Electrical Calculators
Free electrical calculators for circuits and wiring: Ohm's law and power from any two quantities, resistor colour codes in both directions, equivalent capacitance and inductance, the LED series resistor with the preferred value that really exists, loaded and unloaded voltage dividers, current dividers, wattage and running cost, battery runtime and capacity conversion, and the conductor size a run needs once ampacity and voltage drop are both checked. Every result names the preferred value it snapped to and the power it dissipates, because a calculated resistance is not a part and a part that gets hot is a part that fails. Ohm's law, electrical power and equivalent resistance are the Physics batch's engines, reused rather than reimplemented, so a number here and a number there can never disagree.
Most popular
- electricalResistor● ready
Enter any two of voltage, current, resistance and power to get the other two, then the nearest preferred value, its dissipation and the power rating to fit.
- electricalResistor Color Code● ready
4, 5 or 6 bands in either direction — with the resistor drawn in its actual colours, the tolerance range, and the temperature coefficient when a sixth band is present.
- electricalLED Resistor● ready
R = (Vs − Vf) ÷ I for a single LED or a series string, rounded up to a preferred value, with the dissipation and the resistor rating to fit.
- electricalVoltage Divider● ready
Vin × R2 ÷ (R1 + R2), plus the output once a real load is attached — so the sag that breaks dividers in practice is visible, not hidden.
- electricalWire Size● ready
Ampacity from the AWG table and voltage drop from the conductor resistance, checked together over the run — with every gauge considered listed.
- physicsOhm's Law● ready
Enter any two of voltage, current and resistance and get the third — with millivolts, milliamps and kilo/mega-ohms selectable per field.
Ohm's Law & Power
- physicsOhm's Law● ready
Enter any two of voltage, current and resistance and get the third — with millivolts, milliamps and kilo/mega-ohms selectable per field.
- physicsElectrical Power● ready
Enter any two of P, V, I and R and the other two are derived — the tool picks the arrangement that matches the pair you gave.
- electricalResistor● ready
Enter any two of voltage, current, resistance and power to get the other two, then the nearest preferred value, its dissipation and the power rating to fit.
Resistors
- electricalResistor Color Code● ready
4, 5 or 6 bands in either direction — with the resistor drawn in its actual colours, the tolerance range, and the temperature coefficient when a sixth band is present.
- physicsEquivalent Resistance● ready
Add as many resistors as you like, choose series or parallel, and get the equivalent resistance plus each one's share of the voltage or current.
- electricalLED Resistor● ready
R = (Vs − Vf) ÷ I for a single LED or a series string, rounded up to a preferred value, with the dissipation and the resistor rating to fit.
Capacitors & Inductors
- electricalCapacitor● ready
Q = C·V solved for whichever quantity is missing, in mC, V and µF, with the stored energy in joules and millijoules.
- electricalCapacitors in Series● ready
Add as many capacitors as you need and read the equivalent — with the rule spelled out, because capacitors combine the opposite way round from resistors.
- electricalInductor● ready
½LI² for the energy held at a current, and the series/parallel equivalent for a set of inductors — which combine like resistors, not like capacitors.
Dividers
- electricalVoltage Divider● ready
Vin × R2 ÷ (R1 + R2), plus the output once a real load is attached — so the sag that breaks dividers in practice is visible, not hidden.
- electricalCurrent Divider● ready
Each branch current, the shared voltage and the parallel equivalent — where the smaller resistor carries the larger current.
Power, Batteries & Wiring
- electricalWattage● ready
P = V × I for the load, then kilowatt-hours and a cost estimate from the hours per day, the number of days and your tariff.
- electricalBattery Runtime● ready
Capacity ÷ current, then the same figure after the depth of discharge the chemistry tolerates and the converter's efficiency — in hours and minutes.
- electricalBattery Capacity● ready
Convert charge units directly, and to watt-hours once the pack voltage is known — the figure that makes two batteries comparable.
- electricalWire Size● ready
Ampacity from the AWG table and voltage drop from the conductor resistance, checked together over the run — with every gauge considered listed.
All tools
- electricalResistor● ready
Enter any two of voltage, current, resistance and power to get the other two, then the nearest preferred value, its dissipation and the power rating to fit.
- electricalResistor Color Code● ready
4, 5 or 6 bands in either direction — with the resistor drawn in its actual colours, the tolerance range, and the temperature coefficient when a sixth band is present.
- electricalCapacitor● ready
Q = C·V solved for whichever quantity is missing, in mC, V and µF, with the stored energy in joules and millijoules.
