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The Galpin Equation – How to Calculate It & Why It’s Important

The Galpin Equation is a method for calculating how much fluid you should be consuming to stay hydrated intense activities. These can be both physical and mental, such as running a marathon or studying a demanding and technical topic.

So, how does it work?

The formula for the Galpin Equation is as follows: Take your body weight (in pounds), divide that by 30, and consume that number (in ounces) every 15 minutes.

You can do the calculation manually, or use the calculator below:

Galpin Equation Calculator

Amount to drink:

The Galpin Equation – Metric Version (For those outside the US)

For those using the metric system, the equation is as follows: per Kilogram of body weight drink 2 milliliters of fluid (kg of body weight multiplied by 2).

So, as an example, an 82kg person would aim to drink around 164ml of fluid every 15 minutes.

Remember that this isn’t a general guideline for hydration – this is for more demanding activities only. Research shows that even mild dehydration – a body water loss of 1–2% – can impair performance1The Hydration Equation: Update on Water Balance and Cognitive Performance – 2014 | Riebl et al. | ACSMs Health Fitness Journal.

The image above illustrates some popular drink sizes and how much liquid they contain. It isn’t however an endorsement of Gatorade 😅 There are less artificial drinks out there for replacing fluid and electrolytes.

Where does the Galpin Equation come from?

It’s natural to ask what the source of this data is. According to Andy Galpin’s slides from his “Optimizing Hydration for Athletes” YouTube video it comes from a study performed by Fallowfield et al. in 19962Effect of water ingestion on endurance capacity during prolonged running – Fallowfield et al. | 1996 | Journal of Sports Sciences.

Image source – Dr Andy Galpin YouTube

I won’t re-hash the whole abstract of the study, but in essence 4 men and women who train in physical exercise regularly ran on a treadmill until exhaustion under 2 different sets of conditions.

Their first run was without any fluid intake, and their second run was with 3 ml per kg of body-weight consumed prior, and then 2 ml per kg of bodyweight for every 15 minutes they ran. The image above shows how they were, on average, able to run for 26 minutes (33%) longer with the added hydration.

Whilst this is definitely a useful data point, I think it’s fair to ask the question whether the study fully explores what the optimal intake of fluid is. Given that the baseline intake it compares against is zero.

Further studies could look at:

  • The effect of different intakes of fluid, such as 1 ml & 3 ml per kg of body-weight.
  • The effect of adding electrolytes at different ratios to the fluids.
  • The optimal pre-exercise hydration level.
  • Looking at hydration levels on a per exercise basis. For example, perhaps weight lifting benefits from more liquid than soccer.
  • Adjusting fluid amounts based on how easily a person sweats.

It’s possible data for some of the above already exists.

3 General Hydration Principles from Andy Galpin

Above we’ve looked at Dr. Galpin’s formula for how to stay well hydrated during intense activities.

Next we’ll look at Dr. Galpin’s 3 general hydration principles. Which in a nutshell are:

  1. Daily Hydration Targets – as a rough guide, aim for half an ounce of fluid, per pound of body-weight, per day. However, everyone’s different, so some may need more, others less.
  2. Hydration & Performance – loss of fluid impacts performance, so pay special attention to hydration in this context.
  3. Understanding Sweat – we lose much more than just water when we sweat, so it’s important to replace these compounds also.

Below we expand on these points in more detail. The source for Andy discussing these topics is here.

Guideline 1 – Daily Hydration Targets

  • Aim for half an ounce of fluid, per pound of bodyweight, per day

Dr. Galpin states that this isn’t a hard-and-fast rule, but rather, a rough guideline to target. This would be 100 ounces (almost 3 liters of fluid per day) for a person weighing 200 pounds.

However, it’s important to note that food also provides a lot of hydration, so drinking that amount of water/fluid in a day is not required. Foods like eggs and meat are around 65% to 75% composed of water and most fruits and vegetables hover over 90%.

Image source

He also emphasizes that there’s room to maneuver – some people just need less, especially if you’re not a professional athlete. The more active you are the more fluid that your body uses up to maintain said activity.

