Voltage divider calculator

Two resistors in series carry the same current, so the supply splits between them in proportion to their resistance — that is the whole idea, and it is why the ratio matters and the absolute values do not. What the absolute values decide is everything else: how much current the pair wastes continuously, how much heat each one makes, and how badly the output sags the moment anything is connected to it.

How it is calculated

output = supply × bottom ÷ (top + bottom)

The current through the pair is the supply divided by the sum of both resistors, and the output is that current through the bottom one. Doubling both resistors leaves the output untouched and halves the current, which is the trade this page is really about.

Source: Resistors in series carry the same current, so each takes a share of the supply in proportion to its resistance

Questions people ask

Why does my divider give the wrong voltage under load?

Because whatever you connected is now part of the divider. The load sits in parallel with the bottom resistor, lowering the effective resistance and pulling the output down. The usual rule is to make the divider stiff enough that its current is at least ten times what the load draws — or to buy a voltage regulator, which is what a divider is a poor substitute for.

Should I use large or small resistors?

Large ones waste less: a 9 V supply across two 10 kΩ resistors draws 0.45 mA continuously, while two 100 Ω resistors would draw 45 mA and warm up doing it. Large ones are also more easily upset by load and by noise. Tens of kilohms is the usual compromise for a signal, hundreds of ohms when something has to be driven.

Can I use this to power something from a higher supply?

No, and this is the most common misuse. A divider holds its output only while nothing draws current from it. Powering a circuit through one means the output moves with the load — it is a reference, not a supply. Anything that draws real current wants a regulator or a buck converter.

Which resistor is the top one?

The one between the supply and the output point; the bottom one goes from the output to ground. The output is measured across the bottom resistor, which is why making the bottom one larger raises the output.

What about the power figures?

They tell you which part gets hot, and they are rarely equal: the resistor with more resistance carries the same current across a larger drop, so it dissipates more. Both are usually tiny in a signal divider, and both matter once you are dividing a supply rail with low-value resistors.

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