About

Signal attenuation determines whether data arrives intact or vanishes into noise. Every meter of cable, every connector mate, and every fusion splice subtracts power from the transmitted signal. A miscalculation of 0.5 dB in a fiber link budget can push a receiver below its sensitivity threshold, causing bit errors and link failure. This calculator computes total path attenuation A_total using published coefficients from ITU-T G.652, G.655, and TIA-568 standards for single-mode fiber, multimode fiber, copper twisted pair, and coaxial cable. It accounts for cable length, connector insertion loss, and splice loss individually.

The tool assumes linear, frequency-independent loss per unit length at the specified operating wavelength or frequency. It does not model chromatic dispersion, polarization mode dispersion, or non-linear effects such as stimulated Brillouin scattering. For fiber links exceeding 80 km without amplification, verify that your receiver sensitivity accommodates the calculated output power P_out. Pro tip: always add a 3 dB maintenance margin to your power budget to account for aging, temperature drift, and future splices.

Formulas

Total path attenuation sums cable loss, connector insertion losses, and splice losses into a single figure in decibels:

A_total = α × L + n_conn × A_conn + n_splice × A_splice

Where α = attenuation coefficient (dB/km for fiber or dB/100m for copper/coax), L = cable length in the matching unit, n_conn = number of connectors, A_conn = insertion loss per connector (dB), n_splice = number of splices, A_splice = loss per splice (dB).

Output power at the receiver is derived from input power minus total attenuation:

P_out = P_in − A_total

Where P_in and P_out are expressed in dBm. The linear power ratio represents how much of the original signal power survives the link:

Ratio = 10^−A_total10

A ratio of 0.5 means 50% of input power reaches the receiver. The power budget margin indicates remaining headroom:

Margin = Budget − A_total

A negative margin means the link will fail. Industry practice requires at least 3 dB positive margin.

Reference Data

Cable Type	Standard	Frequency / Wavelength	Attenuation Coefficient	Typical Connector Loss	Typical Splice Loss
SMF-28 (Single-Mode)	ITU-T G.652D	1310 nm	0.35 dB/km	0.30 dB	0.05 dB
SMF-28 (Single-Mode)	ITU-T G.652D	1550 nm	0.20 dB/km	0.30 dB	0.05 dB
NZDSF (Non-Zero DSF)	ITU-T G.655	1550 nm	0.22 dB/km	0.30 dB	0.05 dB
OM1 (62.5/125 Multimode)	TIA-568	850 nm	3.50 dB/km	0.75 dB	0.10 dB
OM1 (62.5/125 Multimode)	TIA-568	1300 nm	1.50 dB/km	0.75 dB	0.10 dB
OM2 (50/125 Multimode)	TIA-568	850 nm	3.50 dB/km	0.75 dB	0.10 dB
OM3 (50/125 Multimode)	TIA-568	850 nm	3.50 dB/km	0.75 dB	0.10 dB
OM4 (50/125 Multimode)	TIA-568	850 nm	3.50 dB/km	0.50 dB	0.10 dB
OM5 (50/125 Multimode)	TIA-568	953 nm	3.50 dB/km	0.50 dB	0.10 dB
Cat5e (UTP Copper)	TIA-568-C.2	100 MHz	22.0 dB/100m	0.40 dB	-
Cat6 (UTP Copper)	TIA-568-C.2	250 MHz	33.0 dB/100m	0.40 dB	-
Cat6a (UTP Copper)	TIA-568-C.2	500 MHz	46.0 dB/100m	0.40 dB	-
Cat7 (STP Copper)	ISO/IEC 11801	600 MHz	50.0 dB/100m	0.40 dB	-
Cat8 (STP Copper)	TIA-568-C.2-1	2000 MHz	98.0 dB/100m	0.40 dB	-
RG-6 (Coaxial)	MIL-C-17	100 MHz	6.6 dB/100m	0.50 dB	-
RG-6 (Coaxial)	MIL-C-17	1000 MHz	20.0 dB/100m	0.50 dB	-
RG-11 (Coaxial)	MIL-C-17	100 MHz	4.0 dB/100m	0.50 dB	-
RG-59 (Coaxial)	MIL-C-17	100 MHz	11.0 dB/100m	0.50 dB	-
LMR-400 (Coaxial)	Times Microwave	900 MHz	6.8 dB/100m	0.20 dB	-
POF (Plastic Optical Fiber)	IEC 60793	650 nm	150.0 dB/km	1.00 dB	0.30 dB

Frequently Asked Questions

Fiber and copper links degrade over time. Temperature cycling stresses connectors, dust accumulates on endfaces, and future repairs introduce additional splices. ITU-T G.671 recommends a maintenance margin of 3 dB for links expected to operate for 20 years or more. Without this margin, a link that passes acceptance testing may fail within a few years.

Silica glass fiber has an absorption minimum near 1550 nm (approximately 0.20 dB/km) and a secondary minimum at 1310 nm (approximately 0.35 dB/km). At 850 nm, Rayleigh scattering dominates and attenuation rises to 2 - 3.5 dB/km. Always select the coefficient matching your laser or LED wavelength.

They are cumulative. Each mated pair adds its insertion loss to the total. A link with 6 connectors at 0.3 dB each adds 1.8 dB. Note that the values in the reference table are typical maximums. Actual connectors may perform better (0.1 - 0.15 dB) when factory-polished with APC or UPC endfaces.

No. Bend-induced loss depends on bend radius, fiber geometry, and wavelength in a complex relationship described by coupled-mode theory. This calculator models only linear intrinsic attenuation plus discrete connector and splice losses. For tight-bend scenarios (radius < 15 mm), consult the fiber manufacturer's bend loss specification and add it manually to the total.

Decibels (dB) measure a ratio between two power levels. It is dimensionless. Decibel-milliwatts (dBm) measure absolute power referenced to 1 mW. When you specify input power as 0 dBm, that equals 1 mW. The calculator subtracts attenuation (dB) from input power (dBm) to yield output power (dBm).

Copper attenuation coefficients are specified in dB/100m and increase with frequency due to skin effect and dielectric losses. A Cat6 cable rated at 33 dB/100m at 250 MHz will have lower loss at 100 MHz. The calculator uses the coefficient at the rated maximum frequency. For intermediate frequencies, the actual loss will be lower than calculated.