This guideline also shows us that there is no specific fluid quantity that works for everyone. To state the obvious, a person weighing 125 pounds doesn’t need to consume nearly as much as a 200-pound person. This is especially true when you factor in differences in activity level.

Dr. Galpin mentions that a simple way to check hydration is to do the following:

  • Look at how thirsty you feel and what the color of your urine is.
  • The darker your urine is the more dehydrated you are.
  • He also states that urine that is crystal clear is a sign of over hydration!

Guideline 2 – Staying Hydrated

Dr. Galpin mentions here that it’s important to stay hydrated throughout the day – a loss of just 1 – 3 percent of body weight from water loss can reduce exercise capacity by around 10 percent and increase your perception of effort.

If you’re forgetting to drink fluids throughout the day the stresses could compound and lead to lower performance overall. A 2007 study showed that basketball players progressively lose skill as dehydration went from 1 – 4 percent (source).

If you’re an athlete or engage in lots of focused mental work/stresses and you’re sweating more then following the Galpin equation ensures that you’re getting the fluid you need to replace what’s lost.

However, for the average person who perhaps isn’t a full-time athlete or locked in a 5-hour do-or-die chess match, it comes down to simply trying to consume enough fluids, checking in with how thirsty you are plus looking at the color of your urine.

Guideline 3 – Understanding Sweat

It’s not just how much fluid you’re consuming that’s important but also what you’re drinking and what nutrients it contains.

Dr. Galpin states that in a liter of sweat, you can lose up to:

  • 500 – 2000 mg of Sodium (Na)
  • 100 – 500 mg of Potassium (K)
  • 500 – 3000 mg of Chloride (Cl)
  • 0 – 100 mg of Calcium (Ca)
  • 0 – 100 mg of Magnesium (Mg)
Image source

While most people vary a lot in the ranges above, on a hot day, or when engaging in intense training over a prolonged period of time it’s easy to see how the loss of minerals in sweat begins to add up.

The Best Beverage Choices For Hydration

So while it’s important to understand how much fluid you’re losing and how much you should be drinking on average, it’s also equally important to replace the minerals and electrolytes that you’re losing in your sweat.

Drinking water alone will replace lost fluids, but it doesn’t give you many minerals and you can easily end up overdoing it on the water and simply urinating more frequently.

Dr. Galpin talks about more common sports drinks here but the general consensus is that they don’t provide enough minerals, and the carbs in these drinks are all heavily processed sugars that contribute to poor metabolic health.

Natural Sports Performance Drinks

There are some good “natural” drinks that you can use that provide all the minerals and fluid that your body needs.

Dr. Galpin recommends two items to people that want to avoid more processed foods – Milk and Coconut water.

Both of these drinks contain sodium and plenty of potassium, although Dr. Galpin recommends that you might have to add additional salt to these beverages.

Coconut Water – You can add salt and lime to coconut water for a little extra flavor and it can make a fantastic homemade sports performance drink. An added benefit of coconut water is that it contains a lot of Potassium – 2 liters can contain around 4,000 mg (4 grams).

Milk – has an added advantage over coconut water in that it also contains magnesium. However, he states that milk isn’t something that most people want to drink while exercising or running a marathon. However, it can be a great idea to replenish with milk once your training is finished.

Sports Drinks Dr. Galpin Recommends

Another option is to use sports hydration packets that you can add to water that contains all the minerals you need. Usually, one packet is mixed with around 16 ounces/475 ml of water.

He recommends the following:

  • Ceralyte 70
  • Nuun
  • Drip Drop
  • LMNT

Who Is Dr. Andy Galpin?

Dr. Andy Galpin is a man of many talents – an assistant professor at California State University, he holds a degree in Exercise Science and a Master’s degree in Human Movement Sciences.

He’s worked as a strength and conditioning coach for various professional MLB and NFL athletes, and he’s also competed as a weightlifter and maintains a passionate interest in combat sports.

He holds a Ph.D. in human bioenergetics and has various published peer-reviewed studies. He’s also coauthored a book called Unplugged – on the subject of using technology in sports performance.

Andy Galpin is the founder of the Biochemistry and Molecular Exercise Physiology Lab – you can find out more about himself and his work here.

Further Reading


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