How the budget is built
A cable plant's loss budget is the sum of its parts: fiber attenuation times length, plus the loss of every connection (a mated pair of connectors) and every splice, plus anything else in the path such as a splitter. Compare it with your optics' power budget (transmitter output minus receiver sensitivity) to see how much margin is left, and with your test results after installation.
Values used
| Component | Typical (editable) | TIA-568 maximum |
|---|---|---|
| Multimode fiber, 850 nm | 3.0 dB/km | 3.5 dB/km |
| Multimode fiber, 1300 nm | 1.0 dB/km | 1.5 dB/km |
| Singlemode fiber, 1310 / 1550 nm | 0.4 / 0.3 dB/km | 1.0 dB/km premises, 0.5 dB/km outside plant |
| Connection | 0.3 dB multimode; 0.2 dB singlemode (factory or fusion splice-on) | 0.75 dB |
| Splice | 0.3 dB multimode; 0.05 dB singlemode fusion | 0.3 dB |
Typical values and the TIA-568 maximums as published in the Fiber Optic Association's free guide, Calculating Fiber Optic Loss Budgets. We don't reproduce the standard itself.
Worked example
A 0.2 km multimode link at 850 nm with 3 connections and 1 splice: fiber 0.2 × 3.0 = 0.6 dB, connections 3 × 0.3 = 0.9 dB, splice 0.3 dB, total 1.8 dB typical. At the TIA maximums it's 0.7 + 2.25 + 0.3 = 3.25 dB. With optics at −13 dBm out and −21 dBm sensitivity (an 8 dB power budget), the margin is 6.2 dB typical and 4.75 dB worst case, both above a 3 dB safety margin. At 1300 nm the same link is 1.4 dB typical and 2.85 dB at the maximums. These are the FOA guide's numbers and our automated tests.
Tips
- Count connections, not connectors. Include both ends if you test with a one-cable reference, which is the most common method.
- Fast multimode links (1G and up) lose part of their budget to dispersion, so the usable budget can be far smaller than the optics' specs suggest. Check your vendor's link-length table.
- Your pass/fail number is the loss budget plus your test set's measurement uncertainty.
Planning the IP side too? Use the subnet calculator or the VLSM planner.
Sources
- Fiber Optic Association: Calculating Fiber Optic Loss Budgets (method, typical values and TIA-568 maximums quoted there)
- FOA: insertion loss testing (reference methods)
Formulas checked against the sources above and our automated test cases on 2026-10-18. Planning aid, not legal or engineering advice.