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4.7k, 1M, 4k7, 470R.Use Decode when you've pulled a through-hole resistor out of a drawer and need to read its value: pick 4 bands for standard ±5% / ±10% parts, or 5 bands for precision ±1% / ±2% parts. Each band selector shows the colour live on the resistor preview.
Use Reverse lookup when you have a target value (say 4.7k or 1M) and need to know what colour bands to look for. The closest representable value is chosen, and any rounding is reported.
Use this whenever you need an equivalent resistance from multiple resistors — e.g. you don't have the exact value in your kit, or you're combining several on a board.
You can paste a list of values like 10k, 4.7k, 470 (or one per line) and click Apply. The result panel shows the working, including the average for parallel pairs and the smallest value (which bounds parallel results).
Use this to scale a voltage down to a known fraction, typically for an ADC input. Pick R1/R2 so the standing current is low (less waste) but not so high-impedance that whatever you connect to Vout drags the ratio off.
Enable RL to model the loaded output, plus the Thevenin equivalent (Rth = R1 ∥ R2) — a useful sanity check for whether the load you have in mind will distort the ratio. Power dissipation is shown per resistor so you can pick a part with adequate wattage.
Don't use a divider as a power supply for anything that draws meaningful current — use a regulator instead.
Use this for any timing or filtering question involving an R and a C: debounce delay, soft-start, charge/discharge, or first-order RC low-pass cut-off. Switch solve for to pick which unknown to compute — e.g. given τ and C, find R.
The Target charge level field lets you ask "how long to reach 90%?" (≈ 2.3 τ), "95%?" (≈ 3 τ), or a custom percentage — handy when a signal needs to settle to within a known error before sampling.
Fill any two of V, I, R, P and the rest are solved automatically. Useful for sizing a part you have a current and voltage for, picking a shunt resistor, or checking the wattage rating you need. If you provide three or four values they're checked for consistency.
Pick the current-limiting resistor for an LED powered from a supply. Enter the supply voltage, the LED forward voltage (Vf), the target current, and how many LEDs are wired in series. The calculator returns the ideal resistor, the power it dissipates, and the nearest E12 value (with the resulting actual current). Aim for at least 2× the dissipated power as the resistor's wattage rating.
Given a calculated resistor value, find the nearest standard part you can actually buy. Pick the E-series (E6 / E12 / E24 / E48 / E96) that matches the tolerance of your kit, and the tool returns the nearest catalog value, the error %, and the next lower and next higher candidates — handy when you have a tight constraint in one direction.
Calculate XC = 1 / (2πfC). Useful for picking a coupling/bypass capacitor (you want XC low at the signal frequency), or for working out the impedance presented by a capacitor at AC. The short interpretation tells you whether the cap looks like a short, a meaningful impedance, or essentially open at that frequency.
Resonant frequency of an ideal LC tank: f₀ = 1 / (2π·√(LC)). Useful for RF tank circuits, filter design, oscillator targets, and rough ballparking of where an LC pair will peak.
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