The key to making perfect Maple Syrup lies in achieving the correct sugar content. It is imperative that you stop boiling at just the right time in order to avoid syrup that is either too watery, or syrup that begins to crystalize.

Using a refractometer allows you to achieve perfectly finished Maple Syrup, by providing simple, accurate measurements of the sugar content.

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Making Your Own Maple Syrup

Check out these links to the other articles in this series:

Measuring Sugar Content of Maple Syrup

In this article, we are assuming that you have already performed the majority of the sap boiling on an outdoor fire, and you are now nearing the final stage of the boiling process.

The final stage of boiling down sap, often called the ‘finishing boil,’ is best done in a controlled way (i.e., on a stove) so that you can maximize the likelihood of producing perfect maple syrup that hasn’t been overdone (creating crystals or ‘rock-candy’) or underdone (watery syrup). It can also be more conducive to bottling the syrup in a food-safe way.

While performing the finishing boil, it can be helpful to know the sugar content of the syrup periodically so that you know when to stop boiling. When the syrup reaches a sugar content (brix) of 66%, you have achieved Maple Syrup, stop boiling!

What You’ll Need:

Refractometers, Sugar Content, and Maple Syrup

What is a Refractometer?

A refractometer is a device that measures the refractive index of a liquid. It does this by measuring the angle at which light is bent by passing through a liquid media. A higher angle indicates a higher refractive index, and a lower angle indicates a lower refractive index. 

Maple Syrup is a mixture of water, sugar, and other constituent parts. As water boils away from the syrup, the sugar remains, and the resulting mixture has an increased proportion of sugar in it. The sugar concentration in Maple Syrup is directly related to the refractive index. Therefore, we can use the refractive index to provide a measurement of the Brix value (sugar content) of the Maple Syrup.

At What Temperature Does Maple Syrup Boil?

Maple syrup boils at a higher temperature than water due to its sugar content. Water boils at 212°F, while maple syrup boils at 219°F. For this reason, you want to continue boiling your sap until it reaches a temperature of 219°F.

Note: In general, you want to boil the sap until it reaches a temperature that is 7°F above the boiling temperature of water. So, if your area differs significantly from 212°F, add 7°F to the boiling point of water at your location.
Boiling Maple Sap to Make Maple Syrup
Maple Syrup

What Does Boiling Maple Syrup Look Like?

A couple of clues will occur as you approach the time when the maple syrup is done. You will notice that the consistency of the syrup seems to change as you approach the 219°F temperature. Watch for these tell-tale signs:

  • The syrup will suddenly want to bubble over! The heat setting that you have been using on your stove all along, will suddenly be too hot.  Often this will happen the instant you put a spoon or stirring stick into the syrup. Turn the heat down little by little to keep the boiling going but to stop the syrup from boiling over.
  • The bubbles will change consistency and turn into larger, more viscous, more spherical, and stickier bubbles than before (not foam), like below.
Maple Syrup Boiling

What Happens if the Sugar Content of Maple Syrup is Too High?

There may be a temptation to go to higher temperatures in order to achieve thicker syrup. While this is the case, and the syrup will become thicker, it will also become more prone to crystallization. Maple Syrup will begin to crystallize when the sugar content is above 68.9 °Brix.

Below is some maple syrup that was heated to 230°F. After several weeks on the shelf, significant ‘rock-candy’ crystallization had occurred.

Crystalized Maple Syrup

How Do You Know When Maple Syrup is Done Boiling?

Once the sap has reached 219°F, let it boil at this level for another minute or so, then it is done. I find that knowing exactly when to stop boiling can seem a bit arbitrary!

It depends on how accurate your thermometer is, how quickly you get the syrup off the heat, and it’s easy to end up just ‘guessing’ when it is done. That is why I use a refractometer.

What is the Sugar Content of Real Maple Syrup?

According to the International Maple Syrup Institute, real maple syrup has between 66% and 68.9% sugar content. 

You want the syrup to be between 66-68.9% on the brix scale (66-68.9% sugar content) because lower than 66% and your syrup will be too watery.

And above 68.9%, your syrup will start to crystalize in the jars. So ideally, you measure the sugar content directly instead of only going by temperature.

How to Use a Refractometer to Measure the Sugar Content of Maple Syrup

Here is a picture of my refractometer. I was actually surprised at how inexpensive they are, you can get one very reasonably priced at Amazon

They cost less than a hydrometer, the traditional way of measuring Maple Syrup, which relies on density.

