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The ultimate VPD calculator & environmental timeline.

Tom Starley
February 9, 2021
10 min read
Screenshot of our VPD chart and environmental calender
Screenshot of our VPD chart and environmental calender
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VPD calculator video walkthrough

When you’re a professional or hobby grower trying to grow the best quality plants, a VPD calculator can certainly help you on your way.

But we recognise that there are limitations to most VPD calculators out there.

These limitations restrict their usability, their usefulness and the benefits you can get from them.

That is why we created the ultimate VPD calculator and environmental timeline.

Our mission is to provide growers like you with the tools you need, to help you get the most out of your grow room, whilst growing greener. 😊

In this post I’ll cover

  • What a VPD calculator is.
  • Why VPD calculators are important.
  • How VPD calculators work.
  • What a VPD chart is and how to read it.
  • The limitations of traditional VPD calculators & VPD charts.
  • The 10 ways our VPD calculator is different (& better!)
  • How to use our VPD calculator.
  • The Grow Sensor & Grow App.

Although VPD calculators can be useful for fine tuning your grow space whilst growing a wide variety of different plants, please note:

Disclaimer: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis you’re doing so in accordance with the law and subject to appropriate permissions and licenses of the applicable country.

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What is a VPD calculator?

A VPD calculator is a tool that enables you to improve and fine tune the environment within your grow space, so that you can provide your plants with optimal conditions to grow.

VPD calculators enable you to determine the ideal range of temperatures and humidities to try to maintain in your grow space.

Why are VPD calculators important?

VPD or vapour pressure deficit describes the difference between the vapour pressure inside the leaves of your plants, and the vapour pressure of your grow environment (outside the leaves of your plants.)

Vapour pressure deficit is a key indicator of how happy or stressed your plants are.

Using a VPD calculator, you can work out what environmental conditions to provide to your plants to keep them stress free, healthy and photosynthesising at maximum efficiency.

The values a VPD calculator provides, should also enable you to avoid pest problems and evade mould and mildew.

When you use a VPD calculator, you maximise your chances of growing the best plants.

If you grow as a business, top quality plants translates to top quality product and improved yields. This in turn means improved profitability and customer satisfaction. 😁

How do VPD calculators work?

To use a VPD calculator, you need to take measurements of both the air temperature and relative humidity inside your grow space.

Normally, you drop your ambient air temperature and humidity into a calculator and it generates VPD value. This is usually the vapour pressure deficit of your environment, measured in kilopascals (kPa.)

Some calculators also have a field where you can put in your leaf surface temperature (either instead or as well as) your ambient grow space temperature.

Leaf surface temperature (LST) is accepted as being a very accurate measure of plant health. To measure your leaf temperatures, you’ll need an infrared thermometer.

Some calculators also take your temperature and humidity data and generate a ‘leaf VPD’ value.

This is because the VPD that the plant experiences is different to the VPD of the air.

The inside of your plants leaves are wet and whilst the plant transpires, the water exiting the leaves through the stomata evaporates and cools the leaves which results in a slightly different (normally lower) VPD.

Unless you’ve used an infrared thermometer to give your leaf surface temperature, if a VPD calculator generates a leaf VPD value, it uses an ‘offset.’

This takes VPD value created from the ambient air temperature figure you put in and offsets it by a given amount to give you an estimated leaf VPD value.

Often (but not always) when you put your data into a VPD calculator, it’ll generate a chart or graph for you, to help you visualise your VPD, relative to the ideal VPD.

VPD chart example

What’s a VPD chart?

A vapour pressure deficit chart visually shows you how temperature and relative humidity interact with one another to provide environmental conditions for your plants.

Relative humidity is normally shown on the horizontal axis, and temperature is usually on the vertical axis.

Plants perform best at different VPD values throughout their life cycle.

The ideal VPD for a clone or seedling is different from the ideal VPD for a plant in vegetative growth, early flower, or late flower.

Checkout these previous posts for more detail about VPD in different stages of the growth cycle:

The different types of VPD chart.

There are two types of VPD charts.

1.Charts that advise on VPD values specific to the stage of your plants’ growth.

This means that you’ll need to use different charts depending on whether you’re propagating plants, or if you have plants in vegetative growth of bloom.

