Hello…
When using E-Series Resistor calculator I miss the option to select only serial or only parallel connection of multiple resistors to get precise value.
Usually I do serial connection of two resistors if I use THT resistors, but I do use parallel connection if I use SMD resistor. So forcing calculator to use just serial or parallel could be fine.
There are tools on the web like http://sim.okawa-denshi.jp/en/teikokeisan.htm but keep in mind that E-Series are built such that tolerances overlap. E.g. E24: a 470k 5% resistor can have from 446.5 to 493.5, the next bigger one, 510k can have from 484.5 to 535.5k and in practice may even be lower than the 470k so combining resistors to get more precision is very limited. It is better to switch to a higher, finer graded E-Series to get precision you can rely on.
Usually I stock E12 1% values. And online calculators are great but the Kicad calculator is offline and handy. I prefer offline tools to keep job running even when web dies.
I understand, I have the same stock in my cellar, although only 5% (reminder of my youth). When I needed ‘odd’ values at work, I always put them on stock. It werent so many in my case and resistors (especially SMD) are dirt cheap. Anyhow, here I have an offline tool for you:
http://jahonen.kapsi.fi/Electronics/ResOptimizer/
I didn’t try it but it looks good.
BTW: If you ever need to calculate a voltage divider, I have one on my homepage (online, only 
).
I often parallel or series resistors. Mostly for flexibility tweaking regulator output voltages. I think the main decision is whether you want a high or low resistance value. If you want to get to 2.03 Meg ohms, it will probably be cheaper/easier with series connections. Start with the highest available value which is lower than the target, and then tweak it with a lower value in series.
And if you want to get to 3.79 ohms, parallel will be easier. Start with the lowest available value which is higher than your target, and then add a higher value in parallel to tweak it closer to your target.
To get to values of 1K to 10K you might go either way, depending on what you have. Series connections are better for high voltage and parallel is better for high current.
I do the calculating either with my ~35 year old HP 11C calculator or an Excel spreadsheet.
I know how to calculate and why. I just think that kicad calculator should offer to select if we want parallel or serial connection.
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