Use this calculator to determine your kettle's boil-off rate. Knowing your specific boil-off rate is crucial for hitting target volumes and planning water additions accurately.

Measure at start of boil (boiling temperature)
Measure at end of boil (boiling temperature)
Total boil time in minutes

📚 Understanding Boil-Off Rate

Boil-off rate is the volume of liquid that evaporates per hour during a vigorous boil. This rate varies based on several factors and is specific to your brewing system.

Factors Affecting Boil-Off Rate:

  • Kettle diameter: Wider kettles = more surface area = higher evaporation
  • Boil vigor: More aggressive boiling increases evaporation
  • Altitude: Higher altitudes have lower boiling points and faster evaporation
  • Humidity: Low humidity increases evaporation rate
  • Lid usage: Partially covered kettles reduce evaporation
  • Heat source: Gas vs electric can affect boil intensity

Typical Boil-Off Rates:

  • 5-gallon batches: 0.5-1.0 gal/hr (2-4 L/hr)
  • 10-gallon batches: 1.0-2.0 gal/hr (4-8 L/hr)
  • 15-gallon batches: 1.5-3.0 gal/hr (6-11 L/hr)
  • BIAB (covered): 0.25-0.5 gal/hr (1-2 L/hr)

How to Measure Your Rate:

  1. Fill kettle with known volume of water
  2. Bring to a vigorous, rolling boil
  3. Start timer once at full boil
  4. Boil for exactly 60 minutes (or your typical boil time)
  5. Measure remaining volume at boiling temperature
  6. Use this calculator to determine your rate
  7. Repeat 2-3 times and average for best accuracy

Why It Matters:

  • Hit target volumes: Essential for batch size accuracy
  • Gravity calculations: Affects final wort concentration
  • Water planning: Determine total water needed for brew day
  • Hop utilization: Correct volumes = accurate IBU calculations
  • Efficiency: Consistent volumes improve repeatability

Pro Tips:

  • Document your rate for each system/setup you use
  • Re-check your rate if you change kettles or heat sources
  • Some software (BeerSmith, Brewfather) can track this for you
  • Consider seasonal variations (summer vs winter humidity)
  • If using BIAB, test with and without lid

❓ Frequently Asked Questions

What is boil-off rate and why does it matter?

Boil-off rate is the volume of liquid that evaporates per hour during boiling. It's crucial for hitting target volumes, calculating correct gravities, planning water additions, and achieving accurate IBU calculations. Your specific rate depends on your kettle size, boil vigor, and environmental conditions.

How do I measure my system's boil-off rate?

Fill your kettle with a known volume of water, bring to a vigorous boil, start a timer, boil for exactly 60 minutes, then measure the remaining volume at boiling temperature. The difference divided by time gives your hourly rate. Repeat 2-3 times and average for best accuracy.

What's a typical boil-off rate for homebrewing?

For 5-gallon batches, typical rates are 0.5-1.0 gallons per hour (2-4 liters per hour). Ten-gallon systems usually see 1.0-2.0 gal/hr. BIAB systems with partial lids may have lower rates around 0.25-0.5 gal/hr. Rates vary significantly based on kettle diameter and boil vigor.

What factors affect my boil-off rate?

Major factors include kettle diameter (wider = more evaporation), boil vigor (more aggressive = higher rate), altitude (higher altitude = faster evaporation), humidity (lower humidity = more evaporation), lid usage (covered = less evaporation), and heat source (gas vs electric affects boil intensity).

Do I need to remeasure if I change equipment?

Yes! Your boil-off rate is specific to your kettle and setup. If you change kettles, heat sources, or modify your system (like adding/removing a lid), you should remeasure your rate. Even seasonal changes in humidity can affect your rate slightly.

How do I use my boil-off rate for brew day planning?

Once you know your rate, multiply it by your boil time to calculate total evaporation loss. Add this to your target post-boil volume to determine how much wort you need at the start of the boil. This helps you plan total water needs and hit your target volumes consistently.