With this type of chart, you aim to keep your VPD values within the (usually) green band. The more extreme your conditions, the further you’ll move away from the green band. This indicates less optimal conditions inside your grow environment.

Often the less favourable temperatures and humidities are shown in colours that represent danger - oranges and reds.

The areas on the chart furthest away from the green band represent very high or low VPD’s.

If you're providing your plants with environmental extremes like these, your plants won’t thrive.

2.Charts that show the different plant growth stages in bands together on one chart.

The different coloured bands on this sort of chart represent the different optimal conditions for the different phases of growth; propagation, vegetative growth and flowering.

Extreme or less than ideal conditions for each of the stages of growth are again normally indicated in red to represent danger to your plants.

Reading VPD charts.

How you read or interpret VPD charts is the same regardless of the type of chart.

You need to look for the point at which the relative humidity of your grow space intersects with the temperature inside your grow space.

This intersection will show you whether the VPD in your grow space is where it needs to be, or if it is either side of the optimal range.

Your VPD value shown on a VPD chart highlights any action you need to take.

It’ll show you whether you need to increase or decrease the relative humidity of your space in conjunction with increasing or decreasing the temperatures in your environment to move your VPD into the ideal range.

You can find our more about How to use a VPD chart to grow your best plants here.

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The limitations of traditional VPD calculators & VPD charts.

As you know, the two measurements that generate vapour pressure deficit values are temperature and humidity.

But that isn’t to say that temperature and humidity are the only factors that influence VPD.

And this is where traditional VPD calculators and charts are limited, because....

The amount of carbon dioxide and light available to your plants (alongside temperature) are factors, which then have a knock on effect on relative humidity and therefore VPD in your grow room. 🤯

Plants can use more light for photosynthesis at higher concentrations of CO2. (And higher light intensities normally = hotter temperatures.)

Photosynthesis increases alongside an increase in temperature and CO2. (And higher temperatures influence humidity and VPD.)

You can find more detailed information on the interaction between light, temperature and CO2 here: What’s the ideal grow room temp with CO2?

When consulting a VPD calculator or a VPD chart, you really want it to provide you recommendations that take into consideration all factors... which they don’t.

Which is where our VPD calculator and environmental timeline comes in!

Image showing how our VPD calculator is different to others.

10 ways our VPD calculator is different (& better!)

1. It’s more than just a ‘VPD calculator,’ it’s a calculator and timeline combined.

2. It’s highly customisable. Input the specifics about your grow; what species and strains you’re growing, what lights you use, whether you supplement CO2 and more.

3. You get your very own, personalised, FREE VPD grow calendar.

4. With week by week recommendations for optimal growth conditions, at every stage of your plants' growth cycle.

5. The customisable timeline settings and resultant chart and graph will give you visual clarity about how to stay in your VPD sweet spot.

6. You’ll get a deeper understanding of YOUR setup’s ideal temperature range and relative humidity.

7. Learn about day AND night time temperatures and humidities specific to you.

8. Stay within the recommendations to prevent mould and evade pests.

9. Discover how to induce colour.

10. It gives you actionable information and enables you to grow healthier plants whilst factoring in your exact setup!

The temperature, humidity, VPD, PPFD and CO2 figures that our VPD calculator creates for your grow are a product of complex calculations.

The formulas that generate your suggested grow timeline values take into consideration how temperature, humidity, light intensity and CO2 all have an impact on one another.

This information and these interactions are reflected in your suggested timeline.

Which is why it’s such a comprehensive and useful resource!

How to set up your VPD calculator

How to use our VPD calculator.

We hope you’ll find the VPD calendar and environmental timeline super intuitive to use, but here’s a little run down of how it works just in case...

1. Download your VPD calculator and environmental timeline for FREE. (You can do that at the top or bottom of this page!)

2. Once you’ve opened the document, head down to the section on the VPD calendar called ‘Adjust your timeline settings.’

3. Select your answers to all of the questions from the list of choices in the corresponding drop downs.

4. Scroll back up to the top of the page and review your optimised grow timeline!

5. You’ll now have recommended figures for...

  • Daytime VPD.
  • Ambient temperature during the day.
  • Daytime estimated leaf surface temperature.
  • And daytime relative humidity.
  • Ideal CO2 ppm (if you’ve chosen to supplement.)
  • Nighttime VPD.
  • Ambient night time temperature.
  • Nighttime relative humidity.
  • All for every week of your grow cycle!

