Tools · Loss budget
Fiber link loss budget calculator
Enter the length, the connectors and the splices, and the calculator adds up the loss of the link. Give it the transmitter power and the receiver sensitivity, and it tells you whether the link fits and how long it could be. The defaults are typical figures with their sources below. Type your own over any of them.
Both come from the datasheet of your transceivers, and the sensitivity is a negative number. Leave them empty to get only the loss.
Link loss
With margin
The figures the calculator starts from
Every default is a typical figure, not a limit for your project. Your specification, the cable datasheet or a measurement takes its place.
| Quantity | Default | Where it comes from |
|---|---|---|
| Fiber attenuation at 1310 nm | 0.36 dB/km | Cable datasheets for G.652.D fiber, stated as a maximum |
| Fiber attenuation at 1550 nm | 0.22 dB/km | Cable datasheets for G.652.D fiber, stated as a maximum |
| Loss per connector | 0.5 dB | ITU-T G.671 (11/2025), the maximum for a mated pair |
| Loss per splice | 0.1 dB | ITU-T G.671 gives the typical loss of a fusion splice; rounded up |
| Margin | 3 dB | Design practice. No ITU recommendation gives a number |
ITU-T G.652 allows cabled fiber more attenuation than cable makers state in their datasheets, so the defaults sit below the ceiling of the recommendation.
The link standards, the reflection of each connector polish, the mechanical splice and the multimode figures are typical published values, not quotations from the standards. Check them against the datasheets of your parts and type your own over them.
How the calculator works
Link loss is a sum. Length times attenuation gives the fiber. The number of connectors times the loss of one gives the connectors, and the splices are counted the same way. Anything else on the path is added as it is.
The budget is the transmitter power minus the receiver sensitivity, both in dBm. The link fits when its loss plus the margin is no more than the budget.
The longest link is what remains of the budget after the connectors, the splices and the margin, divided by the attenuation.
The defaults cover single-mode fiber at its two usual wavelengths. For multimode or another wavelength, type the attenuation from the cable datasheet.
Where the calculation can differ from the field
Typical figures describe an average connector and an average splice. A dirty or worn connector can lose several times more, and one bad splice can take the whole margin. A measurement settles it.
The margin covers what the drawing does not show: ageing, repairs, a splice redone after a cable cut. The budget is the limit of the optics, not a target to fill.
A receiver has an upper limit too. On a short link a strong transmitter can overload it, and the link then needs an attenuator. With a link standard picked, the calculator checks that limit and names the attenuator. With your own transceivers, the receiver datasheet gives its maximum input power.
Once the link is built, set the calculation against a measurement. Open the OTDR trace in OTDR Master: the span loss on the screen is the figure to compare with the calculated one, and the event table shows which joint takes more than its share.
Questions
What is the formula for fiber link loss?
Length × attenuation, plus connectors × loss per connector, plus splices × loss per splice. The calculator above does this sum and shows each part.
How much loss per km does single-mode fiber have?
It depends on the wavelength: less at 1550 nm than at 1310 nm. The defaults in the table above are the maximums that cable makers state for G.652.D fiber, and a cable in good condition measures at or below them.
What is an acceptable loss for a fiber link?
One that leaves margin inside the budget of your transceivers, or one that meets the limit in your project specification. There is no single number for every link, which is why the calculator asks for the transmitter and the receiver.
What is span loss?
The total loss between the two ends of a link, the figure a loss test set reports. On an OTDR trace it is the drop in level from the start of the span to its end.