If your tap water contains chlorine or, more commonly, chloramine, those disinfectants can react with malt and hop compounds and create a medicinal, plastic, Band-Aid character that does not fade with time.
This calculator tells you exactly how much Campden (metabisulfite) to add to your total brewing water before you mash and sparge, so your water is neutralized before it ever touches grain.
To get the best results, treat the full volume you will use on brew day (mash water plus sparge water plus any top-up), crush the tablet or measure powder so it dissolves fast, stir thoroughly, then give it a couple of minutes to do its job before you start.
If you have a water report, use the Advanced section to enter your ppm for a tighter estimate; if you do not, the default setting is designed to be conservative and safe.
If your dose comes out as a tiny fraction of a tablet, use powder or the stock-solution helper so you are measuring accurately instead of trying to cut crumbs.
Campden Water Treatment Calculator
Dechlorinate your brewing water in minutes, avoid Band-Aid like chlorophenols forever.
This is for treating brewing water before mash and sparge. It is not intended for dosing finished beer.
Inputs
Include mash water, sparge water, and any top-up water.
Chloramine is harder to remove with simple filtering. Campden is the reliable fix for both.
Pick what matches your packet, then confirm the mg field below.
If your packet lists grams, multiply by 1000.
Either is fine for dechlorination. This selector is here for labeling and notes, not to complicate the dose.
Advanced
If you enter ppm, the tool scales the dose linearly from the baseline assumption. Leave blank to use typical levels.
3.0 ppm is a conservative “typical” assumption for municipal disinfectant. Adjust if your report suggests otherwise.
Adds a small buffer for uncertainty in ppm and tablet mass.
If your dose is a tiny fraction of a tablet, dissolve 1 tablet into this many liters of water, then measure mL instead of cutting tablets.
Results
Enter your water volume
Brew-day checklist
Quick scale (same settings):
What problem does this solve?
Chlorine and chloramine can react with malt and hop phenols to form chlorophenols. These show up as Band-Aid, plastic, antiseptic flavors.They do not age out.
Treating your brewing water first is the simplest prevention you can do for flavor.
Notes that save batches (and beer maker hearts)
If you use a carbon filter, it may reduce chlorine, but chloramine often slips through unless the filter is designed for it and you run the flow rate slowly.
Campden is cheap insurance.
If your dose is under one eighth of a tablet, powder or a stock solution is more accurate than trying to cut tablets into dust.
How to use this tool successfully
Start by entering the total water you will actually use on brew day, not just your mash water. That means mash water plus sparge water plus any top-up you plan to add to the kettle or fermenter. If you brew from a kit and top up with tap water, treat that volume too, it is a common way to accidentally dose only half the water and still get the dreaded off-flavor (see how to brew with beer kits).Leave the disinfectant setting on chloramine if you are unsure, because it is the more persistent disinfectant and the one most likely to cause trouble if you skip treatment.
Confirm your tablet strength from the packet, then follow the brew-day checklist: crush or measure, stir into cold water, wait a few minutes, then mash and sparge as normal.
If the result is a tiny fraction of a tablet, do not play surgeon with a crumbly tablet, switch to powder or use the stock-solution helper so the dose is measured accurately instead of guessed.
Use the outputs to make process decisions, not just to spit out a number. The fraction and quick-scale list help you sanity-check your inputs so you do not under-treat because you forgot sparge water.
The Advanced ppm field is there for when you have a water report and want tighter dosing, and the safety margin is there for real-world uncertainty in tablet size and municipal dosing swings. If you want the deeper background on what Campden is doing and where else brewers use it (water treatment, fruit, cider, and more), read using Campden tablets to clean water and sanitise brewing equipment.
The stock-solution recommendation is a practical workflow upgrade: it turns a hard-to-measure tablet fraction into an easy milliliter measurement, which improves repeatability.
Over time, that repeatability matters, because consistent water treatment removes a major source of random, "mystery" off-flavors and lets you judge recipe changes, hop timing, yeast character, and fermentation control on their own merits.
Why this matters, how problems happen, and how to stay ahead of them
The classic trap is thinking water chemistry only matters when you are chasing a perfect mash pH or building sulfate to sharpen hops. In reality, chlorine and chloramine can ruin a beer that is otherwise flawless, because they create chlorophenols that read as Band-Aid, antiseptic, plastic, and once they are there, they are there for good.
This is why experienced brewers treat water as a non-negotiable ingredient, not a background detail.
Campden is not a luxury step. It is cheap insurance against a flavor fault that makes people push the glass away after one sip.
These faults happen most often through under-treatment, not because the brewer did nothing. The usual mistake is volume blindness: only treating the mash water, then sparging with untreated tap water, or topping up the kettle with untreated water to hit pre-boil volume. Another common miss is assuming a small jug filter is enough.
Many filters reduce chlorine fairly well, but chloramine is designed to survive distribution systems, and it can slip through unless the filter is built and run for it. If you ever tasted chlorophenols once, you know the pain. The cause is rarely a single dramatic error, it is a few small exposures that add up.
The best fix is boring and repeatable.
Treat all brewing water up front, stir it in, give it a couple of minutes, then forget about it and focus on the mash and fermentation. This tool is designed to keep you honest on total volume, and the quick-scale list is there so you can double-check your number at a glance. When you have a water report, the ppm input lets you tighten dosing, but do not overthink it.
In practice, the goal is not theoretical perfection, it is reliable removal of disinfectant before the water touches grain.
Accuracy becomes tricky when the dose is tiny.
Cutting tablets into eighths works sometimes, but it turns into guesswork fast when you are below that. If the result is under one eighth tablet, a powder dose or stock solution is the grown-up move.
The stock solution method also reduces user error on brew day, because measuring milliliters with a syringe is more repeatable than trying to split a crumbly tablet with a knife. If you brew often, consider making a small, clearly labeled stock on brew day only, then discard leftovers. Fresh, clear, simple.
Finally, keep your troubleshooting instincts sharp.
If a beer tastes medicinal, do not immediately blame infection, hops, or the fermenter. First, audit your water treatment. Then audit your cleaning and sanitizing, because sloppy sanitation can layer weird phenolic notes on top of already-stressed beer. If you need a refresher on what "clean" really means in brewing, start with why you need to sanitise beer brewing equipment, then tighten your routine with how to use sodium percarbonate to clean beer equipment, and finish with a true no-rinse approach from best no rinse brewing sanitizers.
Once you lock these fundamentals down, your brewing becomes calmer. Recipe tweaks become meaningful, hop experiments make sense, and you stop chasing ghosts that were really just tap water doing what it always does.