Brew In A Bag Calculator (BIAB)

Calculate strike water, mash thickness, and other key volumes for your Brew In A Bag (BIAB) brewing session. Get accurate measurements for a successful brew day.

BIAB Water Volume Calculator

Water Volumes at Different Stages

Stage	Volume (Liters)
Total Water Needed (Strike Water)	0.00
Water Absorbed by Grain	0.00
Pre-Boil Volume (in kettle, above dead space)	0.00
Total Boil Off	0.00
Post-Boil Volume (in kettle, above dead space)	0.00
Trub & Hop Loss	0.00
Kettle Dead Space Loss	0.00
Volume into Fermenter	0.00

Summary of water volumes throughout the BIAB process.

What is a Brew In A Bag Calculator?

A brew in a bag calculator is a specialized tool designed for homebrewers who use the Brew In A Bag (BIAB) method for all-grain brewing. This method simplifies the traditional mashing process by using a single kettle for both mashing and boiling, with the grains held in a large, fine-mesh bag. The brew in a bag calculator helps brewers accurately determine the total amount of water needed (often called strike water for full volume BIAB), predict pre-boil and post-boil volumes, and understand the resulting mash thickness based on their specific equipment and recipe parameters.

Anyone practicing the BIAB method, from beginners to experienced brewers, should use a brew in a bag calculator. It eliminates guesswork, ensuring you start with the right amount of water to hit your target batch size after accounting for losses to grain absorption, boil-off, trub, hop absorption, and kettle dead space. A common misconception is that you just add water until it looks right, but using a brew in a bag calculator leads to more consistent and predictable results, especially when trying to replicate recipes.

Brew In A Bag Calculator Formula and Mathematical Explanation

The core of a brew in a bag calculator for full-volume mashing is to sum all the water volumes that will be lost or remain outside the fermenter, and add that to the target batch size.

The primary calculation is:

Total Water Needed (Strike Water) = Target Batch Size + Total Boil Off + Trub Loss + Hop Absorption Loss + Kettle Dead Space + Grain Absorption Loss

Where:

• Total Boil Off = (Boil-off Rate per hour / 60) * Boil Time in minutes
• Hop Absorption Loss = (Total Hop Weight / 100) * Hop Absorption Rate per 100g
• Grain Absorption Loss = Grain Weight * Grain Absorption Rate

From the total water, we can estimate other volumes:

• Resulting Mash Thickness = Total Water Needed / Grain Weight (for full volume BIAB)
• Pre-Boil Volume = Total Water Needed – Grain Absorption Loss – Kettle Dead Space
• Post-Boil Volume = Pre-Boil Volume – Total Boil Off
• Volume Into Fermenter = Post-Boil Volume – Trub Loss – Hop Absorption Loss

Variables Table:

Variable	Meaning	Unit	Typical Range
Target Batch Size	Desired volume of beer into the fermenter	Liters	5 – 50
Grain Weight	Total weight of grains used	kg	1 – 15
Grain Absorption	Water absorbed by grain	Liters/kg	0.6 – 1.2
Kettle Dead Space	Unrecoverable volume below kettle outlet	Liters	0 – 5
Boil-off Rate	Evaporation rate during boil	Liters/hour	1 – 8
Boil Time	Duration of the boil	minutes	30 – 90
Trub Loss	Volume lost to sediment and hop debris	Liters	0.2 – 2
Hop Weight	Total weight of hops	grams	0 – 500
Hop Absorption	Water absorbed by hops per 100g	Liters/100g	0.05 – 0.15

Practical Examples (Real-World Use Cases)

Example 1: Standard 19L Pale Ale

A brewer wants to make 19 Liters of Pale Ale using 5 kg of grain. They have a kettle dead space of 1 Liter, a boil-off rate of 3 L/hour for a 60-minute boil. They expect 0.5 L trub loss and will use 100g of hops with 0.1 L/100g absorption. Grain absorption is 0.8 L/kg.

• Batch Size: 19 L
• Grain Weight: 5 kg
• Grain Absorption: 0.8 L/kg (Total = 4 L)
• Kettle Dead Space: 1 L
• Boil Off Rate: 3 L/hr, Boil Time: 60 min (Total Boil Off = 3 L)
• Trub Loss: 0.5 L
• Hop Weight: 100g, Hop Absorption: 0.1 L/100g (Total = 0.1 L)

Total Water Needed = 19 + 3 + 0.5 + 0.1 + 1 + 4 = 27.6 Liters

Mash Thickness = 27.6 / 5 = 5.52 L/kg

Pre-Boil = 27.6 – 4 – 1 = 22.6 L

Post-Boil = 22.6 – 3 = 19.6 L

Into Fermenter = 19.6 – 0.5 – 0.1 = 19.0 L

The brew in a bag calculator shows 27.6 Liters of strike water are needed.

Example 2: Small Batch 10L Stout

A brewer is making a 10L batch of stout with 3 kg of grain. Dead space is 0.5L, boil-off is 2 L/hr for 60 mins, trub is 0.3L, hops 50g at 0.1 L/100g, grain absorption 0.9 L/kg.

