HOW-TO GUIDE
How to Measure Voltage With an Oscilloscope
By the OhmSavvy Research Team ·
Safety first: the ground clip is not a free-floating lead
- On a mains-powered bench scope, the probe's ground clip is normally connected to mains earth through the power cord. Clipping it to any point that isn't at earth potential, such as the inside of a mains-connected, non-isolated power supply, shorts that point to earth through the ground lead. That can destroy the probe, the scope or the circuit, start a fire or injure you.
- Never remove or defeat the scope's earth connection ("floating" the scope) to get around this. To measure voltages that aren't referenced to earth, use an isolation transformer on the device under test, a differential probe, or an isolated-channel or handheld scope rated for the job.
- Check the voltage and CAT ratings printed on the probe and the scope input before you connect. Standard 1x/10x passive probes are not for mains measurements unless their rating explicitly covers it. If you only need a mains voltage reading, a properly rated multimeter is the right tool (see how to use a multimeter).
1Set up the channel before you read anything
Most voltage-reading mistakes happen before the signal is even on screen. Three settings decide whether the numbers you read are correct:
Probe attenuation
Set the channel to match the probe switch (1x or 10x). A 10x probe on a 1x channel setting reads 10 times too low.
Ground reference
Note where the channel's ground marker sits, usually an arrow at the left edge. All DC levels are read from that line.
Coupling
DC coupling shows the whole signal including its DC level. AC coupling removes the DC part. Start on DC.
If you haven't checked your 10x probe yet, do that first: a badly compensated probe distorts square-wave tops and makes amplitude readings unreliable. Our guide to compensating an oscilloscope probe takes a couple of minutes. Then adjust the volts-per-division (V/div) knob until the waveform fills as much of the screen height as possible without running off the top or bottom.
2Read peak-to-peak voltage from the grid
Peak-to-peak (Vpp) is the distance from the highest point of the waveform to the lowest. Count the vertical divisions between them and multiply by V/div. Most scope screens have 8 vertical divisions, each split into five minor ticks of 0.2 division, so you can usually read to the nearest fifth of a division. If the grid itself is new to you, start with how to read an oscilloscope screen.
Worked example: the trace above spans 6 divisions at 1 V/div, so Vpp = 6 × 1 V = 6 V. The same signal at 2 V/div would span only 3 divisions. The voltage hasn't changed, but the reading is coarser because each minor tick is now worth 0.4 V instead of 0.2 V. That is why you zoom in vertically as far as the screen allows.
3Turn peak-to-peak into peak and RMS
For a waveform centred on 0 V, the peak (amplitude) is half the peak-to-peak value. For a pure sine wave only, the RMS value, which is what a multimeter's AC voltage range is trying to report, is the peak divided by √2 (about 1.414).
| Value | Formula (centred sine) | Example (6 V p-p sine) |
|---|---|---|
| Peak-to-peak (Vpp) | divisions × V/div | 6 div × 1 V/div = 6 V |
| Peak (Vpk) | Vpp ÷ 2 | 6 V ÷ 2 = 3 V |
| RMS (Vrms) | Vpk ÷ √2 (≈ Vpk × 0.707) | 3 V × 0.707 ≈ 2.12 V |
| Average (Vavg) | 0 for a sine centred on 0 V | 0 V |
Side by side, the three numbers for the same signal look very different. That is why a voltage value means little until you know which one it is: 6 V, 3 V and 2.12 V can all describe this one sine wave.
This is also why a scope and a multimeter can disagree about the same signal without either being wrong. Below, the 6 V peak-to-peak sine from step 2 is shown at 2 V/div, where it spans 3 divisions, next to what a True-RMS multimeter on its AC voltage range would report for it.
The √2 rule only works for sine waves
4Measure a DC level or a signal that sits above 0 V
Many real signals don't sit around 0 V. A logic output or PWM signal, for example, switches between 0 V and a supply voltage. With DC coupling, measure each level from the ground marker: count the divisions from the ground line up to the trace and multiply by V/div.
Here Vmax = 5 V, Vmin = 0 V and Vpp = 5 V. A steady DC voltage, such as a battery or regulator output, shows up as a flat line, and you read it the same way: a line 1.5 divisions above ground at 2 V/div is 3 V. If the ground marker has been moved off the center line, always count from the marker, not from the center of the screen.
5Use AC coupling to see small ripple on a DC rail
Suppose you want to see the ripple on a 5 V supply. At 2 V/div, DC coupled, the rail sits 2.5 divisions above ground and 200 mV of ripple is only a tenth of a division tall, which is invisible. Turning the V/div down far enough to see the ripple would push the 5 V level off the top of the screen.
AC coupling solves this by blocking the DC part of the signal, so only the variation is shown, centred on the ground line. Now you can turn the sensitivity right up:
| Coupling | Shows | Use it for |
|---|---|---|
| DC | The full signal, including its DC level | Most measurements: DC levels, logic signals, Vmax/Vmin |
| AC | Only the changing part; DC is blocked | Small ripple or noise riding on a large DC voltage |
Two cautions. AC coupling also attenuates very low-frequency signals, and the cutoff point varies by model, so don't use it to measure slow signals. And only DC coupling shows the true DC level, so switch back to DC when you're done. Some scopes also let you shift the trace with a large vertical offset while staying DC coupled; the available offset range depends on the model and V/div setting.
