Alcohol dilution calculator

Alcohol and water don’t mix in a simple, additive way. Pour 500mL of pure alcohol into 500mL of water and you don’t get exactly 1000mL — the two liquids bond at a molecular level and the combined volume actually contracts slightly, a phenomenon distillers and chemists call “volume contraction.” For most practical dilution work this contraction is small enough to ignore, but it’s worth knowing it exists if your numbers come out a touch off from a lab-grade hydrometer reading.

The Dilution Formula Itself

The core equation is straightforward: C1V1 = C2V2. That’s starting concentration times starting volume equals target concentration times target volume. Rearranged to solve for the volume you end up with: V2 = (C1 × V1) ÷ C2. Subtract your starting volume from that result and you’ve got the amount of water to add.

Run through an example. You’ve got 1000mL at 70% ABV and you want to bring it down to 40%. V2 = (70 × 1000) ÷ 40 = 1750mL. Subtract the original 1000mL and you need to add 750mL of water. Your final 1750mL batch sits at 40% ABV.

Why the Order of Operations Trips People Up

A common mistake is calculating water as a percentage of the starting volume rather than the final volume. Adding “75% more water” sounds like it should match the example above, but 75% of 1000mL is only 750mL — which happens to be correct here by coincidence of the numbers, though it won’t hold for every ratio. Always work from the formula, not a rough percentage guess, because the relationship isn’t linear once you change either ABV figure.

Temperature’s Effect on the Result

Alcohol and water both expand and contract with temperature, but at different rates. A hydrometer or alcoholmeter reading taken at 30°C will read differently than the same liquid measured at the standard reference temperature, typically 20°C. If you’re working from a hydrometer reading rather than a known starting ABV, you’ll want a proper temperature correction reference before trusting the figure that goes into this calculator.

For most home and small-batch contexts, measuring at or near 20°C and skipping the correction step entirely is the simpler and more reliable path — fewer variables means fewer places for error to creep in.

A Worked Real-World Example

Say you’ve got 2 litres (2000mL) of a spirit base sitting at 65% ABV, and the finished product you’re aiming for needs to land at 37% ABV — a common bottling strength. V2 = (65 × 2000) ÷ 37 = 3,513.5mL. Water to add: 3,513.5 − 2000 = 1,513.5mL, or just over 1.5 litres.

That’s nearly double the original volume in water alone, which surprises a lot of first-timers. The higher your starting ABV and the lower your target, the more dramatically the final volume grows — it’s not a one-to-one swap.

Working Backwards From a Target Bottle Count

If you need a specific final volume rather than just a target ABV — say, exactly six 750mL bottles — calculate the required starting volume in reverse: V1 = (C2 × V2) ÷ C1. Plug in your desired final volume and ABV, and solve for how much full-strength liquid you need before dilution.

FAQs

How much water do I add to dilute alcohol to a specific ABV?

Use V2 = (C1 × V1) ÷ C2 to find your final volume, then subtract the starting volume. The difference is the water to add. The calculator above does this instantly from your starting volume, starting ABV and target ABV.

Does mixing water and alcohol change the total volume?

Slightly, yes. Water and ethanol contract a small amount when mixed, known as volume contraction, but it’s minor enough that standard dilution calculations ignore it for practical purposes.

What’s the formula for alcohol dilution?

C1V1 = C2V2, where C1 and V1 are the starting concentration and volume, and C2 and V2 are the target concentration and final volume.

Why does temperature matter for ABV measurements?

Alcohol and water expand and contract at different rates with temperature changes, so a hydrometer reading taken away from the standard reference temperature will be inaccurate unless corrected.

Can I dilute alcohol with anything other than water?

The standard formula assumes you’re diluting with pure water, since other liquids carry their own density and composition that the simple C1V1=C2V2 equation doesn’t account for.

How do I work out how much full-strength spirit I need for a target final volume?

Rearrange the formula to V1 = (C2 × V2) ÷ C1, plugging in your desired final volume and target ABV to solve for the starting volume needed.

Get the order of operations right and the maths is genuinely simple, even if the result occasionally surprises you with how much water a high-proof starting point actually needs. Double-check your starting ABV figure before you commit to adding water; that single input is where almost every real-world dilution error actually comes from.