This worksheet below will help you to find out about how far optics will actually reach. Keep in mind that the example here is under perfect conditions. Meaning no splices, kinks, or any other damage to the cable.

• The first thing we need to know is what wavelength are we working with? For this example, we will use SFP-10G-ZR-GT which runs at 1550nm. 
• Next up we make a note of the optics minimum output power. In this case, it would be 0dBm. We can find this value from our data sheets on the manufacturer's website, in this example, GigaTech's website. 
• Now we need to know the receiver sensitivity, also found on the same datasheet. For SFP-10G-ZR-GT this value is -24dBm. 
• The last thing we need to remember is that every connection has a typical loss of 0.75dB.

First up we find out how much power we have to work with:

Minimum Tx Power            0dBm

Receiver Sensitivity        -  24dBm

Available Power:            =  24dB

Connection loss              -   0.75dB

Available Power              =  23.25dB

Now we take the available power and divide that by the loss per km.

23.25dB  /  0.25dB   =  93km

So from this, we see that under perfect conditions SFP-10G-ZR should be able to reach 93km. As a standard manufactures subtract 3dB from the available power as a safety margin which is why this part is listed at 80km.