I have a simple question on resistance calculation but I can't work it out. The diagram is from the site All About Circuits:

enter image description here

I can't figure out why the top parallel total resistance for example is 71 ohms. I thought the calculation for this would be:

$$1/Rt=1/100+1/250$$ $$1/Rt=0.1+0.04=0.104$$ $$Rt=1/0.104=9.6$$

Have I missed something obvious? Appreciating that the answer is probably simple but if I don't ask I'll never find out...

  • \$\begingroup\$ 1/100=0.01 and 1/25=0.004 0.01+0.004=0.014 1/0.014=71.42 \$\endgroup\$
    – cventu
    Nov 9, 2015 at 14:20
  • \$\begingroup\$ For R1 and R2 parallel resistance = \$\dfrac{1}{\dfrac{1}{R1}+\dfrac{1}{R2}}\$ \$\endgroup\$
    – Andy aka
    Nov 9, 2015 at 14:23

1 Answer 1


The way you do it is correct, but the numbers don't check out:

\$\frac{1}{100 \Omega}\$ is 0.01 S and not 0.1. \$\frac{1}{250 \Omega}\$ is 0.004 S and not 0.04. So you end up with 0.0104 S, of which the inverse is 71.429 \$\Omega\$ (which has way to many digits).

  • \$\begingroup\$ Aha. So it is because I'm stupid after all....Thanks for your patience \$\endgroup\$
    – user91280
    Nov 9, 2015 at 15:02
  • \$\begingroup\$ You're not stupid, you just weren't careful or concentrated enough @user91280. \$\endgroup\$
    – Arsenal
    Nov 9, 2015 at 15:06

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