Brewing Evaporation Rate Calculator

Enter your pre-boil volume, post-boil volume, and boil duration to calculate evaporation rate, percent boil-off per hour, and total wort loss.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter the Pre-Boil Volume
Input the total wort volume in the kettle before the boil begins, in gallons.
2
Enter the Post-Boil Volume
Enter the wort volume remaining in the kettle after the boil ends, in gallons.
3
Enter the Boil Duration
Input the total length of the boil in hours (e.g., 1 for a 60-minute boil).
4
Review your results
The calculator displays six cards: Evaporation Rate, Evaporation % per Hour, Total Volume Lost, Total % Lost, Wort Retained, and Post-Boil Yield.

Example Calculation

A homebrewer measures a 7-gallon pre-boil volume and 5.5 gallons post-boil after a 60-minute boil.

Pre-Boil Volume

Post-Boil Volume

5.5

Boil Duration

Results

Evaporation Rate

1.50 gal/hr, Evaporation % per Hour: 21.4%, Total Volume Lost: 1.50 gal, Total % Lost: 21.4%, Wort Retained: 78.6%, Post-Boil Yield: 5.50 gal

Tips

Account for Kettle Geometry

Wider, shallower kettles typically exhibit higher evaporation rates due to a larger surface area exposed to heat. Consider this when scaling recipes or comparing rates.

Consistency is Key for Repeatability

Maintaining a consistent boil intensity and lid position (if used) across batches will stabilize your evaporation rate, making future gravity and volume predictions more accurate. Aim for a steady, rolling boil.

Adjust for Altitude

Brewers at higher altitudes will experience lower boiling points, which can lead to slightly reduced evaporation rates compared to sea level. Factor this into your calculations if brewing above 2,000 feet.

Calculating Your Brewing System's Evaporation Loss

Accurately predicting the final volume and original gravity of your homebrew is a cornerstone of consistent brewing. The Evaporation Rate Calculator helps brewers quantify the liquid lost during the boil, a critical factor for hitting target specifications. Understanding this rate allows for precise adjustments to pre-boil volumes, ensuring that a desired 5-gallon batch doesn't accidentally become 4.5 gallons with an unintended higher gravity. Many brewers experience an evaporation loss of around 10-15% of their pre-boil volume per hour, making this metric essential for repeatability.

The Logic Behind Evaporation Rate Measurement

The evaporation rate is a direct measure of how much liquid volume is lost from your wort during the boiling process, typically due to steam escaping. This calculation is vital because it directly impacts your final batch volume and the concentration of sugars, which in turn determines your beer's original gravity and potential alcohol content. Without knowing your system's specific evaporation rate, it becomes challenging to consistently hit your target numbers, leading to variations in beer strength and yield from one batch to the next.

The Formula for Brewing Evaporation

The Evaporation Rate Calculator uses a straightforward set of calculations to determine both the volume of liquid lost per hour and that loss as a percentage of your initial volume. This helps brewers understand their system's efficiency and plan for future brews.

First, the total volume lost is calculated:

lost volume = pre-boil volume - post-boil volume

Next, the evaporation rate in gallons per hour is determined:

evaporation rate (gal/hr) = lost volume / boil duration

Finally, the percentage of volume lost per hour is calculated:

percent per hour = (evaporation rate (gal/hr) / pre-boil volume) × 100

Here, pre-boil volume is the wort volume before boiling, post-boil volume is the volume after boiling, and boil duration is the total time the wort was boiled in hours.

💡 Once you've dialed in your pre-boil gravity using an accurate evaporation rate, our IBU (International Bitterness Units) Calculator can help you fine-tune your hop additions for optimal bitterness.

Determining Evaporation for a 90-Minute Brew

Let's walk through an example for a homebrewer setting up a new system. They start with a pre-boil volume of 6.5 gallons and, after a 1.5-hour boil, measure a post-boil volume of 5.5 gallons.

