Resistive divider calculator

One problem with resistive dividers is to find a couple of resistors that will give a required voltage division ratio. Of course this comes from the fact that resistors only exists in discrete sets of values depending on their tolerance. Those sets are called "E series", and denoted as E followed by the number of resistors in one decade. Well, you probably know all this already if you arrived on this page... Anyway, with only dicrete values availables it's not trivial to find the couples of resistors that yield a ratio close to the one required. Hence this little tool.

Select the E series you're working with, the type in your required ratio. A list of the best 12 matches will be computed. Simple and easy!


  1. You probably shouldn't use resistive dividers with ratios close to zero or close to 1 (e.g. 0.01, 0.98) . This will likely result in inaccurate results. It's best to keep the ratio within [0.05 ~ 0.95]. If you really need a very large/small ratio, it's probably a good idea to rethink your design.
  2. Ratios should be within ]0,1]; zero is thus not allowed. Don't put something over 1 either: resistors can't amplify, and weird things may happen. Like the destruction of the universe. Math is powerful, be careful.


Vout = ------- Vin
       R1 + R2

Bootnote: I did this small program for a problem with an analog/digital converter (ADC). It was a 10bit ADC fed by the output of a resisitve divider (factor 1/10). Maximum input for the ADC was 10V. 10 bits means 1023 steps, so the LSB of the results was close (but not equal!) to 10mv (10V/1023 ~ 10mV). To have it equal to 10mv I wanted to add the 1000/1023 factor in the upstream divider. So instead of a 1/10 divider I was now looking for 1/10.23 = 0.097752. Which, it turns out, can be obtained almost exactly by using two simple 3K9 and 36K resitors.

Thanks to Uwe Schueler for spotting a bug that limited the range of proposed values.