The Sensirion SGP40 is one of the easiest ways to add VOC monitoring to Home Assistant with an ESP32 and ESPHome.

It connects over I²C and ESPHome handles the important part automatically: converting the sensor’s raw signal into Sensirion’s VOC Index.

Quick answer

For a standard ESP32 DevKit:

ESP32	SGP40
3.3V	VIN / VCC
GND	GND
GPIO21	SDA
GPIO22	SCL

Then use ESPHome’s current sgp4x component:

sensor:
  - platform: sgp4x
    voc:
      name: "VOC Index"

Once the ESPHome device is added to Home Assistant, the VOC Index appears automatically through the native ESPHome integration.

The SGP40 uses I²C address 0x59. Sensirion specifies the sensor as a digital VOC sensor for indoor-air-quality and air-treatment applications.

The important thing to understand is that the SGP40 does not measure VOC concentration in ppm.

Instead, Sensirion’s algorithm produces a relative VOC Index from 1 to 500, where approximately 100 represents the average VOC conditions the sensor has learned from its recent environment.

What Is the SGP40?

The SGP40 is a metal-oxide gas sensor made by Sensirion for detecting changes in volatile organic compounds in indoor air.

VOCs can come from sources such as:

cooking
cleaning products
perfumes
paint
solvents
furniture and flooring
adhesives
human activity
smoke and combustion products

The sensor does not identify individual chemicals.

Instead, it responds to a broad range of VOC gases and produces a raw signal called SRAW_VOC.

That raw signal is then processed by Sensirion’s Gas Index Algorithm to generate the much more useful VOC Index. Sensirion recommends providing the raw measurement to the algorithm once per second.

ESPHome performs that processing for you, which is why using SGP40 with ESPHome is considerably easier than implementing the complete sensor algorithm yourself.

What Does the SGP40 VOC Index Mean?

This is where a lot of SGP40 tutorials get it wrong.

The VOC Index is not an absolute air-quality score where a fixed number always represents the same concentration of pollutants.

It is adaptive.

Sensirion’s algorithm learns the normal VOC background in the room and maps the recent average to approximately 100.

A useful way to read it is:

VOC Index	Meaning
1–99	VOC conditions are better than the recent average
~100	Typical recent VOC background
Above 100	VOC levels have increased relative to the recent average
150+	Significant VOC event and a useful starting point for automation
Up to 500	Very strong VOC event relative to the learned baseline

Sensirion specifically gives VOC Index 150 as an example threshold for activating an air purifier. It is an example control point rather than a universal health limit.

VOC Index is not ppm

An SGP40 reading of 200 does not mean:

200 ppm
200 ppb
200 µg/m³
AQI 200

It means the sensor is currently seeing substantially more VOC activity than its learned normal environment.

This relative behaviour is intentional.

For example, opening a window may drive the VOC Index below 100, while cooking, cleaning or spraying perfume can cause a sharp increase above 100.

SGP40 vs a CO₂ Sensor

The SGP40 is not a CO₂ sensor.

This distinction matters for indoor-air-quality projects.

A CO₂ sensor such as an SCD40/SCD41 measures carbon dioxide concentration.

The SGP40 measures changes in volatile organic compounds.

You can therefore have:

high CO₂ but relatively low VOC
low CO₂ but a large VOC spike
both high at the same time

For a more complete Home Assistant air-quality station, combining CO₂ + VOC + particulate matter + temperature/humidity gives you much more information than any single sensor.

See our Home Assistant Air Quality Monitor guide for a multi-sensor setup.

Hardware Required

For the basic project you need:

ESP32 development board
SGP40 breakout board
jumper wires
USB cable

For better compensation, also add a temperature and humidity sensor such as:

SHT40
SHT41
SHT45
BME280
another ESPHome temperature/humidity sensor

If you’re already using an SHT45, see our ESP32 SHT45 ESPHome and Home Assistant guide.

ESP32 SGP40 Wiring

The SGP40 uses I²C, so only four connections are required.

