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Inputs
Example data
| Measured time (µs) | Interpretation | Fiber n | Offset (m) | Slack (%) | Adjusted distance (m) |
|---|---|---|---|---|---|
| 10.000 | Round-trip | 1.468 | 0 | 0 | 1,021.1 |
| 12.500 | Round-trip | 1.468 | 5 | 3 | 1,317.8 |
| 8.000 | One-way | 1.468 | -2 | 2 | 1,640.3 |
Formula used
OTDR distance is based on time-of-flight through fiber:
- v = c / n
- d = v × t / k (k = 2 for round-trip, k = 1 for one-way)
- d_adj = (d + offset) × (1 + slack/100)
Here, c is the speed of light in vacuum (299,792,458 m/s), n is the refractive index, and t is the measured time in seconds.
How to use this calculator
- From your OTDR trace, place markers around the segment or event.
- Enter the measured time and select the correct unit.
- Keep “Round-trip” if using standard OTDR time-to-event.
- Enter fiber refractive index (or velocity factor from the cable spec).
- Add an offset for launch/receive cables, if required.
- Apply a slack percentage for routing and service loops.
- Press Calculate, then export CSV or PDF for documentation.
Practical OTDR distance estimation for construction projects
Optical time-domain reflectometry is widely used during campus builds, roadway ducting, industrial plants, and high-rise projects to confirm fiber lengths, locate events, and document acceptance testing. An OTDR does not “measure distance” directly; it measures the travel time of a light pulse and converts that time into distance using the assumed propagation speed of the fiber. Because propagation speed varies by fiber type and wavelength, using the correct refractive index (or velocity factor) is the foundation of reliable results. When the index is incorrect, the OTDR trace can still look clean while the reported distance silently drifts.
In the field, distance accuracy also depends on how the time marker is chosen. Standard OTDR time-to-event values are typically round-trip, meaning the pulse travels to the event and back. This calculator supports both round-trip and one-way interpretation so you can match the readout from your instrument or software. It also supports a fixed event offset to account for launch cables, receive cables, patch leads, or known reference segments. Finally, a slack allowance is included for planning: installed route length is often longer than the straight-line path due to tray routing, vertical drops, bends, service loops, and riser transitions.
Use the results in three practical ways. First, verify delivered spool or reel length against design assumptions before large pulls. Second, cross-check as-built cable route length against drawings for handover packages. Third, standardize reporting across crews by exporting the same unit set (meters, kilometers, feet, miles) into your daily test log. Consistent distance reporting reduces rework, helps identify mislabeled fibers, and speeds up troubleshooting when multiple trades share pathways.
Example calculation
Suppose an OTDR shows 12.5 µs to a splice event (round-trip). The fiber index is 1.468. You want to add a 5 m offset for launch/patch leads and apply 3% slack for routing. The calculator returns an adjusted one-way distance of approximately 1,317.8 m (about 1.318 km).
Tip: Always confirm the fiber parameter from the cable datasheet and keep it consistent across all test sets and reporting templates.
FAQs
1) Why does refractive index matter for distance?
Distance is computed from pulse travel time and fiber speed. Fiber speed equals the speed of light divided by refractive index. A small index error causes a proportional distance error across the entire trace.
2) Should I select round-trip or one-way time?
Most OTDR time-to-event readings are round-trip because the instrument measures the return signal. If your software already reports one-way distance or one-way time, choose one-way to avoid dividing by two twice.
3) What is velocity factor and how is it used?
Velocity factor (VF) is the fraction of light speed that a pulse travels in the fiber. It is approximately VF = 1/n. If your cable datasheet provides VF, use it directly for consistent results.
4) What does “event offset” represent?
Event offset is a fixed correction applied to the calculated distance. Use it for known launch/receive cable lengths, patch leads, or reference segments so your reported distance matches the portion of the link you intend to document.
5) When should I add slack allowance?
Slack is helpful for planning installed route length, especially in trays, risers, and congested pathways. Use it when estimating cable purchase length or verifying that an installed route reasonably matches design intent.
6) Why might my OTDR distance differ from tape or GIS distance?
OTDR distance follows optical path length, which includes extra routing, coils, and service loops. Tape or GIS may reflect straight-line or duct-path estimates. Differences also come from incorrect fiber parameters or marker placement.
7) How can I improve repeatability across crews?
Standardize the fiber parameter, wavelength/test profile, launch cable method, and time interpretation. Use consistent naming for events and export CSV/PDF reports from the same template so logs remain comparable across teams and dates.