How the calculation works
Link loss = fiber loss + connection loss + splice loss + other losses, all in dB:
- Fiber = length in km × attenuation in dB/km
- Connections = mated connector pairs × loss per connection
- Splices = splices × loss per splice
- Other = splitters, couplers or other passive parts you enter
Power budget = the optic’s minimum transmit power (dBm) − its receiver sensitivity (dBm).
Margin = power budget − link loss. The link passes when the margin is at least your safety margin. The calculator also shows the longest run the budget allows for attenuation: (power budget − safety margin − connection, splice and other losses) ÷ attenuation.
Defaults are the TIA-568 maximums, as given by the Fiber Optic Association’s loss budget guide (read October 2026), with its typical values for comparison:
| TIA-568 maximum | Typical | |
|---|---|---|
| Multimode, 850 nm | 3.5 dB/km | 3 dB/km |
| Multimode, 1300 nm | 1.5 dB/km | 1 dB/km |
| Single-mode, 1310 nm | 1.0 dB/km inside plant, 0.5 outside plant | 0.4 dB/km |
| Single-mode, 1550 nm | 1.0 dB/km inside plant, 0.5 outside plant | 0.3 dB/km |
| Mated connection | 0.75 dB | 0.3 dB |
| Splice | 0.3 dB | 0.3 dB multimode, under 0.05 dB single-mode fusion |
The 3 dB default safety margin follows the FOA’s guidance that margin should be more than about 3 dB. Planning with maximums is conservative: if a link passes on maximums, it should pass when built properly. Testing the installed link with a light source and power meter, or an OTDR, is what proves it.
The check covers attenuation only. Modal bandwidth (multimode) and dispersion (long single-mode runs) can limit reach before loss does, so compare the length with the optic’s rated reach for your fiber type.
Worked example
The FOA’s example: 200 m of multimode at 850 nm, 3 mated connections and 1 splice, with an optic that transmits at least −13 dBm and a receiver sensitivity of −21 dBm:
| TIA-568 maximum | Typical | |
|---|---|---|
| Fiber: 0.2 km × 3.5 (or 3) dB/km | 0.7 dB | 0.6 dB |
| Connections: 3 × 0.75 (or 0.3) dB | 2.25 dB | 0.9 dB |
| Splice: 1 × 0.3 dB | 0.3 dB | 0.3 dB |
| Link loss | 3.25 dB | 1.8 dB |
| Margin against the 8 dB power budget | 4.75 dB | 6.2 dB |
On maximums the link passes the 3 dB safety margin with 1.75 dB to spare. The budget would cover (8 − 3 − 2.25 − 0.3) ÷ 3.5 = 0.7 km, or 700 m, of this fiber with the same connections and splice.
A campus run: 1,500 m of outside-plant single-mode at 1310 nm, 4 connections and 2 splices, with an optic rated at −8.2 dBm minimum transmit and −14.4 dBm sensitivity (a 6.2 dB budget):
- On TIA maximums: 0.75 + 3 + 0.6 = 4.35 dB of loss, a 1.85 dB margin. It fails the 3 dB safety margin by 1.15 dB.
- On typical values (0.4 dB/km, 0.3 dB connections, 0.05 dB fusion splices): 0.6 + 1.2 + 0.1 = 1.9 dB, a 4.3 dB margin. It passes with 1.3 dB to spare.
Most of the difference is the four connections. A link like this is worth testing before ordering optics; if it tests close to the maximums, an optic with a larger power budget avoids the risk.