For a normal ESP32 DevKit:

SGP40	ESP32
VIN / VCC	3.3V
GND	GND
SDA	GPIO21
SCL	GPIO22

GPIO21 and GPIO22 are the conventional I²C pins on the original ESP32, but ESP32 peripherals are flexible and other pins can be selected if required.

SGP40 I²C address

The SGP40 has a fixed I²C address:

0x59

Sensirion specifies support for standard-mode and fast-mode I²C at up to 400 kHz.

If ESPHome’s I²C scanner detects:

0x59

your ESP32 is communicating with the SGP40.

Complete ESPHome Configuration for SGP40

Current ESPHome uses the sgp4x sensor platform for both SGP40 and SGP41 devices. ESPHome detects which sensor is connected.

A complete basic configuration for an original ESP32 DevKit looks like this:

esphome:
  name: esp32-sgp40
  friendly_name: ESP32 SGP40 VOC Sensor

esp32:
  board: esp32dev

logger:

api:

ota:
  - platform: esphome

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

i2c:
  sda: GPIO21
  scl: GPIO22
  scan: true

sensor:
  - platform: sgp4x
    voc:
      name: "VOC Index"
    update_interval: 30s

ESPHome’s current ESP32 configuration places the board under the esp32: component, while the current SGP40 implementation uses platform: sgp4x.

After compiling and installing the firmware, open the ESPHome logs.

You should see the SGP40 detected on the I²C bus and the VOC Index entity created.

Why ESPHome Uses `sgp4x` Instead of `sgp40`

Older tutorials may contain:

platform: sgp40

Current ESPHome uses:

platform: sgp4x

The sgp4x component supports both:

SGP40 → VOC Index
SGP41 → VOC Index + NOx Index

ESPHome automatically detects the sensor type.

For SGP40 you only need:

voc:
  name: "VOC Index"

You do not need to configure a NOx sensor.

Why `update_interval` Does Not Mean the SGP40 Is Sampled Every 30 Seconds

This is an important ESPHome detail.

Sensirion’s VOC algorithm requires the SGP40 to be driven at 1 Hz, meaning one measurement every second.

ESPHome therefore reads the SGP4x sensor internally once per second regardless of the configured update_interval.

The update_interval controls how often the calculated value is published to ESPHome/Home Assistant.

ESPHome’s documentation explicitly notes that the device continues to be sampled at 1 Hz while network updates can occur less frequently to reduce Wi-Fi traffic and power use.

So:

update_interval: 30s

does not break the VOC algorithm.

It simply sends the current VOC Index to Home Assistant every 30 seconds.

For most Home Assistant installations, 30–60 seconds is plenty.

SGP40 Temperature and Humidity Compensation

The SGP40 can compensate its raw measurement using the actual temperature and relative humidity of the air.

Sensirion supports humidity compensation directly in the measurement process, and ESPHome exposes this through the compensation: configuration.

If temperature and humidity sensors are not supplied, ESPHome can still operate the SGP40 using default compensation values.

However, if you already have a good temperature/humidity sensor in the same enclosure, providing the real measurements is preferable.

SGP40 + SHT45 ESPHome Configuration

A very good combination is:

SGP40 + SHT45 + ESP32

The SHT45 provides temperature and humidity, while the SGP40 provides VOC sensing.

Because both use I²C, they can share GPIO21 and GPIO22.

Example:

esphome:
  name: esp32-air-quality
  friendly_name: ESP32 Air Quality Sensor

esp32:
  board: esp32dev

logger:

api:

ota:
  - platform: esphome

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

i2c:
  sda: GPIO21
  scl: GPIO22
  scan: true

sensor:
  - platform: sht4x
    temperature:
      name: "Room Temperature"
      id: room_temperature
    humidity:
      name: "Room Humidity"
      id: room_humidity
    update_interval: 10s

  - platform: sgp4x
    voc:
      name: "VOC Index"
    compensation:
      temperature_source: room_temperature
      humidity_source: room_humidity
    update_interval: 30s

The current ESPHome SGP4x component expects the fields:

temperature_source:
humidity_source:

inside the compensation: block.

This replaces older examples that use different compensation syntax.

