Free electronics calculators
Small, sharp tools for the parts of circuit design that are easy to get subtly wrong. Every one is free with no signup, runs entirely in your browser, and never uploads what you type — there is no server for it to go to.
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Resistor color code & Ohm's law calculator
Decode a 4-band or 5-band resistor into its resistance and tolerance, and solve Ohm's law and power — give it any two of V, I, R, and P and it fills in the rest.
"What is brown-black-red-gold?" → 1 kΩ ±5%.
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LED series resistor calculator
Size the current-limiting resistor for one or several LEDs in series, snap it to a resistor you can actually buy (E24), and check the power rating you need so the resistor doesn't run hot.
"Red LED, 5V supply, 20mA?" → 150 Ω dissipating 60 mW, so a ⅛W part is enough.
Several LEDs? Series, one resistor each, or one shared?
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Voltage divider calculator
Compute a divider's output including the sag a real load causes — the loading correction the textbook formula ignores — or design a divider for a target output voltage using a buyable E24 resistor.
"12V, two 10k resistors, 1k load?" → not 6V, and the tool shows how far off.
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555 timer calculator
Astable and monostable, in both directions: analyse the parts you have, or design from a target frequency, duty cycle, or pulse width. Flags the designs a real chip can't run — duty cycles at or below 50%, an R1 that over-currents the discharge pin, drift-prone resistor values.
"1 kHz at 60% duty from a 100nF cap?" → the exact R1/R2 and the E24 pair to buy.
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RC / RL filter calculator
Cutoff frequency and time constant for all four first-order topologies (RC or RL, low-pass or high-pass), plus a design mode that solves for the missing component from a target cutoff and snaps it to a value you can actually buy.
"1 kHz low-pass with a 100nF cap?" → 1.59 kΩ, nearest E24 1.6 kΩ.
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RC charge and discharge time calculator
Find the time to reach a real capacitor voltage threshold, or design a buyable resistor or capacitor for a target delay.
"How long to charge 100 nF through 10 kΩ to 3.16 V from 5 V?" → about 1 ms.
No threshold in mind yet? How long until it's basically fully charged?
Common questions
Why did the voltage divider calculator give a different answer than Vout = Vin × R2 / (R1 + R2)?
Because that textbook formula assumes nothing is drawing current from the output. Add a real load and the divider sags — the calculator's loading correction accounts for the load resistor pulling R2 down, which is why “12V, two 10k resistors, 1k load” does not come out to 6V.
Are the resistor and capacitor values these tools suggest ones I can actually buy?
Where a tool has a design mode — the LED, voltage-divider, 555-timer, RC-filter, and RC-time calculators — it snaps the computed value to the nearest standard E24 part and shows you the exact pair, instead of leaving you to source an odd value like 8,342 Ω.
Can I use these for a safety-critical or production design?
No. They are educational calculators for getting the arithmetic right, not a substitute for reading a datasheet, checking a component's real tolerances, or measuring the circuit you actually built. Treat any output as a starting point to verify, especially anything touching mains voltage or driving real current.
How is this different from a general calculator site like Omni Calculator?
See the full comparison — in short, a general calculator site answers one formula among thousands of unrelated topics, while these six tools snap a design to a resistor or capacitor you can actually buy and flag a 555 design a real chip can't run.
Could I just work these formulas out by hand instead?
Yes — see the full comparison. Every formula here is a standard, published equation; what these tools add is snapping the answer to a resistor or capacitor you can actually buy and flagging a 555 design a real chip can't run, both easy steps to skip when working a formula out on paper.
Does anything I type get uploaded anywhere?
No — there is no server for it to go to. Every calculation runs in your browser, and the only thing stored is an anonymous local count of which features get used, kept in your own browser's storage and easy to turn off on each tool's page.