Grains Per Pound (GPP)

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Grains Per Pound 

 Cannabis Growers Need to Know

Cannabis growers need to understand Grains per Pound (GPP) when they grow cannabis, since GPP is one of properties to help adjust and optimize the humidity and temperature for the indoor cannabis environment.


Chapter 1

What is Grains Per Pound

Chapter 2

How does GPP Come to be Used for Moisture Measurement

Chapter 3

5 Reasons Growers Need

to Know GPP

Chapter 4

Grains Per Pound Measuring & Calculation

Chapter 5

Psychrometrics and

How to Read & Draw

Chapter 6

Necessary to understand GPP

 in HVAC

Chapter 1. What is Grains Per Pound

GPP Chart 01

Grains Per Pound (GPP) is a unit to measure the moisture in the air, and the moisture measurement is based on the weight of dry air. The number of grains per pound of dry air expresses the specific humidity.

So, GPP in cannabis growing presents the actual weight of the water in a pound of dry air.

Along with dew point temperature and relative humidity (%RH), it is one of the most commonly-used measurements of air moisture. 

As you can imagine, this information is an invaluable resource to allow for adjustments in order to optimize the cannabis growth environment. With that in mind, the aim of this article is to provide a comprehensive overview of what GPP means, how it is calculated, and how useful it can be as a resource for indoor cannabis growers.

It would help you find the right dehumidification system by measuring the actual moisture of a grow room.

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Grains Per Pound VS. Relative Humidity (%RH)

GPP & Relateive Humidity

Relative humidity is a measure of the percentage of water present in the air – “how much moisture in the air” VS “how much moisture the air can hold at a drybulb temperature”. While the GPP is a measurement of the specific weight of the moisture in a pound of air.  That is why %RH is referred to as “relative” as opposed to the “absolute” nature of GPP.

You could find more POSTs about “Relative Humidity”

Chapter 2. How does GPP Come to be Used for

Moisture Measurement?

GPP was not used to measure moisture before, here we have to mention the unit “grains”.

The use of grains as a measurement has its origins all the way back in the Bronze Age, when it referred to literal grains of wheat or barley.  According to Wikipedia, we can see

GPP from Wikipedia

Next, we will use two infographics to get you understand the history and current use of grais, and grains per pound. 

The first info graphic tells you the history and development about the use of “grains”. 

The second info graphic lets you know the current usage of Grains Per Pound.

Grains Per Pound History & DevelopmentGrains per pound current usage


Grains per Pound gained its currentdefinition in 1959. These days it is a wholly inflexible and accurate measure. The International Yard and Pound agreement standardized it at its current rate:

1 grains = 64.7989 mg

1 mg = 15.4324 grains

1 pound = 7000 grains

As mentioned, know GPP can help you optimize the growth environment.

How we can understand GPP?

The answer is Psychrometrics Chart, which presents physical and thermal properties of moist air. If you understand Psychrometrics Chart, you can easily understand GPP and optimize humidity and temperature for your grow rooms.

The first figure at the beginning of the article is the Psychrometrics Chart. We will also teach you how to read and draw a Psychrometrics Chart later in this article.

Above all, let us be clear, why we need to understand GPP, especially for growing cannabis.

Chapter 3.  5 Reasons Growers Need to Know the GPP

Grains Per Pound (GPP) measures the actual moisture in the air, an accurate GPP measurement allows growers to gauge the actual indoor humidity of a grow room.

It allows growers to know the exact amount of moisture in their grow rooms, and helps them to choose the right equipment at different stages.

5 illustrations can conclude GGP’s role in cannabis growing.

Visualize with GPP

Visualize Environmental Control

Such as why heated air can hold more moisture or, conversely, how allowing moist air to cool will result in condensation.

Diagnose Environmental Problems  (Humidity and Air Temperature)

A rule of thumb for inside typical greenhouses during winter conditions is that a 10°F rise in air temperature can decrease relative humidity 20 percent.  For example, to decrease relative humidity in a winter greenhouse during a critical time period, we could heat the air.

Diagnose with GPPFind solution with GPP

Find Solutions to Indoor Growing

A GPP Chart presents properties of air in a graphical format, very useful for troubleshooting greenhouse or livestock building environmental problems.

Optimize the Cannabis Growth

Grains per Pound is helpful to adjust and optimize the cannabis growth environment. As such, the value of the GPP figure can be used to determine the actual humidity of a grow room.

Optimize by GPP

For instance,

GPP indicates whether dehumidification needs to take place, or whether humidity needs to be increased to ensure optimum moisture levels for plant growth at different conditions – a sealed grow room, lights on/off mode, and ventilation.

Especially, excess humidity can cause the growth of mold, bud rot and other harmful pathogens.


Chapter 4. Grains Per Pound Measuring & Calculation

In Chapter 2 we know, if we understand Psychrometrics Chart, it is easy for us to understand GPP, and know how we could optimize indoor humidity and temperature.

Psychrometrics Chart (also called Enthalpy Moisture Chart) is the technical term given to study moisture levels in the air. See two figures below:

GPP (water vapor) 01GPP (water vapor) 02

Grais of moisture/water is commonly used in Psychrometrics. The variables for the charts are the air temperature (the dew point saturation temperature) and the relative humidity.


Grains Per Pound is the point at Air Temperature and Relative Humidity intersect.

Therefore, we have the following tables:

GPP (water vapor) 03

Above table is the GPP (Grains of Moisture per Pound of Dry Air) at standard atmospheric pressure at RH from 10% to 90% in °F and °C.

The one below is same in “Pounds of Moisture per Pound of Dry Air” at same condition.

GPP (water vapor) 04

Chapter 5. Psychrometrics and How to Read & Draw

As mentioned previously, “psychrometry” is the technical term given to the study of moisture levels in the air. Therefore calculating GPP requires use of a psychrometric chart.

The variables which will serve as axes for the psychrometric chart are the temperature of the air and the relative humidity, the intersection of these two variables provides the grains per pound. 

This video will help you to understand GPP and how to read it from a psychrometric chart.

Calculating the value of moisture grains per pound of dry air might initially seem like a complex procedure.

However, with the right materials it is in fact a simple process.

8 points you need to know about Psychromrtric chart.

  • Dry Bulb Temperature 
  • Specific Humidity 
  • Dew Point Temperature 
  • Wet Bulb Temperature 
  • Relative Humidity 
  • Specific Volume 
  • Enthalpy 
  • Vapor Pressure Lines

GPP chart 02

you can watch the video about drawing Psychromrtric chart

Chapter 6. The Role of Psychrometrics in HVAC

An HVAC technology website states, “It is the heart of all Heating, Ventilating and Air Conditioning (HVAC) system design.”

GPP in HVAC 01GPP in HVAC 02GPP in HVAC 03


grains per pound, grow room humidity

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