• Batch Size: 10 L
• Grain Weight: 3 kg
• Grain Absorption: 0.9 L/kg (Total = 2.7 L)
• Kettle Dead Space: 0.5 L
• Boil Off Rate: 2 L/hr, Boil Time: 60 min (Total Boil Off = 2 L)
• Trub Loss: 0.3 L
• Hop Weight: 50g, Hop Absorption: 0.1 L/100g (Total = 0.05 L)

Total Water Needed = 10 + 2 + 0.3 + 0.05 + 0.5 + 2.7 = 15.55 Liters

Mash Thickness = 15.55 / 3 = 5.18 L/kg

Pre-Boil = 15.55 – 2.7 – 0.5 = 12.35 L

Post-Boil = 12.35 – 2 = 10.35 L

Into Fermenter = 10.35 – 0.3 – 0.05 = 10.0 L

The brew in a bag calculator indicates 15.55 Liters of strike water.

How to Use This Brew In A Bag Calculator

1. Enter Target Batch Size: Input the volume of beer you want to end up with in your fermenter (in Liters).
2. Input Grain Weight: Enter the total weight of all malted grains and adjuncts (in kg).
3. Set Grain Absorption: Adjust the grain absorption rate based on your grain bill and crush (Liters per kg).
4. Enter Kettle Dead Space: Measure and input the unrecoverable volume at the bottom of your kettle (Liters).
5. Input Boil-off Rate and Time: Enter your kettle’s evaporation rate (Liters per hour) and the planned boil duration (minutes).
6. Estimate Losses: Input expected trub loss, total hop weight, and hop absorption rate.
7. Calculate: The calculator automatically updates, but you can click “Calculate” to refresh.
8. Review Results: The “Strike Water Volume” is the total water you need to start with for a full volume mash. Note the “Resulting Mash Thickness,” “Estimated Pre-Boil Volume,” and “Estimated Post-Boil Kettle Volume” to guide your brew day.

The results from the brew in a bag calculator help you prepare the correct amount of water and anticipate volumes at each stage. If the resulting mash thickness is too far from your desired range, you might consider adjusting your total water or grain bill, or planning for a small sparge (which this full-volume calculator doesn’t explicitly include but you can adapt).

Key Factors That Affect Brew In A Bag Calculator Results

• Grain Bill Weight and Type: Heavier grain bills absorb more water. Different grains might have slightly different absorption rates. More grain requires more water for the same mash thickness using a mash volume calculator component.
• Grain Crush: A finer crush might lead to slightly higher absorption and also affect mash efficiency, though the brew in a bag calculator focuses on volume.
• Boil Intensity: A more vigorous boil increases the boil-off rate, requiring more starting water. Accurately knowing your boil-off rate is crucial.
• Kettle Geometry: A wider kettle may have a higher boil-off rate than a narrower one. Kettle dead space is also equipment-specific.
• Hop Load and Type: Large hop additions, especially whole cone hops, absorb more water than small additions of pellets.
• Squeezing the Bag: How thoroughly you squeeze the grain bag after mashing significantly affects the amount of water recovered and thus the pre-boil volume. This calculator assumes reasonably efficient squeezing, reflected in the grain absorption rate. If you don’t squeeze, effective absorption is higher. Our strike water calculator considers these factors implicitly.
• Ambient Humidity: Higher humidity can slightly reduce boil-off rate.
• Trub Management: How much trub and hop material you leave behind affects the volume transferred to the fermenter.

Understanding these factors helps you fine-tune the inputs for the brew in a bag calculator for your specific system and process.

Frequently Asked Questions (FAQ)

Q: What is “full volume” BIAB?
A: Full volume BIAB means you add all the water needed for the entire process (mashing and boiling, accounting for all losses) at the start of the mash. This brew in a bag calculator is designed primarily for this method.

Q: What if I want to sparge (rinse the grains)?
A: If you plan to sparge, you’d start with less water in the mash (e.g., calculated using a target mash thickness) and add the remaining water as a sparge. This calculator gives total water; you’d need to split it between mash and sparge.

Q: How do I measure my boil-off rate?
A: Measure the volume of water in your kettle, boil it vigorously for 60 minutes, let it cool, and measure the volume again. The difference is your hourly boil-off rate.

Q: Is kettle dead space always the same?
A: Yes, for a given kettle and outlet, the dead space (volume below the tap/port) is constant.

Q: How accurate is the grain absorption value?
A: It’s an estimate. 0.6-1.0 L/kg is typical, but it can vary. Squeezing the bag well helps recover more water, lowering effective absorption towards the lower end of the range.

Q: Does this calculator account for water added for temperature adjustments?
A: No, it calculates volumes based on the inputs. If you add extra water to adjust mash temperature, you’d need to factor that in separately or adjust your initial strike water.

Q: Why is my pre-boil volume different from what the calculator predicted?
A: Usually due to differences in actual vs. estimated grain absorption (how much you squeezed) or boil-off rate starting during the ramp to boil if not accounted for. The BIAB brewing method has some variability here.

Q: Can I use this calculator for extract brewing?
A: No, this brew in a bag calculator is specifically for all-grain BIAB, dealing with grain absorption and mashing volumes. Extract brewing has different water calculations. Check our all-grain brewing guide for more.

Related Tools and Internal Resources