How to Use a Refractometer

The maple syrup will boil for a long time at 212ºF, then it will start to climb. I watch the candy thermometer as a reference, and when it is indicating about 215ºF I start sampling with the refractometer.


Drip Some Syrup on the Viewing Slide

Take a few drops from the pot and drip the syrup onto the viewing slide, then close the lid.

Using a refractometer to accurately determine the sugar content of maple syrup
Adding Maple Syrup to the slide.
Maple Syrup Refractometer

After that, wait a few seconds for the syrup to cool to room temperature. It doesn’t take long because it is only about 5 drops, which cool very quickly.

Read Off the Sugar Content from the View Port

Look into the viewport (like a telescope) to read off the % sugar concentration (brix value). The line between blue and orange indicates the % sugar in your syrup.

In the image below, I was at about 66.4% which is about perfect. (If the scale is all blue – keep boiling, you are below the minimum reading.)


In the picture below, I show 2 different batches. The one on the left is at the maximum (68.9%) and the one on the right is at the minimum (66%) brix values to technically be considered maple syrup.

68.9% and 66% Sugar Content – Read from a Refractometer.

This is not a ‘digital’ refractometer, it is analog (doesn’t require batteries). I read somewhere that a person thought it required sunlight – it doesn’t. All I use is the normal overhead room light, and then look in the viewport (like a telescope), and it looks like the images I’ve pasted here.

I wrote a whole article on How to Perfect the Finishing Boil for Maple Syrup, check it out!

Cleaning the Refractometer

To clean the refractometer, use a wet dishtowel and wipe off the slide and slide cover with warm soapy water. Then try it off with a dry towel.

For more information, check out my List of Most Frequently Asked Questions About Making Maple Syrup.


And that’s it! This is a super easy way to directly measure the sugar content of you Maple Syrup. I usually sample multiple times as I am boiling, so that I have an idea of when the syrup will be done. 

The final product is incredibly beautiful; it looks like a jewel.

Crystal Clear Maple Syrup
Crystal Clear Maple Syrup

Congratulations! Enjoy your self-made maple syrup!


I hope you found this helpful! I know that harvesting maple sap and boiling it into fresh maple syrup is a highlight of the spring for my family! I hope it is for you and yours as well!

I love getting comments and especially hearing stories about how other people boil and make maple syrup! Please leave a comment on any tips and tricks that have worked well for you!

Supplies mentioned in this Article:

Check out these Details on my YouTube Video on Refractometers

How to Make Maple Syrup on YouTube - Practical Mechanic
Watch on YouTube!

Check out the “How to Make Maple Syrup Podcast!”

Listen to the “How to Make Maple Syrup Podcast!

Top 5 Lesser-Known Mistakes When Making Maple Syrup (Rebroadcast) How to Make Maple Syrup!

In this episode I go over 5 lesser known, yet still somewhat common mistakes people tend to make when making homemade maple syrup. We start with several mistakes related to boiling the sap into syrup and how that can be messed up! We then cover the final filtering stages when everything happens so fast! Don’t miss this episode to improve your Maple Syrup making operation! Below are links to equipment that I use and recommend for making Maple Syrup: Propane Stove: ⁠⁠ Maple Syrup Refractometer: ⁠⁠ Dial Thermometer: ⁠⁠ Digital Candy Thermometer: ⁠⁠ Non-woven mesh Filters: ⁠⁠ Synthetic Maple Syrup Filter: ⁠⁠ Filter Holder Wire: ⁠⁠ Maple Syrup Canning Bottles: ⁠⁠ For more details on these topics, visit the website: ⁠⁠ Download a free eBook as well as other resources from around the web at: ⁠⁠ Here’s a link to my YouTube Video on How to Make Maple Syrup in 4 Minutes: ⁠⁠ Send me an email at: ⁠⁠ I’d love to hear from you, leave me a voicemail! ⁠⁠ Leave a review for this podcast, it helps get the word out! ⁠⁠ Thanks for Listening to the: How to Make Maple Syrup Podcast Podcast Host: Richard Tomlinsen Episode 022 As an Amazon Associate, I earn from qualifying purchases. — Send in a voice message: Support this podcast:
  1. Top 5 Lesser-Known Mistakes When Making Maple Syrup (Rebroadcast)
  2. What are the Top 5 Most Common Mistakes When Making Maple Syrup? (Rebroadcast)
  3. Hard Maplin' – Behind the Music (Rebroadcast)
  4. Finishing Boil for Maple Syrup – Top 5 Tips (Rebroadcast)

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