HOW-TO GUIDE
Why Does an Oscilloscope Show Different Voltage Readings from a Multimeter?
By the OhmSavvy Research Team ·
Compare on safe signals
- Compare on low-voltage signals, such as a function generator. A standard scope probe is not a mains test lead.
- A bench scope's ground clip is earthed through its power cord. Clip it only to the circuit's 0 V on an isolated, low-voltage supply, never to a non-isolated mains circuit.
Why an oscilloscope shows different voltage readings from a multimeter: the usual causes
Before blaming either instrument, check they are measuring the same thing. One of these usually explains the gap:
Different quantity
Peak-to-peak or peak on the scope, RMS on the meter.
Wave shape
A meter that is not True RMS misreads non-sine waves.
Frequency
The meter's AC range rolls off far below the scope's bandwidth.
Probe ratio
A 1x/10x mismatch makes the scope read ten times off.
Coupling
One instrument includes the DC level and the other doesn't.
Accuracy and loading
A few percent of scope error, or a probe loading a weak node.
1Peak-to-peak vs RMS
Feed a 10 Vpp sine to both and the scope shows 10 V crest to trough while the meter shows 3.54 V. Neither is wrong: for a pure sine, RMS is the peak divided by √2, so Vpp = 2√2 × Vrms, roughly 2.83 times.
Compare like with like by turning on the scope's Vrms measurement, as shown in how to measure voltage with an oscilloscope.
2True RMS, average-responding meters and crest factor
Many lower-cost meters are average-responding: they rectify the signal, average it, and multiply by about 1.11, a ratio that is correct only for a sine. On a ±5 V square wave the true RMS is 5.00 V, but that shortcut displays about 5.55 V, around 11% high. A True RMS meter calculates the real RMS instead.
True RMS meters have a limit too: crest factor, peak divided by RMS. A sine's is 1.41 and a square wave's 1.0, but narrow pulses go much higher. Meters specify a maximum, often about 3 near full scale; beyond it even a True RMS reading loses accuracy.
3The multimeter's AC bandwidth is low
A handheld meter's AC voltage accuracy is typically specified only up to a few hundred hertz or a few kilohertz, depending on model and range. A 20 kHz sine may read well under its true RMS on the meter while the scope shows it correctly, and square or PWM waves suffer at lower frequencies because their harmonics fall outside that range. The scope reads low too near its own, far higher limit; see what oscilloscope bandwidth means.
4A probe attenuation mismatch
A factor of exactly ten between the instruments almost always means the channel's probe setting doesn't match the 1x/10x switch on the probe. Fix the channel menu, then check the probe: a miscompensated 10x probe rounds or peaks square-wave corners, which shifts Vpp. See how to compensate an oscilloscope probe.
5DC vs AC coupling
On AC volts most meters report only the AC component; on DC volts, the average. The scope follows its coupling setting. Take a 5 V rail with 100 mVpp of triangular ripple: the meter on DC shows about 5.00 V, the meter on AC shows about 29 mV (the RMS of a triangle is Vpp ÷ 2√3, if the ripple is within its bandwidth), and the scope on AC coupling shows 100 mVpp.
6Scope vertical accuracy vs meter accuracy
Entry-level scopes typically quote DC gain accuracy of around ±3% of full scale. At 1 V/div, full scale is 8 divisions or 8 V, so a 5.00 V rail could legitimately show roughly 4.76 V to 5.24 V. A typical handheld meter is accurate to a fraction of a percent on DC, so trust it for the number.
7Input impedance and loading
Meters typically present about 10 MΩ on DC volts; a scope input is 1 MΩ, or about 10 MΩ through a 10x probe. On high-impedance nodes that matters. Picture a divider of two 1 MΩ resistors across 10 V, which should give 5 V. A 1x probe puts 1 MΩ in parallel with the lower resistor, leaving 0.5 MΩ, so the node falls to 10 × 0.5 ÷ 1.5 = 3.33 V. With 10 MΩ, as from the meter or a 10x probe, the node reads about 4.76 V.
8Noise and averaging
A meter integrates its input over a fraction of a second, so noise averages away. A scope's Vpp and Vmax include every spike, so peak figures read high on a noisy signal. Use Vavg or Vrms, turn on averaging or the bandwidth limit, and see measuring the noise floor to judge how much noise comes from the setup itself.
What each instrument shows for a sine, a square and a PWM signal
Worked figures at 1 kHz. Many handhelds are not accurate on square or PWM harmonics at that frequency, so treat the meter columns as best case.
| Signal | Scope Vpp | Scope Vrms | True RMS meter, AC V | Average-responding meter, AC V | Meter, DC V |
|---|---|---|---|---|---|
| Sine, 10 Vpp | 10.0 V | 3.54 V | 3.54 V | 3.54 V | 0 V |
| Square, ±5 V | 10.0 V | 5.00 V | 5.00 V | ≈ 5.55 V | 0 V |
| PWM, 0–5 V, 20% duty | 5.00 V | 2.24 V | 2.00 V | ≈ 1.78 V | 1.00 V |
In the PWM row, the scope's Vrms includes the 1 V DC average (√(2² + 1²) ≈ 2.24 V), while an AC-coupled meter sees only the 2.00 V AC component.
What to check first
01
Quantity
Vrms on the scope, not Vpp
02
Probe ratio
Channel matches the probe
03
Coupling
Both AC or both DC
04
Frequency
Inside the meter's AC range?
05
Loading
10x probe on weak nodes
Use a known reference
Meters from our picks, and how they read AC
True RMS, CAT III 600V, auto-ranging, with built-in non-contact voltage detection.
Auto-ranging, CAT III 600V. Extech doesn't state True RMS, so check before relying on it for non-sine AC.
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Frequently asked questions
Which is more accurate, an oscilloscope or a multimeter?
For a steady DC voltage, or the RMS of a low-frequency AC signal within its rated bandwidth, the multimeter is usually more accurate. The scope wins when the signal is fast, non-sinusoidal or changing, because it measures the actual waveform rather than one filtered number.
Why does my scope read 2.83 times my multimeter on a sine wave?
Because the scope is showing peak-to-peak and the meter is showing RMS. For a pure sine, peak-to-peak equals 2√2, about 2.83, times the RMS value. Both readings are correct. Switch the scope to its Vrms measurement, or divide the peak-to-peak figure by 2.83, and the two should agree closely.
Why does my multimeter read low on a high-frequency signal?
Most handheld meters are only specified for AC voltage up to somewhere between a few hundred hertz and a few kilohertz, depending on model and range. Above that the reading falls away. A scope's bandwidth is far higher, so it keeps showing the true amplitude. Check your meter's manual.
Can a scope probe set to the wrong ratio cause a 10 times error?
Yes. If a 10x probe is used with the channel set to 1x, the scope shows a tenth of the real voltage; the other way round, it shows ten times too much. Match the channel's probe setting to the switch on the probe, then compare again.
Why do the readings disagree on a 5 V DC rail?
Usually by only a little, and that is expected. An entry-level scope's DC accuracy is often around ±3% of full scale, which can be over 0.2 V at 1 V/div, while a handheld meter is typically far tighter. Ripple and noise also lift the scope's peak values. Trust the meter for the DC number.
