Free calculators that turn your multimeter readings into clear answers. Each tool runs in your browser, with nothing to download and no sign-up.
Ohm’s Law Calculator
Enter any two of volts, amps, ohms or watts and get the other two, with the formula used and which multimeter input to use.
Open the calculatorResistor Color Code Calculator
Pick the 4, 5 or 6 color bands to get a resistor’s value, tolerance and the reading your multimeter should show, or type a value to see its colors.
Open the calculatorCar Battery Health Checker
Enter your resting, cranking and running voltages to find out if the battery is weak, discharged, or if the alternator is the real problem.
Open the checkerBattery Voltage Checker
Enter your multimeter reading for an AA, AAA, 9V, CR2032 coin cell or 18650 battery to see if it is full, weak or dead, with a voltage chart for each type.
Open the checkerCapacitor Code Calculator
Type the code printed on a capacitor, like 104 or 2A473J, to get its value in pF, nF and µF, its tolerance and voltage, and what your multimeter should read.
Open the calculatorCapacitance Converter
Convert any value between pF, nF, µF, mF and F as you type, with charts for nF to µF, µF to nF and pF to nF.
Open the converterWatts to Amps Calculator
Turn watts into amps or amps into watts for DC, single-phase and three-phase circuits, with power factor and a quick circuit check.
Open the calculatorVoltage Divider Calculator
Find Vout, R1, R2 or Vin for a two-resistor divider, get standard resistor values, and see what your multimeter will actually read.
Open the calculatorSeries and Parallel Resistor Calculator
Add up to 12 resistors in series or parallel to get the total, find the resistor that makes an odd value, and see the current and power in each one.
Open the calculatorLED Resistor Calculator
Pick the LED colour and your supply voltage to get the right resistor, the next standard value, its wattage and the real LED current.
Open the calculatorWhich Tool Should I Use?
- You know two values and need the others: use the Ohm’s Law Calculator. Enter any two of volts, amps, ohms or watts, such as the voltage and wattage printed on an appliance label, and it works out the current and resistance.
- You have a resistor and can’t read its value: use the Resistor Color Code Calculator. Pick the color bands and it shows the value, the tolerance and the reading your multimeter should show.
- Your car is slow to start: use the Car Battery Health Checker. Enter three voltage readings (resting, cranking and running) to see whether the battery or the alternator is the problem.
- A remote, toy or smoke alarm has stopped working: use the Battery Voltage Checker. Measure the battery on DC volts, pick its type and see whether it is full, weak or dead.
- You have a capacitor marked 104 or 2A473J and need its value: use the Capacitor Code Calculator. Type the code to get the value in pF, nF and µF, the tolerance and the voltage rating.
- Your meter shows nF but the part says µF (or the other way round): use the Capacitance Converter to change between pF, nF, µF, mF and F.
- You want to know how many amps an appliance draws: use the Watts to Amps Calculator. Enter the watts and the voltage to see the current and whether it fits your circuit.
- You need to drop a voltage with two resistors: use the Voltage Divider Calculator to find the output voltage or the resistor values, for example 12 V down to a 5 V input.
- You need to combine resistors: use the Series and Parallel Resistor Calculator to get the total of up to 12 resistors, or to find the one to add in parallel for a value you do not have.
- You are wiring an LED: use the LED Resistor Calculator to get the resistor value and wattage for your supply voltage and LED colour.
How the Tools Work With Your Multimeter
Each tool turns a reading from your meter into a clear answer. For example, read the bands on a resistor with the color code calculator, measure it with your multimeter to confirm the value, then use the Ohm’s law calculator to work out how much current will flow through it at your circuit’s voltage.
The calculations run in your browser, so the numbers you type are not sent anywhere. The tools also work on a phone, which makes them easy to use at the workbench or next to the car.
The tools follow the same checks used in the step-by-step guides on this site. If there is a tool you would like to see here, let me know through the contact page.