Calculate boil-off volume, hourly evaporation rate, percentage loss, and the required pre-boil volume to hit your target post-boil batch size. Designed for homebrewers and craft brewers who want repeatable brew day results.
Enter your measured volumes and boil time. You can switch between gallons and liters at any time.
A boil off calculator helps brewers answer one of the most practical questions on brew day: how much liquid will disappear during the boil, and how much wort should you start with to finish at your target volume. Whether you brew all-grain in a garage, run a small pilot system in a professional brewhouse, or are simply tuning your setup for better consistency, understanding boil-off is a direct path to better beer and cleaner brew day execution.
In simple terms, boil-off is evaporation. When wort boils, water vapor escapes from the kettle and volume drops over time. That loss is not random. It follows a repeatable pattern in each brewing system when process variables stay stable. Once you can measure and predict that loss, you can set your pre-boil volume with confidence, hit your gravity more consistently, and reduce surprise corrections late in the session.
Boil-off rate is the amount of wort volume lost per hour during boiling. It is usually shown as:
Both matter. Absolute rate helps you predict exactly how many gallons or liters you will lose at a given boil length. Relative rate helps compare performance between different batch sizes and systems.
When your boil-off estimate is wrong, a chain reaction can happen:
A reliable boil-off calculation stabilizes all of that. It also improves recipe repeatability because your process inputs become predictable instead of reactive.
Total Boil-Off = Start Volume − End Volume
Boil-Off Rate (vol/hr) = Total Boil-Off ÷ (Measured Boil Time in hours)
Boil-Off Rate (%/hr) = (Total Boil-Off ÷ Start Volume) ÷ (Measured Boil Time in hours) × 100
Projected Boil-Off = Boil-Off Rate (vol/hr) × Planned Boil Time (hours)
Estimated Hot Post-Boil Needed = Target Post-Boil Volume ÷ (1 − Shrinkage Fraction)
Required Pre-Boil = Estimated Hot Post-Boil Needed + Kettle/Trub Loss + Projected Boil-Off
Great calculations start with accurate measurements. If your baseline data is noisy, your results will be noisy too. Use a repeatable process every time:
Many brewers see better consistency when they standardize burner setting, keep lid position unchanged, and avoid large swings in boil intensity. If you brew in different seasons, record temperature and humidity too. Environmental factors can shift evaporation more than expected.
Wide kettles typically evaporate more per hour than tall narrow kettles because surface area is larger. Surface area is one of the strongest drivers of evaporation.
A stronger boil increases vapor production and drives higher evaporation. If you adjust burner output from batch to batch, your boil-off rate will move accordingly.
Partially covering the kettle can reduce evaporation, but most brewers avoid full lid coverage during the boil due to DMS-related concerns. Whatever your choice, keep it consistent.
Dry air and ventilation can increase evaporation. Humid environments may reduce it. Outdoor brewing in winter and summer can produce different loss rates.
At higher elevations, boiling behavior changes and may alter evaporation dynamics. This is another reason to rely on measured system-specific data instead of generic assumptions.
These are broad ranges. Your true number can be outside them and still be completely normal for your setup.
| System Type | Typical Volumetric Boil-Off | Typical Percent/Hour | Notes |
|---|---|---|---|
| Small homebrew electric kettle | 0.4–0.9 gal/hr (1.5–3.4 L/hr) | 6–12%/hr | Depends heavily on power setting and kettle width. |
| Homebrew propane kettle | 0.8–1.5 gal/hr (3.0–5.7 L/hr) | 8–16%/hr | Wind, ambient temperature, and burner control are major variables. |
| Pilot/professional brewhouse | System specific | 4–12%/hr | Often monitored with strict process controls and SOPs. |
If you only focus on final volume, you often discover problems late. Pre-boil volume is your early warning and correction point. When you know your boil-off rate, you can set pre-boil volume to absorb expected losses and land exactly where your recipe expects.
A good boil off calculator turns pre-boil planning into a simple equation instead of guesswork. This means less topping up, fewer last-minute gravity adjustments, and cleaner process control from mash runoff to fermenter fill.
Imagine you measured a previous batch:
Results:
Now plan today’s batch:
Projected boil-off for 75 minutes = 1.1 × 1.25 = 1.375 gal
Hot post-boil needed = 5.5 ÷ (1 − 0.04) = 5.73 gal (approx)
Required pre-boil = 5.73 + 0.25 + 1.375 = 7.355 gal
So you should target roughly 7.36 gallons pre-boil for this process to hit your finished volume target.
Boil-off is not just about volume. Because sugars remain while water evaporates, concentration rises during the boil. That affects original gravity and therefore final ABV potential. In hop-forward beer, volume concentration also changes perceived bitterness and flavor balance.
If you consistently over-evaporate, wort can become denser than intended, creating stronger bitterness impact and a potentially heavier profile. If you under-evaporate, gravity may finish low, body may thin out, and hop expression can feel softer than recipe targets. Reliable boil-off control helps lock these outcomes into your design rather than chance.
Small process discipline here pays off across your whole brewing workflow. You will also find that mash and sparge planning becomes easier once your post-boil expectations are stable.
Check if boil intensity was stronger than normal, or if boil time drifted beyond plan. Also verify whether transfer losses increased due to trub bed or hop load.
Likely under-evaporation, shorter boil duration, or lower heat input. Confirm burner output and whether weather conditions changed significantly.
Most often caused by inconsistent heat settings, unclear volume measurements, or changing lid behavior. Standardize your process and log every session.
Use both. Gallons/liters per hour are easier for direct planning. Percent per hour is useful for comparison across different batch sizes.
Either can work, but be consistent and account for shrinkage when converting between hot and cold values.
Any time you change kettle, heat source, boil vigor, typical batch size, or brew in a very different environment. Otherwise, check quarterly with spot measurements.
Yes. Accurate boil-off assumptions are essential when scaling recipes because concentration behavior changes with volume and kettle geometry.
Usually yes when boil vigor is stable. If intensity changes throughout the boil, the relationship may not be perfectly linear, so track your real-world numbers.
A boil off calculator is one of the highest-impact brewing tools for process control. Once your evaporation rate is measured and repeatable, you can plan pre-boil volume precisely, hit target gravity more reliably, and improve consistency from batch to batch. The best results come from pairing this calculator with disciplined measurement and recordkeeping. If your process is consistent, your numbers become trustworthy—and your beer quality follows.