Adding SGP40 to Home Assistant

If your ESP32 is using the native ESPHome API, you do not need to manually create an MQTT sensor in configuration.yaml.

Once the device is added through the ESPHome integration, Home Assistant receives its entities directly.

You should see an entity named something similar to:

VOC Index

The exact Home Assistant entity ID depends on your device and existing entity names, but it may look like:

sensor.esp32_sgp40_voc_index

You can rename the entity in Home Assistant without changing the ESPHome configuration.

For more ESP32/Home Assistant projects, see our Home Assistant ESP32 guides.

Home Assistant VOC Index Gauge

A gauge works well for seeing short-term VOC events.

Replace the example entity with your own VOC Index entity:

type: gauge
entity: sensor.esp32_sgp40_voc_index
name: VOC Index
min: 0
max: 500
severity:
  green: 0
  yellow: 100
  red: 150

Home Assistant’s gauge card currently supports numeric severity thresholds directly in YAML.

There is one caveat:

do not interpret the colours as official health classifications.

The SGP40 VOC Index is adaptive.

The gauge is useful for making VOC events obvious, but 150 is better understood as an automation/action threshold than a regulatory air-quality limit.

Home Assistant VOC History Graph

The VOC Index becomes even more useful when you can see its history.

A typical day might show:

baseline around 100
spike while cooking
reduction after opening windows
another spike while cleaning
gradual return toward the learned baseline

A history graph makes these patterns much easier to recognise than a single live number.

You can simply add your VOC Index entity to a Home Assistant history graph or statistics card.

Home Assistant Automation: VOC Alert

A notification when the VOC Index rises significantly above normal can be useful.

For example:

alias: VOC High Alert
triggers:
  - trigger: numeric_state
    entity_id: sensor.esp32_sgp40_voc_index
    above: 150
    for: "00:02:00"

actions:
  - action: notify.mobile_app_your_phone
    data:
      title: "VOC level increased"
      message: "The room VOC Index has been above 150 for two minutes."

mode: single

Current Home Assistant automation YAML uses triggers: and actions: with entries such as trigger: numeric_state and action:.

Waiting two minutes helps avoid notifications for very brief spikes.

Automatically Turn On Ventilation

If you have an extractor fan, ventilation system or smart relay controlled by Home Assistant, VOC monitoring can be used to automate ventilation.

Example:

alias: Ventilate on High VOC
triggers:
  - trigger: numeric_state
    entity_id: sensor.esp32_sgp40_voc_index
    above: 150
    for: "00:02:00"

actions:
  - action: fan.turn_on
    target:
      entity_id: fan.ventilation

mode: restart

Then switch it off again once VOC levels have remained lower:

alias: Stop VOC Ventilation
triggers:
  - trigger: numeric_state
    entity_id: sensor.esp32_sgp40_voc_index
    below: 110
    for: "00:10:00"

actions:
  - action: fan.turn_off
    target:
      entity_id: fan.ventilation

mode: restart

The gap between the ON and OFF thresholds prevents the fan constantly switching on and off around one value.

Why Is My SGP40 VOC Index Around 100 All the Time?

Often, that’s exactly what it should do.

The VOC Index algorithm continuously adapts to the environment.

A value around 100 represents the learned average VOC background, so the sensor naturally tends to return toward that region when conditions remain relatively stable.

The useful information is often the change.

For example:

Normal room:
95–110

Cooking:
180

Window opened:
70

Cleaning spray:
260

Those numbers are examples, not universal thresholds.

Different rooms and different VOC sources can produce different responses because the VOC Index measures changes relative to the recent environment.

Why Does the VOC Index Change After Opening a Window?

This is normal.

If outdoor air contains fewer VOCs than the learned indoor background, the VOC Index can fall below 100.

Sensirion specifically describes values below 100 as conditions with fewer VOCs relative to the recent average, with fresh air from an open window being one example.

This makes the SGP40 particularly useful for checking whether ventilation is actually reducing indoor VOCs.

Why Does Cooking Cause a Large VOC Spike?

Cooking releases a mixture of gases and vapours that can trigger the SGP40.

