# How to calculate coax cable loss for lengths other than 100 feet

Cable loss for coax is mostly given in loss per 100 feet, express in dB. Say I have cable which is 20 dB loss per 100 feet at frequency x. If I use 50 feet (1/2 the length,) would my loss be:

20 dB / 2 = 10 dB

or

2 x the power = 20 dB - 3 dB = 17 dB

• 20dB loss / 100 ft is huge... am guessing the numbers are for convenience and not measured or spec'd? Commented Mar 24, 2022 at 15:36

It's the first one, if you use 50 feet (1/2 the length) then the loss is 20 dB / 2 = 10 dB.

similarly, if you used 200 feet, the loss would be double… 20dB + 20dB = 40dB loss.

there are several online calculators also available, here's a link to one.

• Thanks. That is how I thought it would be, however I read sites that said the cable length was inversely proportional to the power. Thus, 2 x cable length would be 1/2 power (according to some), thus, a -3dB difference (if that were correct.) Commented Mar 29, 2022 at 0:33
• Marking your answer as accepted. Commented Mar 29, 2022 at 0:33

$$20\text{ dB}/100\text{ ft} = 0.2\text{ db/ft}$$
The loss at any length $$d$$ ft will be:
$$L(d) = 0.2\times d$$
At 50 ft, $$d=50$$, so $$L = 10\text{ dB}$$.