A resistor value must be entered in the same resistance unit used by the calculation. A value such as 1.5 kΩ produces the same electrical result as 1,500 Ω; the number changes only because the unit scale changes. Entering 1.5 where a calculator expects ohms instead of kilohms changes the circuit resistance from 1,500 Ω to 1.5 Ω.
Resistance-unit conversion is commonly needed before using Ohm’s law, checking a voltage divider, selecting a resistor from its color bands, or reviewing current through a branch of an electronic circuit. The converted value is used as the resistance term (R) in:
\(\displaystyle V = I \times R\)
where:
- (V) = voltage
- (I) = current
- (R) = resistance
Resistance Unit Scales
The ohm, shown by the symbol Ω, is the base unit of electrical resistance.
| Unit | Symbol | Equivalent in ohms |
|---|---|---|
| Ohm | Ω | 1 Ω |
| Kilohm | kΩ | 1,000 Ω |
| Megohm | MΩ | 1,000,000 Ω |
A kilohm is one thousand ohms:
\(\displaystyle 1\text{ kΩ} = 1{,}000\text{ Ω}\)
A megohm is one million ohms:
\(\displaystyle 1\text{ MΩ} = 1{,}000{,}000\text{ Ω}\)
The uppercase M in MΩ means mega, or (10^6). The lowercase k in kΩ means kilo, or (10^3). Keep the unit symbol with the value when recording a resistor value, entering it into a calculator, or comparing it with a schematic.
Ohm, kOhm, and MOhm Conversion
Move the decimal point three places for each step between ohms and kilohms. Move it six places for each step between ohms and megohms.
Convert Kilohms to Ohms
Multiply the kilohm value by 1,000.
\(\displaystyle \text{Ω} = \text{kΩ} \times 1{,}000\)
For example:
\(\displaystyle 1.5\text{ kΩ} \times 1{,}000 = 1{,}500\text{ Ω}\)
Convert Ohms to Kilohms
Divide the ohm value by 1,000.
\(\displaystyle \text{kΩ} = \frac{\text{Ω}}{1{,}000}\)
For example:
\(\displaystyle 150\text{ Ω} \div 1{,}000 = 0.15\text{ kΩ}\)
Convert Megohms to Ohms
Multiply the megohm value by 1,000,000.
\(\displaystyle \text{Ω} = \text{MΩ} \times 1{,}000{,}000\)
For example:
\(\displaystyle 0.15\text{ MΩ} \times 1{,}000{,}000 = 150{,}000\text{ Ω}\)
Convert Ohms to Megohms
Divide the ohm value by 1,000,000.
\(\displaystyle \text{MΩ} = \frac{\text{Ω}}{1{,}000{,}000}\)
For example:
\(\displaystyle 150{,}000\text{ Ω} \div 1{,}000{,}000 = 0.15\text{ MΩ}\)
Equivalent Resistance Values
The following values show how the same resistance is written at different scales.
| Ohms | Kilohms | Megohms |
|---|---|---|
| 150 Ω | 0.15 kΩ | 0.00015 MΩ |
| 1,500 Ω | 1.5 kΩ | 0.0015 MΩ |
| 150,000 Ω | 150 kΩ | 0.15 MΩ |
| 1,000,000 Ω | 1,000 kΩ | 1 MΩ |
The physical resistor does not change when its value is converted. A 150 kΩ resistor remains 150,000 Ω and 0.15 MΩ.
Calculation Example
Convert 150 Ω, 1.5 kΩ, and 150 kΩ before entering resistance into a circuit calculation.
| Stated value | Ohms | Kilohms | Megohms |
|---|---|---|---|
| 150 Ω | 150 Ω | 0.15 kΩ | 0.00015 MΩ |
| 1.5 kΩ | 1,500 Ω | 1.5 kΩ | 0.0015 MΩ |
| 150 kΩ | 150,000 Ω | 150 kΩ | 0.15 MΩ |
For a 12 V circuit using a 1.5 kΩ resistor, first convert the resistance to ohms if the calculation uses base units:
\(\displaystyle R = 1.5\text{ kΩ} = 1{,}500\text{ Ω}\)
Then apply Ohm’s law:
\(\displaystyle I = \frac{V}{R}\)
\(\displaystyle I = \frac{12\text{ V}}{1{,}500\text{ Ω}} = 0.008\text{ A}\)
\(\displaystyle I = 8\text{ mA}\)
If 1.5 Ω were entered instead of 1,500 Ω, the calculated current would be 8 A rather than 8 mA. That is a 1,000-to-1 error caused solely by using the wrong resistance scale.
Ohm’s Law Verification
Ohm’s law provides a quick check after converting resistance values. Use resistance in ohms when working directly with volts and amperes.
\(\displaystyle R = \frac{V}{I}\)
For a measured 5 V drop and 0.01 A current:
\(\displaystyle R = \frac{5\text{ V}}{0.01\text{ A}} = 500\text{ Ω}\)
Equivalent forms are:
\(\displaystyle 500\text{ Ω} = 0.5\text{ kΩ} = 0.0005\text{ MΩ}\)
The current value may instead be expressed in milliamperes, but its unit must also be handled correctly. A current of 0.01 A equals 10 mA.
Resistor Identification and Circuit Inputs
A resistor color code may identify a value in ohms, kilohms, or megohms depending on its significant figures and multiplier band. Confirm the marked or decoded resistance value, then convert it to the unit required by the circuit calculation.
Use these Elecatrix tools during resistor selection and circuit review:
- Resistor Color Code Calculator for decoding a resistor’s color bands into a resistance value.
- Ohm’s Law Calculator for calculating voltage, current, or resistance after placing all values in compatible units.
- Voltage Divider Calculator for evaluating output voltage from two resistor values.
In a voltage divider, both resistor values must use the same scale. For example, a 1.5 kΩ resistor and a 150 kΩ resistor can be entered as 1.5 and 150 kΩ, or as 1,500 and 150,000 Ω. Mixing 1.5 kΩ with 150,000 kΩ produces an incorrect ratio.
Field Verification
Resistance conversion confirms only the numerical equivalence between Ω, kΩ, and MΩ. It does not verify the physical resistor’s color bands, tolerance, wattage, voltage rating, temperature performance, lead condition, or suitability for the circuit.
A resistor used in a circuit must also be evaluated for the voltage across it and the power it dissipates:
\(\displaystyle P = V \times I\)
or:
\(\displaystyle P = \frac{V^2}{R}\)
A correct resistance value can still be unsuitable if the resistor’s power rating or voltage rating does not match the actual circuit conditions.