The VOC Index may therefore rise sharply during:

frying
baking
boiling certain foods
using cooking oils
burning food

This does not mean the SGP40 has identified a particular chemical.

It simply detects that the overall VOC environment has changed significantly relative to its recent baseline.

Why Does Perfume or Cleaning Spray Trigger the SGP40?

Many perfumes, alcohol-based products, detergents and cleaning products release VOCs.

A strong temporary spike after using them is therefore expected.

This is actually a useful test to confirm that the SGP40 is responding.

Don’t spray anything directly onto the sensor.

Allow the VOC to disperse naturally through the room.

Does the SGP40 Need 24 Hours of Burn-In?

Not in the simplistic way this is sometimes described.

Sensirion specifies that the SGP40 can reliably detect VOC events in under 60 seconds after switch-on, while full specified sensor performance can take up to approximately one hour.

The 24-hour figure relates to the adaptive VOC Index history, not a requirement that the sensor is useless for its first 24 hours.

The algorithm uses recent history to learn the normal VOC background and continuously adapts as conditions change.

ESPHome also supports storing the algorithm baseline so that startup can be faster after a restart, and store_baseline currently defaults to true.

SGP40 Not Detected in ESPHome

If the sensor isn’t working, start with the I²C scanner.

Your configuration should contain:

i2c:
  sda: GPIO21
  scl: GPIO22
  scan: true

In the logs, look for:

0x59

If 0x59 does not appear, check:

SDA and SCL aren’t swapped
the sensor has power
ESP32 and SGP40 share GND
the breakout board accepts 3.3 V
GPIO numbers match your ESP32 board
connections aren’t excessively long
the breakout actually contains an SGP40

The official SGP40 I²C address is 0x59.

SGP40 Readings Are Very High

A high VOC Index means current VOC conditions are substantially above the recent learned background.

Look for obvious sources:

cooking
cleaning
aerosols
perfume
paint
solvents
new furniture
glue
alcohol
smoke

Try opening windows and watch whether the VOC Index falls.

If the sensor remains unexpectedly high, also check its physical placement.

Avoid placing it:

directly beside cleaning chemicals
inside a sealed enclosure
beside an alcohol dispenser
directly over a cooker
where it is exposed to condensation

Sensirion specifies the SGP40 for non-condensing operation and recommends protecting the sensor from unsuitable environmental exposure.

SGP40 Readings Change Slowly

Remember that two different things are happening:

The sensor itself responds quickly to changes.

The VOC Index algorithm also adapts to recent history.

Sensirion specifies a sensor response time below roughly 10 seconds for the 63% response point in its defined ethanol test, while the Gas Index Algorithm continuously learns the environmental background.

So a fast VOC event and the longer-term movement of the baseline are not the same thing.

SGP40 Raw Signal vs VOC Index

This distinction is important if you’re comparing ESPHome with Arduino examples.

The SGP40 hardware outputs:

SRAW_VOC

A raw digital measurement.

It does not directly output the final 1–500 VOC Index.

Sensirion’s Gas Index Algorithm must process the raw measurements to generate:

VOC Index

The ESPHome sgp4x component performs this processing automatically.

This is why Arduino code that simply calls a raw measurement function and publishes that number as a “VOC Index” is incorrect.

If you’re writing custom Arduino firmware, use Sensirion’s sensor driver together with the Gas Index Algorithm implementation rather than treating SRAW_VOC as the final value. Sensirion provides an Arduino Gas Index Algorithm library specifically for converting SGP40 raw signals into VOC Index output.

ESPHome or MQTT for SGP40?

For a normal Home Assistant project, I recommend:

ESPHome native API

It is the simplest setup.

You get:

SGP40 support built in
VOC Index algorithm handled automatically
Home Assistant entities created automatically
OTA updates
logging
optional temperature/humidity compensation
no manual MQTT discovery configuration

If you’re building a broader system that specifically needs MQTT, there is nothing wrong with using MQTT.

But you don’t need MQTT simply to get an SGP40 into Home Assistant.

For a custom MQTT implementation, see our ESP32 MQTT to Home Assistant guide.

SGP40 vs SGP41

The SGP40 and SGP41 are closely related, and ESPHome supports both through the same sgp4x component.

