by step
Values used
Round-Trip Time = 100 µs; Fiber Refractive Index (n) = 1.47
Distance from OTDR round-trip time
distance = (speed_of_light × time) / (2 × refractive_index)
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
How it works
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
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.