How is total brew day water calculated?

The calculator starts from your target fermenter volume and adds back every loss along the way: grain absorption, boil-off, kettle trub, chiller loss, and mash tun dead space. The sum is the total mash plus sparge water you need to heat at the start of the day.

How do I find my boil-off rate?

Boil-off is the litres of water evaporated per hour of vigorous boil, and it depends on your kettle width, heat source, and lid. Measure it once by recording volume before and after a timed boil. Three litres per hour is a common figure, but yours may differ; the calculator scales it to your boil time.

What losses should I include?

The main ones are grain absorption, around 1.04 litres per kilogram, boil-off over the boil, kettle trub and hop debris, chiller and transfer losses, and mash tun dead space below the false bottom. Include yeast cake or break material if you want to be precise, lumped in with trub.

What is the difference between pre-boil and post-boil volume?

Pre-boil volume is the wort in the kettle at the start of the boil, after the mash and sparge. Post-boil volume is what remains after boil-off has evaporated some water. The fermenter volume is the post-boil figure minus trub and chiller losses left behind in the kettle.

Why does my wort end up short of the target?

Coming up short usually means a loss was underestimated, most often boil-off on a hot or windy day, or higher grain absorption with a large grain bill. Re-measure your boil-off rate and absorption, update the inputs, and the prediction will match your actual fermenter volume.

Brew Day Water Loss Calculator

The Brew Day Water Loss Calculator makes sure you finish with the right amount of wort in the fermenter by accounting for every place water disappears during the brew day. Underestimating these losses is the most common reason a batch comes up short or over-strength, and this tool fixes that.

How it works

Water leaves the process at several points between the hot liquor tank and the fermenter. Working backwards from the volume you want in the fermenter, you add each loss back to find the total water to start with:

Boil-off       = (boil minutes / 60) * boil-off rate
Grain absorb   = grain weight * absorption rate
Total losses   = grain absorb + boil-off + trub/chiller + mash dead space
Total water    = fermenter target + total losses

The calculator also reports the intermediate volumes: the pre-boil volume in the kettle, and the post-boil volume after evaporation, so you can check your numbers at each stage of the day.

Worked example

Targeting 23 L into the fermenter, with a 5 kg grain bill at 1.04 L/kg absorption, a 60 minute boil at 3 L/hour boil-off, 1.5 L trub and chiller loss, and 0.5 L mash dead space:

Grain absorption = 5 * 1.04 = 5.2 L
Boil-off = (60 / 60) * 3 = 3 L
Total losses = 5.2 + 3 + 1.5 + 0.5 = 10.2 L
Total water = 23 + 10.2 = about 33.2 L

So you would heat about 33.2 L of water in total across the strike and sparge to land 23 L in the fermenter.

Tips and notes

The single most variable loss is boil-off, which depends on your kettle, heat source, and the weather. Measure it once by recording the volume before and after a timed boil, and use your real figure. Grain absorption rises with a bigger grain bill, so heavy, high-gravity recipes need noticeably more total water. Include yeast cake or hot break in the trub figure if you want a precise number. Once your boil-off and absorption values match your equipment, this calculator will reliably hit your fermenter target batch after batch.