- electricalCapacitors in Series● ready
Add as many capacitors as you need and read the equivalent — with the rule spelled out, because capacitors combine the opposite way round from resistors.
- electricalInductor● ready
½LI² for the energy held at a current, and the series/parallel equivalent for a set of inductors — which combine like resistors, not like capacitors.
- electricalLED Resistor● ready
R = (Vs − Vf) ÷ I for a single LED or a series string, rounded up to a preferred value, with the dissipation and the resistor rating to fit.
- electricalVoltage Divider● ready
Vin × R2 ÷ (R1 + R2), plus the output once a real load is attached — so the sag that breaks dividers in practice is visible, not hidden.
- electricalCurrent Divider● ready
Each branch current, the shared voltage and the parallel equivalent — where the smaller resistor carries the larger current.
- electricalWattage● ready
P = V × I for the load, then kilowatt-hours and a cost estimate from the hours per day, the number of days and your tariff.
- electricalBattery Runtime● ready
Capacity ÷ current, then the same figure after the depth of discharge the chemistry tolerates and the converter's efficiency — in hours and minutes.
- electricalBattery Capacity● ready
Convert charge units directly, and to watt-hours once the pack voltage is known — the figure that makes two batteries comparable.
- electricalWire Size● ready
Ampacity from the AWG table and voltage drop from the conductor resistance, checked together over the run — with every gauge considered listed.
About Electrical Calculators
Electronics calculators fail in three predictable ways. The first is prefixes: a resistor in kΩ against a formula expecting ohms, a capacitor in µF against farads, a battery in mAh against amp-hours — so every field here states its unit and the conversion happens before the arithmetic. The second is that a calculation is not a part: 219.4 Ω does not exist and 220 Ω does, so the resistor pages snap to an E12/E24/E96 preferred value, report which way they rounded and why the direction matters, and name the standard power rating the part needs with a 2× margin. The third is the load: a voltage divider behaves differently the moment something draws current from the tap, so both outputs are shown with the sag between them. Battery runtime is reported ideally and realistically, because depth of discharge and converter efficiency are real losses. Nothing is uploaded, there is no sign-up, and the pages work offline. Mains electricity, charged capacitors and undersized conductors hurt people: these are educational estimates, not a substitute for a licensed electrician or your local electrical code.
Related categories
- Physics Calculators
Kinematics, forces, energy, electricity, waves and fluids — with units per field and the working shown.
- Construction & Engineering Calculators
Beam, bolt, pipe, tank, flow, pressure and heat calculations — textbook formulas with the working shown.
- Home & DIY Calculators
Material and measurement calculators for the house and yard — paint to concrete, stairs to roof pitch.
- Units & Numbers Tools
Convert units and work with numbers — length, weight, temperature, and more.
Frequently asked questions
Why does the resistor calculator round my answer to a value I did not compute?
Because a resistor is a manufactured part from a preferred-value series, and 219.4 Ω is not one of them. The E12, E24 and E96 series space values about as closely as a tolerance band lets two parts sit without being interchangeable, so the calculation is matched to the nearest series member and the current or voltage that value really produces is reported next to the target. Which way it rounds is your choice — up for less current, down for more.
Why is the voltage divider's output lower with a load attached?
Because Vin × R2 ÷ (R1 + R2) assumes nothing draws current from the tap. A real load sits in parallel with R2, lowering the effective resistance of the lower leg, and the output sags in proportion to how heavy the load is against R2. Enter the load resistance to see both figures; if the sag is unacceptable the divider needs a buffer, not different resistors.
Do capacitors really combine the opposite way from resistors?
Yes — parallel capacitors add directly and series capacitors add reciprocals, which is exactly backwards from resistors. To see why: in series every capacitor stores the same charge, so the smallest one develops the largest voltage; in parallel every capacitor sees the same voltage and the charges add. The calculator prints which way the equivalent landed so the rule is visible rather than memorised.
Can I use the wire size result to wire a real circuit?
No. It is an educational estimate of two things — whether the conductor will get too hot and whether too much voltage is lost over the run — and it deliberately does not claim to be code-compliant. Rules differ by country, by installation type, by bundling and by ambient temperature, and they also set the overcurrent protection, which this page says nothing about. Have the circuit designed and inspected by a licensed electrician.
Why is the LED resistor always rounded up?
Because rounding down passes more current than you asked for, and an LED's current is what determines its brightness and its life. Rounding up to the next preferred value gives slightly less current, which is the safe direction to miss in. The page shows the current the chosen value actually produces, and the resistor's dissipation with the smallest standard power rating that covers it with a 2× margin.