# How to calculate dB of a transmitted power

why if I lose 80% power in the output of a system, Does it mean that I had lost 10×log10(0.2) = -7dB ?

• is it just me or does this question seem trivial? – CyberMen Apr 27 '12 at 18:44
• @CyberMen Please see (dsp.stackexchange.com/faq) – Spacey Apr 28 '12 at 2:27
• @0x90: I'm finding it hard to delete this question because the accepted answer has so many upvotes. That means lots of people thought the answer was interesting... so perhaps the question isn't as trivial as CyberMen suggests? – Peter K. Jul 30 '15 at 14:13

$$G = \frac{P_{out}}{P_{in}} =0.2$$ $$G|_{dB} = 10 \log_{10}(G) = 10 \log_{10}(0.2) \approx -7 \text{ dB}$$