All you need to do is check back each week to see what, if any amends needs to be made to your environment to provide your plants with exactly what they need, when they need it.

6. Once you’ve taken in all the information on the VPD Enviro Calculator tab, you can access your VPD chart on Tab 2.

7. Here you can quickly and easily select what week of the growth cycle your plants are in. You’ll also see the recommended VPD, temperature and relative humidity to aim for in your grow space.

8. Beneath this, you also get a personalised VPD chart for whichever week of your grow you’re in. This allows you to cross reference your measurements against the recommendations.

Then what?

You’ll easily be able to see if your VPD is in the sweet spot, (if so, congrats!🎉)

And if not, you can see what alterations you need to make to your temperatures and humidities to be more in keeping with what your plants need at this time.

There's a couple of important things to note here…

Firstly, if you are looking to supplement CO2 in your grow space, it’s definitely worth taking a look at this post and get your FREE CO2 grow room calculator here too.

Secondly, all the tools in the world are no substitute for taking the time to study your plants and visually check they look happy and healthy.

It’s really important that you’re consistently taking measurements, seeing what exactly works the best for your unique setup and circumstances and acting accordingly. 🤓

The Grow App showing data pulled in from The Grow Sensor

The Grow Sensor & Grow App.

Part of the way that we’re helping growers like you get the most from your indoor grow space is through the free tools (like this VPD calculator) that we provide right here on our blog.

The other side of what we do is creating physical products with smart hardware to further lighten the load and help you grow better plants whilst growing greener…

Which is where the Grow Sensor and accompanying Grow App come in!

If you’re new to growing or a little hazy on why using a grow room monitor is such a beneficial thing to do, check out our previous post what is a grow room sensor & why do I need one?

In a nutshell, the Grow Sensor and accompanying App enables you to perfect your growing environment.

A quick rundown of the benefits:

The Grow Sensor & Grow App enable you to monitor your room (or rooms!) 24/7. Giving you the insight to act fast, reduce running costs and optimise your grow room, so you can grow the best product.

The sensor and app work together to send you instant SMS and push notifications for temperature, humidity, CO2, power outages or light spikes and abnormalities. Making sure you never lose a crop again.

The sensor also measures VPD in real time. No more having to measure temperature and humidity before using a VPD calculator to work out what your VPD is!

Grow allows you to monitor and access your data on the go (from anywhere!) via the free Apple and Android Grow App. You’re even able to add multiple growers into your account and share the responsibility!

We’re working hard to provide the most beneficial features alongside crafting the Grow Sensor from the best components and ensuring that it’s built to last.

The Grow Sensor has low power consumption, a long battery life and is built from super high accuracy sensors for reliable and stable data collection.

Checkout the full feature set on the home page for more detailed information. 😊

Full disclosure, there are a number of available sensors out there, suited to different budgets and available in different countries.

So don’t just take our word that the Grow Sensor is THE SENSOR you want monitoring your grow room...

Checkout our FREE top grow room sensors comparison chart and see for yourself what grow room monitor best suits your needs.

… Then come back & sign up for early access if the Grow Sensor’s the product for you!


A VPD calculator can guide you by suggesting temperatures and humidities to aim for in your grow space.

This enables you to provide your plants with optimal conditions for strong growth and pest prevention.

VPD charts are visual representations of how temperature and humidity interact to provide a vapour pressure deficit value.

The VPD of your grow environment directly impacts your plants - either positively or negatively.

Traditional VPD calculators have limitations. They only take into consideration temperature and humidity.

But CO2 concentration and light intensity also play a significant role in VPD.

Our combined VPD calculator and environmental calendar does take these additional important factors that influence VPD into consideration, and is highly customisable to your unique set up.

Using it enables you to determine and work towards providing optimal conditions for your specific plants, grown with your specific equipment at every stage of growth.

The Grow Sensor and Grow App measure and report on VPD (amongst other things!) 24/7 to give you real time visibility.

This enables you to cross reference your VPD against the recommendations from your unique personalised VPD chart and environmental timeline, so you stay in that sweet spot!

Get your VPD and environmental calculator below, you’re going to love it! ✌

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Tom Starley
February 9, 2021
10 min read

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