How to use this calculator
A 60-second walkthrough — match each input to what's on your fiber, then read the cards 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. Drag the second slider 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 6 splices and 2 connectors.
- Splitters. Pick the type (1×2 through 1×128) and quantity. Tap + Add splitter to cascade a second splitter — common in tap-and-pass PON designs (e.g. a 1×2 feeding a 1×32).
- Wavelength. Tap any blue wavelength chip on the result cards to cycle 1310 → 1490 → 1550 → 1625 nm. All three cards update with the new attenuation figure.
- Read the cards. Total loss is the dB the link burns through. Test point is source minus loss — what your meter should read. Loss/km is the fiber attenuation figure for the selected wavelength.
- 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. Wavelength defaults to 1550 nm — tap any blue wavelength chip on the result cards to switch.
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 default to 1550 nm?
1550 nm is the most common wavelength for downstream video in HFC/PON deployments and the dominant choice for long-haul. It also has the lowest typical attenuation of the four wavelengths, so a 1550 nm budget is usually the tightest constraint in a design — if it passes at 1550, the upstream 1310 nm and downstream 1490 nm data wavelengths typically pass too.
Off-by-a-dB? Missing a splitter ratio? Tell us.
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