How to use this calculator
A 60-second walkthrough — match each input to what's on your fiber, then read the boxes at the top.
Watch the walkthrough, or follow the steps below.
- Set the source level. Drag the top slider to your transmitter's spec output — default is 0 dBm, range −25 to +25 dBm.
- Set the distance. Type the span length and pick the unit (m km ft kft mi). Switching units converts the value automatically — the physical distance stays the same.
- Splices & connectors. Type the count of fusion splices on the path and the count of mated connector pairs. Defaults are 4 splices and 2 connectors.
- Splitters. None by default. Tap + Add splitter to add one, then pick the type (1×2 through 1×128) and quantity. Add more rows to cascade — common in tap-and-pass PON designs (e.g. a 1×2 feeding a 1×32).
- Wavelengths. The three result boxes default to 1310, 1550, and 1625 nm. Open any box's wavelength menu to change it — the list groups Standard, CWDM (1271–1611 nm) and DWDM. Each box recalculates on its own.
- Read the boxes. Every box shows Test point (source minus loss — what your meter should read at that wavelength), Total loss (the dB the link burns through), and Loss/km (the fiber attenuation figure). Compare the three side by side.
- Start over. Tap Clear all to return every input to its default.
Frequently asked questions
Defaults and assumptions baked into the calculator.
What fiber attenuation values does this calculator use?
Industry-standard design figures by wavelength:
- 1310 nm — 0.35 dB/km (typical SMF, PON upstream)
- 1490 nm — 0.30 dB/km (PON downstream data)
- 1550 nm — 0.25 dB/km (downstream video / long-haul)
- 1625 nm — 0.27 dB/km (out-of-band monitoring / fault test)
Real-world fiber usually measures tighter than these design figures. Each result box has its own wavelength menu, so you can switch any box to a CWDM channel (1271–1611 nm, ITU-T G.694.2) or a DWDM (C-band) channel. CWDM channels inherit their band's design figure — 0.35 dB/km in the O and E bands, 0.30 in the S band, 0.25 in the C band, 0.27 in the L band.
Note on the E-band (≈1360–1440 nm): these figures assume low-water-peak G.652.D fiber. Legacy fiber installed before ~2003 has a strong 1383 nm water-absorption peak and runs much higher across the E-band — don't budget CWDM E-band channels at 0.35 dB/km on older plant.
What splice loss does the calculator assume?
0.025 dB per fusion splice. This is a typical design / acceptance figure across the 1310-1625 nm range. Mechanical splices and lower-quality fusion splices run noticeably higher in practice.
What connector loss does it assume?
0.5 dB per mated pair — the standard design figure for SC/UPC and SC/APC. Higher-grade APC pairs measure lower in practice; field connectors with contamination can run much higher. Always clean both endfaces before measuring against the budget.
Where do the splitter values come from?
ITU-T G.671 typical figures, which include both the intrinsic split (10·log10(N) dB) and a typical excess loss of ~0.6 dB:
- 1×2 — 3.6 dB
- 1×4 — 7.4 dB
- 1×8 — 10.7 dB
- 1×16 — 13.9 dB
- 1×32 — 16.6 dB
- 1×64 — 20.1 dB
- 1×128 — 23.3 dB
Cascaded splitters add: a 1×2 feeding a 1×32 totals 3.6 + 16.6 = 20.2 dB of splitter loss, plus whatever fiber and connector loss sits between them.
Should the calculated level match what my meter reads?
Within tolerance, yes. The calculator gives you a design-target level — a measured value within roughly 0.5 dB on each side of the design figure usually means a clean install.
If your meter reads more than ~1 dB worse than the budget, the problem is real (bad splice, dirty connector, macrobend). If your meter reads way better than the budget, double-check your meter's reference and wavelength — most "too good to be true" readings turn out to be a 1310 nm reference on a 1550 nm meter, not a magic install.
If your source level is already low or the design is out of spec, no amount of field re-splicing fixes the result. The calculator helps you tell those cases apart — see the guide for examples.
Why three wavelength boxes, defaulting to 1310 / 1550 / 1625?
A real link carries more than one wavelength, and the fiber attenuates each differently — so the loss and test-point level are different at each. Showing three side by side lets you check the whole plan at once instead of toggling back and forth.
The defaults cover the three you most often test: 1310 nm (PON upstream / OTDR), 1550 nm (downstream video / long-haul, the lowest-attenuation window and usually the tightest budget), and 1625 nm (out-of-band monitoring and fault-finding). Point any box at a CWDM or DWDM channel when you're planning a WDM system.
Off-by-a-dB? Missing a splitter ratio? Tell us.
✉ Send feedback