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Fibre Optic Technician calculations with worked answers

Updated 16 September 2026 · 2 min read

Fibre work is measured in decibels, and decibels are where technicians make mistakes, because they add rather than multiply and because a small number represents a large change in power. The examples below show each calculation step by step, and every figure was recalculated by independent code before publication.

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Worked examples

Try each one on paper first. Every figure is generated and then recalculated by separate code before it is published.

  1. Will this fibre link work?

    A 12 km single-mode link runs at 1550 nm. Assume 0.25 dB/km fibre attenuation, 2 connectors at 0.5 dB each and 5 splices at 0.1 dB each. The transmitter launches -3 dBm and the receiver needs at least -28 dBm. What is the total link loss and the remaining margin?

    Show the working
    1. Fibre loss: 12 km × 0.25 dB/km = 3 dB
    2. Connectors: 2 × 0.5 = 1 dB; splices: 5 × 0.1 = 0.5 dB
    3. Total loss = 3 + 1 + 0.5 = 4.5 dB
    4. Power budget = -3 − (-28) = 25 dB
    5. Margin = 25 − 4.5 = 20.5 dB, comfortably above the 3 dB many designers keep in reserve for ageing and repairs.

    Answer: Loss 4.5 dB, margin 20.5 dB

  2. Converting dBm to milliwatts

    A radio or optical transmitter is rated at 27 dBm. What is that in milliwatts?

    Show the working
    1. dBm is power relative to 1 mW on a log scale: P(mW) = 10^(dBm ÷ 10).
    2. P = 10^(27 ÷ 10) = 10^2.7
    3. P = 501 mW. Every +3 dB roughly doubles the power; every +10 dB multiplies it by ten.

    Answer: 501 mW

  3. How long a file takes to transfer

    How long does a 250 MB file take to copy over a 100 Mbps link? Use decimal units (1 GB = 1,000 MB) and ignore protocol overhead.

    Show the working
    1. Storage is sized in bytes, link speed in bits. Multiply megabytes by 8 to get megabits.
    2. 250 MB × 8 = 2000 Mb
    3. Time = 2000 Mb ÷ 100 Mbps = 20 s ≈ 20 seconds
    4. Real transfers take longer: TCP/IP headers, retransmissions and disk speed all eat into the link rate.

    Answer: 20 seconds

  4. Will this fibre link work?

    A 18.5 km single-mode link runs at 1310 nm. Assume 0.35 dB/km fibre attenuation, 2 connectors at 0.5 dB each and 8 splices at 0.1 dB each. The transmitter launches 2 dBm and the receiver needs at least -24 dBm. What is the total link loss and the remaining margin?

    Show the working
    1. Fibre loss: 18.5 km × 0.35 dB/km = 6.47 dB
    2. Connectors: 2 × 0.5 = 1 dB; splices: 8 × 0.1 = 0.8 dB
    3. Total loss = 6.47 + 1 + 0.8 = 8.28 dB
    4. Power budget = 2 − (-24) = 26 dB
    5. Margin = 26 − 8.28 = 17.73 dB, comfortably above the 3 dB many designers keep in reserve for ageing and repairs.

    Answer: Loss 8.28 dB, margin 17.73 dB

  5. Converting dBm to milliwatts

    A radio or optical transmitter is rated at -17 dBm. What is that in milliwatts?

    Show the working
    1. dBm is power relative to 1 mW on a log scale: P(mW) = 10^(dBm ÷ 10).
    2. P = 10^(-17 ÷ 10) = 10^-1.7
    3. P = 0.02 mW. Every +3 dB roughly doubles the power; every +10 dB multiplies it by ten.

    Answer: 0.02 mW

  6. How long a file takes to transfer

    How long does a 12 GB file take to copy over a 1000 Mbps link? Use decimal units (1 GB = 1,000 MB) and ignore protocol overhead.

    Show the working
    1. Storage is sized in bytes, link speed in bits. Multiply megabytes by 8 to get megabits.
    2. 12000 MB × 8 = 96000 Mb
    3. Time = 96000 Mb ÷ 1000 Mbps = 96 s ≈ 1.6 minutes
    4. Real transfers take longer: TCP/IP headers, retransmissions and disk speed all eat into the link rate.

    Answer: 1.6 minutes

  7. Will this fibre link work?

    A 4 km single-mode link runs at 1550 nm. Assume 0.25 dB/km fibre attenuation, 2 connectors at 0.5 dB each and 2 splices at 0.1 dB each. The transmitter launches 0 dBm and the receiver needs at least -30 dBm. What is the total link loss and the remaining margin?

    Show the working
    1. Fibre loss: 4 km × 0.25 dB/km = 1 dB
    2. Connectors: 2 × 0.5 = 1 dB; splices: 2 × 0.1 = 0.2 dB
    3. Total loss = 1 + 1 + 0.2 = 2.2 dB
    4. Power budget = 0 − (-30) = 30 dB
    5. Margin = 30 − 2.2 = 27.8 dB, comfortably above the 3 dB many designers keep in reserve for ageing and repairs.

    Answer: Loss 2.2 dB, margin 27.8 dB

  8. Converting dBm to milliwatts

    A radio or optical transmitter is rated at 3 dBm. What is that in milliwatts?

    Show the working
    1. dBm is power relative to 1 mW on a log scale: P(mW) = 10^(dBm ÷ 10).
    2. P = 10^(3 ÷ 10) = 10^0.3
    3. P = 2 mW. Every +3 dB roughly doubles the power; every +10 dB multiplies it by ten.

    Answer: 2 mW

What these calculations tell you

A loss budget answers the only question that matters before you build: will this link work, and by how much margin? Decibel and milliwatt conversion lets you move between an instrument reading and the actual optical power, which matters when you compare a measurement with an equipment specification. Transfer time turns a link speed into a number a customer understands when they ask how long a backup will take.

Habits that keep the numbers right

Write down every element of the route before adding anything: the length, the number of splices, the number of connector pairs, and the loss you are assuming for each. State your assumptions, because two technicians using different per-splice figures will disagree by a margin that matters. Keep a few decibels of headroom for repairs: a cut repaired with two extra splices must still work.

Common questions

Why is 3 dB significant?

A loss of 3 dB means roughly half the optical power is gone. It is a useful mental checkpoint when a measurement looks wrong.

Should I use the same attenuation figure for every wavelength?

No. Fibre attenuates more at 1310 nanometres than at 1550, so the wavelength you test at changes the expected result. Use the figure the design or the standard specifies.

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