Calculate the lost volume: lost volume = 6.5 gal - 5.5 gal = 1.0 gal
Determine the evaporation rate in gallons per hour: evaporation rate (gal/hr) = 1.0 gal / 1.5 hr = 0.666... gal/hr ≈ 0.67 gal/hr
Calculate the percentage lost per hour: percent per hour = (0.67 gal/hr / 6.5 gal) × 100 ≈ 10.31%

Based on these inputs, the brewer's system has an evaporation rate of approximately 0.67 gallons per hour, which translates to about 10.31% of the pre-boil volume lost per hour. This data is invaluable for scaling recipes or adjusting future pre-boil volumes to achieve precise final batch sizes and gravities.

💡 After mastering your boil-off rate, consider how other factors like oxidation might impact your beer. Our Free SO2 Calculator can help you manage sulfite additions to protect against unwanted flavor changes.

Practical Application Context

The evaporation rate calculation is a cornerstone for brewers focused on consistency and quality. Firstly, it's crucial for recipe scaling and adjustment. If a brewer wants to brew a 5-gallon batch with a specific original gravity, they must account for the volume lost during the boil. Without this, a target 1.050 OG might end up as 1.055 or higher, altering the final beer. Secondly, it aids in troubleshooting batch variations. If a brewer consistently misses their gravity or volume targets, a fluctuating evaporation rate is a prime suspect. Identifying and stabilizing this rate (e.g., by ensuring a consistent boil intensity) can bring consistency back to the brew day. Finally, it’s essential for optimizing hop additions, particularly for bittering hops. While hop isomerization occurs throughout the boil, the concentration of wort changes as water evaporates. A higher evaporation rate means a more concentrated wort earlier, potentially affecting hop utilization.

What evaporation rate results look like in practice

In the brewing world, evaporation rates vary significantly based on equipment, boil intensity, and even atmospheric conditions. For homebrewers using a standard 5-gallon kettle on a propane burner, a typical evaporation rate ranges from 0.75 to 1.5 gallons per hour, or roughly 10-15% of the pre-boil volume per hour. This provides a baseline for common setups. Commercial breweries, operating with much larger kettles and often more controlled heating elements, tend to have lower percentage-based evaporation rates, frequently in the range of 5-8% per hour. This is due to factors like kettle geometry (larger surface area to volume ratio) and often more sophisticated heat management. For distillers or those making very high-gravity wort, evaporation rates can be intentionally higher, sometimes exceeding 20% per hour, to achieve maximum concentration, though this requires careful monitoring to prevent scorching. Finally, experimental brewers might intentionally manipulate their boil intensity to achieve specific effects, with rates from a gentle simmer (2-5% per hour) to a vigorous rolling boil (15-20% per hour) depending on the desired outcome for a specific beer style.

Frequently Asked Questions

Why is knowing my brewing evaporation rate important?

Understanding your evaporation rate is crucial for hitting target original gravities and final batch volumes. A typical homebrew system might lose 10-15% of its volume per hour during a vigorous boil, directly impacting alcohol content and yield.

How can I reduce the evaporation rate during brewing?

You can reduce the evaporation rate by lowering your boil intensity, partially covering your kettle (though this can lead to off-flavors from DMS), or using a narrower kettle. Reducing boil vigor from a 'rolling' to a 'simmering' boil can decrease evaporation by 20-30%.

What is a typical evaporation rate for homebrewing?

A common evaporation rate for homebrewers using a standard electric or propane burner setup is between 0.75 to 1.5 gallons per hour for a 5-gallon batch, or roughly 10-15% per hour. This can vary significantly based on equipment and boil intensity.

Does evaporation rate affect hop utilization?

While evaporation itself doesn't directly alter hop utilization, the boil intensity that drives evaporation can. A more vigorous boil, leading to higher evaporation, can slightly increase hop isomerization and thus IBU extraction, potentially by 5-10% in extreme cases.