Millivolts to Volts

Turning a Millivolt Sensor Reading into Volts

Most sensors speak in millivolts. A thermocouple, a strain-gauge bridge, a current shunt or a pH electrode puts out a few millivolts to a few hundred millivolts — far below the volt-level range a data logger or analog input card expects. Converting that figure to volts is the first step before anything else can be scaled.

Conversion factor: 1 mV = 0.001 V, so divide the millivolt reading by 1,000. A K-type thermocouple at 100 °C puts out 4.096 mV, which is 0.004096 V — the number you type into a 0–0.1 V range or a gain calculation.

Where the Millivolts Come From

Thermoelectric signals

Two dissimilar metals joined at a hot junction give only tens of microvolts per degree — about 41 µV/°C for a K-type — so a full working range still fits in tens of millivolts.

Bridge transducers

Load cells and pressure sensors return a fraction of their excitation voltage, quoted in mV/V — so the real signal depends on the supply feeding the bridge.

Shunt voltage drops

A DC current shunt is built so its drop stays small — a standardised 50, 75 or 100 mV at rated current — keeping power loss and circuit burden low.

Electrochemical probes

pH and ORP electrodes follow the Nernst relation: about 59.16 mV per pH unit at 25 °C, from a source with very high impedance.

From Datasheet Figure to Logger Setting

The workflow is the same whether the number comes off a multimeter in mV mode or off a page of a sensor datasheet.

Type the reading you measured

Enter the millivolt figure in the left field — 4.096, 20, 59.16, whatever the meter says. Volts appear as you type. A comma works as a decimal separator and stray spaces are ignored.

Check it against the input range

0.020 V from a load cell disappears on a ±10 V input but sits comfortably inside a ±25 mV or ±100 mV bridge range.

Copy the bare number into the configuration

The copy button puts the plain number on the clipboard — no unit, no spaces — which is what a scaling dialog or spreadsheet cell expects. Ctrl + C inside a field does the same.

Work backwards from the logger

If the logger reports 0.075 V, press the swap button (↔) to run V → mV and see what the shunt saw. By hand, multiply by 1,000: 0.075 V is 75 mV.

Watch the reference conditions: a thermocouple millivolt value only means a temperature if the cold junction matches the table's reference, and a bridge output only means a load at the datasheet excitation voltage.

Typical Sensor and Transducer Output Levels

The signal levels you meet most often on the bench, as the millivolt figure printed in datasheets and as the volt value used in scaling and gain calculations.

Sensor / signal source	Condition	Output (mV)	Output (V)
K-type thermocouple	100 °C, 0 °C reference junction	4.096 mV	0.004096 V
J-type thermocouple	100 °C, 0 °C reference junction	5.269 mV	0.005269 V
Load cell, 2 mV/V	10 V excitation, rated capacity	20 mV	0.02 V
pH electrode	One pH unit from neutral, 25 °C	59.16 mV	0.05916 V
DC shunt, 50 mV class	At rated current	50 mV	0.05 V
DC shunt, 75 mV class	At rated current	75 mV	0.075 V
Pt100 RTD	100 °C, 1 mA excitation (138.51 Ω)	138.5 mV	0.1385 V

The band is narrow: from a thermocouple at four thousandths of a volt to an RTD at just over a tenth, everything here lands in the first decimal places of a volt. That is why datasheets stay in millivolts and leave the conversion to you.

What This Converter Does for Signal Work

Both fields stay live

Type in either box and the other tracks it, so you can walk a whole span — zero, mid-scale, full scale — without clearing anything.

Reverse for the back-check

The swap button flips to V → mV, the direction you need to verify what a logger reading means at the sensor terminals.

Any voltage unit on either side

Searchable dropdowns cover all twelve units, so the page also handles a µV/°C thermocouple figure or a nanovolt offset spec.

Numbers you can paste

Results carry up to eight decimals and switch to scientific notation for very small values; copy hands over the clean number.

Sensor Signal Questions

What does 2 mV/V on a load cell mean in actual volts?

It is a ratio, not a fixed output: the cell gives 2 mV per volt of excitation at its rated capacity. On 10 V excitation full scale is 20 mV (0.02 V); on 5 V it is 10 mV (0.01 V). Half the rated load gives half of that.

Why is a current shunt labelled 50 mV, and what current does that represent?

The millivolt figure is the drop at rated current — 50, 75 and 100 mV are the common catalogue classes. The current is the other half of the label: a 100 A / 50 mV shunt reads 50 mV (0.05 V) at 100 A, so its resistance is 0.5 mΩ and 25 mV means 50 A.

How many millivolts does a K-type thermocouple give at 100 °C?

4.096 mV — that is 0.004096 V — with the reference junction at 0 °C, per the ITS-90 tables. Sensitivity there is roughly 41 µV per degree, so a degree is about 0.041 mV. The curve is not perfectly linear, so read the published table instead of multiplying across a wide span.

Can a 0–10 V PLC analog input read a millivolt sensor directly?

Not usefully. A 20 mV bridge output is 0.02 V — two thousandths of a 0–10 V span, a handful of counts on a 12-bit card and buried in noise. Add a signal conditioner or instrumentation amplifier (0–50 mV to 0–10 V needs a gain of 200), or use a thermocouple, RTD or bridge module with millivolt ranges on board.

Why does my multimeter show 0.000 on the V range when the sensor is working?

The signal is smaller than the last digit that range resolves: even on a 6 V range a 3½-digit meter steps about 1 mV, so a 4 mV thermocouple output rounds to 0.00. Switch to the dedicated mV range. Also check that you are on DC, and that a pH probe has a high-impedance input so the meter does not load it.