6Use cursors and automatic measurements
Counting divisions is the skill to learn, but a digital scope can do it more precisely. Horizontal voltage cursors (often labeled Y or amplitude cursors) are two lines you place on the levels you care about; the scope displays each level and the difference between them, ΔV. Cursors are ideal when you want a specific feature, such as the height of one step or a single spike.
Automatic measurements go further and calculate values from the captured samples. Names and exact definitions vary between brands, so check your scope's manual, but these are common:
| Measurement | What it reports |
|---|---|
| Vpp | Highest sample minus lowest sample, including spikes and overshoot |
| Vmax / Vmin | The highest and lowest sample values |
| Vamp (on many scopes) | Top level minus base level, ignoring overshoot; useful on square waves |
| Vavg / Mean | The average of the samples: the DC level of the signal |
| Vrms | The true RMS of the samples, valid for any wave shape |
Many scopes offer RMS and average over the whole screen or over a whole number of cycles. The whole-screen version can be slightly off if a partial cycle is on screen, so where both exist, prefer the per-cycle version for repeating signals.
Getting an accurate voltage reading
- Fill the screen vertically. Both your eye and the scope's digitizer resolve finer steps when the waveform uses most of the 8 divisions.
- Know that scopes aren't precision voltmeters. Vertical accuracy is typically specified as a few percent; the exact figure is on your model's datasheet. For a precise DC value, a good multimeter usually wins. Our oscilloscope vs multimeter guide covers when to use which.
- Stay well inside the bandwidth. At the scope's rated bandwidth, a sine wave reads about 70.7% of its true amplitude, and the error starts earlier than that. See what oscilloscope bandwidth means.
- Get a stable trace. A rolling waveform is hard to read; set the trigger as described in how to trigger an oscilloscope.
- Keep the ground lead short. A long ground lead picks up noise and adds ringing to fast edges, which inflates Vpp and Vmax readings.
Which scope do you need for voltage measurements?
Any working scope can measure voltage; the differences are in what signals it can keep up with. A pocket unit like the FNIRSI DSO152 (200kHz bandwidth, 1 channel) handles audio-range and slow signals. Bench scopes such as the Rigol DS1054Z (50MHz, 4 channels), the Hantek DSO2D10 (100MHz, 2 channels) and the Siglent SDS1104X-E (100MHz, 4 channels) cover microcontroller and most hobby work. Compare them in our best beginner oscilloscopes roundup, or read the oscilloscope buying guide first. New to scopes altogether? Start with what an oscilloscope is and how to use one.
Bench scopes in our reviews
50MHz, 4 channels, 1 GSa/s, with a 7-inch display and strong community documentation.
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Frequently asked questions
How do you measure Vpp on an oscilloscope?
Count the number of vertical divisions from the waveform's highest point to its lowest, then multiply by the volts-per-division setting: 6 divisions at 1 V/div is 6 V peak-to-peak (Vpp). Most digital scopes also read Vpp automatically from an auto-measurement menu, which is more precise than counting divisions by eye, especially on a noisy or fast signal.
Can an oscilloscope measure AC voltage?
Yes. An oscilloscope shows the AC waveform itself, so you can read its peak-to-peak and peak values straight from the grid. Most digital scopes also calculate RMS from the samples. For a clean sine you can work it out yourself: a 6 V peak-to-peak sine has a 3 V peak and an RMS of about 2.12 V.
Why does my oscilloscope reading not match my multimeter?
Usually they are reporting different things. A scope's grid gives peak-to-peak or peak voltage, while a multimeter's AC range reports RMS, so a 6 V peak-to-peak sine reads about 2.12 V on the meter. Meters that are not True-RMS also assume a sine wave and misread other shapes, and a scope's vertical accuracy is typically only a few percent.
Can you measure mains voltage with an oscilloscope?
Not with a standard bench scope and ordinary passive probe. The probe's ground clip is tied to mains earth, so clipping it to a mains circuit can short it out, and most 1x/10x probes are not rated for mains. It needs a differential probe, an isolation transformer on the device under test, or a suitably rated isolated scope. For a simple mains reading, use a rated multimeter.
How does a 10x probe affect voltage readings?
A 10x probe divides the signal by ten before it reaches the scope input, which lets the scope handle larger voltages and loads the circuit less. The scope must be told about it: set the channel's probe ratio to 10x and it multiplies the reading back. If the channel is left on 1x, every voltage reads ten times too low.
How accurate is an oscilloscope for measuring voltage?
Good enough for seeing levels, swings, ripple and spikes, but not a precision instrument. Vertical accuracy is typically specified as a few percent, and your model's datasheet gives the exact figure. Filling the screen vertically and staying well inside the scope's bandwidth help. For a precise DC value, such as checking a regulator output, a good multimeter is usually the better tool.
