Proofing and Blending Your Spirit
Reminder before this chapter: home distillation has no federal personal-use exemption in the United States. This chapter covers the math and process of bringing a spirit to strength and combining batches. It isn't legal advice. Check your local, state, and federal rules before you build or run anything.
Chapter 3 ended with a set of jars and a decision about which ones make the cut. What comes out of that decision is the hearts, and hearts off a still are strong. Depending on the still and the run, they commonly land somewhere between 60% and 80% ABV. Nobody bottles at that strength on purpose. This chapter covers getting from raw hearts to a finished spirit at a target strength, and combining batches along the way. It's where this site got its name.
Proof and ABV
Two numbers describe the same thing:
- ABV (alcohol by volume) is the percentage of the liquid that's ethanol.
- Proof, in the US system, is double the ABV. A 40% ABV spirit is 80 proof; 50% ABV is 100 proof.
Most commercial spirits bottle between 40% and 50% ABV. Where a spirit lands in that range is a flavor decision as much as a strength one. Higher proof carries more intensity and aroma. Lower proof reads softer and lets more subtle notes through.
Measuring Strength Accurately
Every calculation in this chapter depends on one starting number: the current ABV of the spirit. Get that wrong and everything downstream is wrong with it.
An alcohol hydrometer (sometimes sold as a proof-and-tralle hydrometer) is different from the one used on a wash in chapter 2. It's calibrated for spirits, not sugar solutions, and it's calibrated at a specific temperature, printed on the instrument. A reading taken far from that temperature drifts, because liquid density changes as it warms and cools. The simplest route to an accurate reading is to bring the sample to the calibration temperature before reading it, rather than correcting afterward.
Why Dilution Isn't Simple Arithmetic
The basic dilution math is straightforward. To bring a spirit down to a lower ABV, the final volume has to grow in proportion to the drop in strength:
final volume = current volume × current ABV ÷ target ABV
One liter at 60% brought to 40% ends up at 1.5 L, so the water to add is about 0.5 L.
Here's the part that surprises people: mixing ethanol and water isn't perfectly additive. Their molecules pack together more tightly than either does alone, so a mix of 500 mL of each comes out a little under a liter. That's volume contraction. In practice, it means the finished volume after dilution runs slightly short of the math, and the true ABV lands slightly above target.
The Dilution tab on our calculators uses the standard volumetric method above and flags this same caveat. For most home proofing, the gap is small enough that the fix is procedural: add a little less water than the calculation says, measure, then close the remaining distance.
Adding Water: Slowly, in Stages
How the water goes in matters almost as much as how much.
Stage it. Distillers commonly bring a spirit down in two or three steps rather than all at once, resting it between additions. Adding water in one shot can shock the spirit and bring out harsh notes that a gradual approach avoids.
Use clean, neutral water. Distilled, reverse-osmosis, or deionized water is the usual choice. Tap water carries minerals and chlorine, and both can show up as off-flavors or as a haze in the bottle.
Watch for cloudiness. A spirit that turns milky as water goes in is showing you oils, mostly from the tails side of the run, dropping out of solution as the alcohol level falls. Some haze is normal in heavily flavored spirits. A lot of it usually points back to the cuts in chapter 3 running a little deep into tails. Resting, chilling, and filtering can all help; a cleaner cut helps more.
Measure after resting, not right away. A freshly diluted spirit is still settling. Give it time before taking the final reading.
Blending Batches
Few finished spirits come from a single jar or a single run. Blending is the process of combining batches deliberately, either to hit a strength or to build a flavor.
The Blend tab on our calculators handles both directions:
- Forward: you have two batches at known volumes and ABVs and want to know what combining them produces.
- Hit a target: you have a low-proof and a high-proof batch and want to know how much of each to combine to reach a target ABV at a total volume. This uses the classic Pearson square method, a standard technique for solving two-component mixing problems.
The same contraction caveat from dilution applies here, so expect the result to land a touch above the calculated ABV.
Blending for Flavor
Hitting a number is the mechanical half of blending. The other half is taste.
This is where the jar method from chapter 3 pays off a second time. Each jar is a slightly different spirit: early hearts are lighter and brighter, late hearts carry more body and weight. A distiller with a row of numbered jars can blend deliberately, pulling a little more from the late jars for a fuller whiskey, or keeping to the early and middle jars for a cleaner, lighter spirit. Small test blends, measured and noted, make that a repeatable decision instead of a guess.
Resting and Marrying
A freshly combined blend isn't finished. Spirits change for days to weeks after dilution and blending, as the components settle and integrate. Distillers call this marrying. Harsh edges soften, aromas come together, and occasionally the ABV drifts a little as well.
For that reason, many distillers leave a final strength check and final taste until after the spirit has rested, rather than treating the moment of mixing as the end.
Storage
Finished high-proof spirit goes into glass. Some plastics can react with or leach into strong alcohol over time, and glass is inert. Sealed and kept out of direct sunlight and heat, a finished unaged spirit is stable indefinitely. Every container gets a label with at least the date, the batch, and the ABV.
Keep a Log
Same habit, third chapter in a row. Starting ABV and temperature, volume, water added per stage, rest times, ending ABV, which jars went into which blend. If you're signed in, the calculators can save a dilution or blend so the exact numbers behind a good batch are there next time.
What's Next
A proofed, rested spirit can be finished as-is, and plenty are. The final chapter covers the alternative: aging, flavoring, and finishing, from oak and time to botanicals and infusions, and how each one changes what's in the glass.
In the US, federal law bans distilling spirits at home, and the Supreme Court is reviewing that ban (DOJ v. McNutt). State and local rules apply too. This content is for informational and educational purposes only and is not legal advice. Calculator outputs are planning estimates, not lab measurements. Read our full Legal Disclaimer for more detail.
A federal appeals court ruling in April 2026 changed part of this picture in three states — see our full breakdown before assuming anything here is settled.