The main difference is straightforward:

Sensor	VOC	NOx
SGP40	Yes	No
SGP41	Yes	Yes

ESPHome automatically detects whether an SGP40 or SGP41 is connected. The SGP41 can expose both a VOC Index and NOx Index, while the SGP40 provides the VOC measurement only.

Choose SGP40 if:

You mainly want VOC monitoring for:

cooking
cleaning
general indoor air quality
ventilation control
Home Assistant automations

Choose SGP41 if:

You also want NOx monitoring.

For many normal residential Home Assistant projects, the SGP40 remains a simple and effective VOC sensor.

Is the SGP40 Good for Home Assistant?

Yes.

The combination of SGP40 + ESP32 + ESPHome works particularly well because ESPHome handles the parts that would otherwise require custom firmware:

I²C communication
1 Hz sensor sampling
Sensirion VOC algorithm
baseline storage
temperature/humidity compensation
Home Assistant integration

ESPHome’s current SGP4x component handles the mandatory one-second sensor measurement cycle independently of the published update interval and supports stored baseline data and external temperature/humidity compensation.

That makes the SGP40 much more useful than simply reading its raw value with a generic Arduino library.

Best SGP40 ESPHome Configuration

For a simple installation:

ESP32 + SGP40

is enough.

For better environmental compensation:

ESP32 + SGP40 + SHT45

is the setup I would choose.

Use:

GPIO21 → SDA
GPIO22 → SCL
SGP40 → VOC Index
SHT45 → temperature + humidity
ESPHome native API → Home Assistant

Then let Home Assistant handle:

graphs
alerts
ventilation
air purifiers
long-term history

The result is a compact indoor-air-quality node with no custom C++ code and no MQTT configuration required.

SGP40 ESPHome FAQ

What is the SGP40 I²C address?

0x59.

What ESPHome platform does SGP40 use?

Current ESPHome uses:

platform: sgp4x

not the older sgp40 platform.

Does SGP40 measure CO₂?

No.

It is a VOC sensor.

Does SGP40 measure TVOC in ppm?

Not directly.

Its raw signal is processed into Sensirion’s adaptive VOC Index.

What does VOC Index 100 mean?

Approximately the average VOC environment learned from the recent history of the sensor.

Is a VOC Index below 100 good?

It means VOC conditions are currently lower than the sensor’s recent average.

It should not be interpreted as an absolute certification that the air is “safe”.

Is a VOC Index above 100 bad?

It indicates VOC levels have increased relative to the recent learned baseline.

The magnitude of the increase tells you how significant the event is.

Can SGP40 detect cooking fumes?

Yes, cooking frequently causes strong VOC events.

Can SGP40 detect perfume and cleaning products?

Yes. Many of these products release VOCs that can cause noticeable index spikes.

Should I use humidity compensation?

If a good temperature/humidity measurement is already available near the SGP40, use it.

ESPHome supports temperature and humidity sources directly.

Does the SGP40 need to run every second?

The Gas Index Algorithm requires a one-second sensor sampling interval.

ESPHome handles this automatically even if you publish the resulting VOC Index to Home Assistant less frequently.

Final Recommendation

For a Home Assistant VOC sensor, the SGP40 + ESP32 + ESPHome combination is simple, inexpensive and flexible.

The most important things to remember are:

connect the SGP40 over I²C
look for 0x59
use ESPHome’s sgp4x component
use the VOC Index, not the raw SGP40 value
understand that 100 represents the recent average, not a fixed pollution limit
use temperature/humidity compensation when available
let ESPHome handle the one-second sampling and Gas Index Algorithm
use Home Assistant for graphs, alerts and ventilation control

For the simplest build, connect an SGP40 directly to an ESP32 and let ESPHome do the processing.

For a better complete indoor-air-quality node, combine it with an SHT45 temperature/humidity sensor and, if required, add dedicated CO₂ and particulate-matter sensors separately.

The key point is simple:

The SGP40 measures VOC changes. ESPHome turns them into a usable VOC Index. Home Assistant turns that index into something you can monitor and automate.