Key takeaways
- Always de-energise and discharge the circuit first.
- Lift at least one leg of the component, or parallel paths corrupt the reading.
- Subtract the test-lead resistance for anything under about 10 Ω.
- OL means open circuit, not a fault with the meter.
Before you touch anything
Never measure resistance on a live circuit. The ohms range injects its own small test current, and applied voltage will give a nonsense reading or destroy the meter. Switch off, unplug, and discharge any capacitors.
Step 1 — set up the meter
- Insert the black lead in COM and the red lead in the VΩmA socket — never the high-current socket.
- Turn the dial to the Ω symbol.
- On an auto-ranging meter that is all. On a manual meter, start above the value you expect and step down.
Reading the display
| Display | Meaning | Action |
|---|---|---|
| OL or 1 | Over limit / open circuit | Range up, or the path is broken |
| 0.00 | Short circuit | Expected when probes touch |
| Drifting | Capacitance charging | Wait for it to settle |
| Steady value | Valid reading | Record it |
Step 2 — compensate the leads
Touch the probe tips together. A typical pair reads between 0.1 Ω and 0.5 Ω. That offset is part of every reading you take.
- Measuring a 10 kΩ resistor? Ignore it — it is 0.003% of the value.
- Measuring a 1 Ω shunt or a motor winding? Subtract it, or the error is 30%.
- Meters with a REL or Δ button will zero the offset for you.
Resistor Color Code CalculatorDecode the bands first, then confirm the measured value falls inside the stated tolerance.
Step 3 — isolate the component
A resistor still soldered into a board is measured in parallel with everything else connected to those two nodes. The meter reports the combination, which is always lower than the true value.
- Lift one leg, or desolder the part entirely.
- Semiconductor junctions elsewhere on the board can conduct at the meter's test voltage.
- If in-circuit is unavoidable, treat the reading as a rough go/no-go check only.
Our parallel resistor calculator shows how quickly a stray path pulls a reading down.
Step 4 — measure and interpret
Press the probes firmly onto clean metal. Hold the plastic, not the tips — your body across a high-value resistor adds a parallel path of a few hundred kilohms.
Comparing against the marked value
Decode the bands with the colour code calculator, then check the tolerance:
| Tolerance | Minimum | Maximum |
|---|---|---|
| ±1% (brown) | 990 Ω | 1010 Ω |
| ±5% (gold) | 950 Ω | 1050 Ω |
| ±10% (silver) | 900 Ω | 1100 Ω |
Things that will fool you
- Temperature. Copper windings rise about 0.4% per °C — see resistivity of copper.
- Filament lamps. Cold resistance can be a tenth of the operating value.
- Long leads. Cheap probes add resistance that varies as they flex.
- Low battery. A weak meter battery skews the ohms range before it warns you.
Interpreting an unexpected reading
When the number is not what you expected, the reading is usually correct and the assumption is wrong. Work through the likely causes in order.
| Symptom | Most likely cause | What to do |
|---|---|---|
| Reads lower than marked | Parallel path on the board | Lift one leg |
| Reads slightly high | Lead resistance or a dirty contact | Zero the leads, clean the joint |
| Drifts upward slowly | Capacitance charging | Wait, or discharge first |
| Reads OL | Open component or wrong range | Range up, then inspect |
| Unstable, jumping | Intermittent contact | Re-probe, check the joint |
A resistor that measures open is genuinely faulty. One that measures low almost never is — resistors fail high or open far more often than they fail low, so a low reading points at the circuit around it rather than the part itself.
Turning the reading into an answer
A measured resistance becomes useful when combined with a voltage or current. Feed it into the V-I-R-P solver to get the current a supply will push through it and the power it will dissipate — the two numbers that decide whether your part survives.
Frequently asked questions
Can I measure resistance in a live circuit?
No. The ohms range applies its own test current and any external voltage invalidates the reading or damages the meter. Always isolate and discharge first.
Why does my multimeter show OL when measuring a resistor?
OL means the value is above the selected range, or the path is open. Range up first; if it still shows OL, the resistor is likely open circuit.
Why is my in-circuit reading lower than the marked value?
Other components connected across the same two nodes form parallel paths, and parallel resistance is always lower than the smallest branch. Lift one leg of the resistor to measure it alone.