Resistance, voltage and current calculators
This page contains a calculator for working out electrical resistance (in ohms, Ω), as well as calculators with the
equation rearranged to make voltage (V) and current (amps, A) the subject. The formulas are shown together with other
useful information and examples to try. Note: Current is denoted by the letter I.
Both ohms (Ω) and volts (V) are derived units in the SI system. You can see how they are derived on this page, together
with other derivations. The ohm (Ω) is the unit of electrical resistance.
Enter all figures without commas. For example, enter 2,400 as 2400.
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Calculate resistance (Ω) from volts (V) and amps (A)
The equation for resistance is given on the right.
Example: If a voltage of 4 V causes a current of 0.1 A to flow through
a bulb, what is the resistance in the bulb circuit?
In this case we simply enter the voltage (4 V) and the current (0.1 A)
into the calculator and click Calculate, to show that the answer is 40
ohms.
Some devices have currents measured in milliamps (mA). There are
1000 milliamps in 1 amp, so 1 milliamp equals 0.001 amps.
Calculate volts (V) from resistance (Ω) and amps (A)
The equation for volts is given above right.
Example: An electrical circuit draws a current of 0.5 amps and has a
resistance of 60 ohms. What is the voltage drop of the circuit?
In this case we simply enter the current (0.5 A) and resistance (60
ohms) into the calculator and click Calculate, so give an answer of 30
(V). Note that voltage is also sometimes called the electromagnetic
force (e.m.f.).
The equation for volts is:
Where:
V = volts (V)
I = current (A)
R = resistance (ohms,
Ω
)
Quick Facts
1 millivolt (mV) is 0.001 volts (V)
1 amp is 1 coulomb per second
The ohm is named after the German
physicist Georg Ohm (1789 - 1854)
Calculate amps (A) from from resistance (Ω) and volts (V)
The equation for ampage is given above right.
Example: An electric fire draws 230 volts and has a resistance of 60
ohms. What is the draw in amps of the device?
The answer can be obtained by entering the voltage (230 V) and the
resistance (60 ohms) into the calculator and clicking Calculate. Doing
so shows that the answer is 3.83 amps.
The equation for ampage is:
Where:
I = current (A)
V = volts (V)
R = resistance (ohms,
Ω
)
The equation for resistance is:
Where:
R = resistance (ohms,
Ω
)
V = volts (V)
I = current (A)
Ohm’s Law calculators
Resistance, voltage and current calculators
This page contains a calculator for working out electrical
resistance (in ohms, Ω), as well as calculators with the equation
rearranged to make voltage (V) and current (amps, A) the
subject. The formulas are shown together with other useful
information and examples to try. Note: Current is denoted by the
letter I.
Both ohms (Ω) and volts (V) are derived units in the SI system.
You can see how they are derived on this page, together with
other derivations. The ohm (Ω) is the unit of electrical resistance.
Enter all figures without commas. For example, enter 2,400 as
2400.
Calculate resistance (Ω) from volts (V) and amps (A)
The equation for resistance is given below.
Example: If a voltage of 4 V causes a current of 0.1 A to flow
through a bulb, what is the resistance in the bulb circuit?
In this case we simply enter the voltage (4 V) and the current
(0.1 A) into the calculator and click Calculate, to show that the
answer is 40 ohms.
Some devices have currents measured in milliamps (mA). There
are 1000 milliamps in 1 amp, so 1 milliamp equals 0.001 amps.
Calculate volts (V) from resistance (Ω) and amps (A)
The equation for volts is given below.
Example: An electrical circuit draws a current of 0.5 amps and
has a resistance of 60 ohms. What is the voltage drop of the
circuit?
In this case we simply enter the current (0.5 A) and resistance (60
ohms) into the calculator and click Calculate, so give an answer of
30 (V). Note that voltage is also sometimes called the
electromagnetic force (e.m.f.).
The equation for volts is:
Where:
V = volts (V)
I = current (A)
R = resistance (ohms,
Ω
)
Calculate amps (A) from from resistance (Ω) and volts (V)
The equation for ampage is given below.
Example: An electric fire draws 230 volts and has a resistance
of 60 ohms. What is the draw in amps of the device?
The answer can be obtained by entering the voltage (230 V)
and the resistance (60 ohms) into the calculator and clicking
Calculate. Doing so shows that the answer is 3.83 amps.
The equation for ampage is:
Where:
I = current (A)
V = volts (V)
R = resistance (ohms,
Ω
)
The equation for resistance is:
Where:
R = resistance (ohms,
Ω
)
V = volts (V)
I = current (A)
Ohm’s Law calculators
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