How to Dilute Anhydrous Ethanol to 70% | How Much Water, and Why the Volume Shrinks
When disinfecting ethanol is hard to find, you can dilute anhydrous ethanol with water instead. But simply thinking “add 30% water for 70%” is slightly off, and alcohol has a peculiar habit of shrinking in volume when mixed. This guide gives tables of how much to use for 70%, 75% and 80%, the calculation for using up a whole bottle, why the volume drops when you mix, how to choose the water, and the precautions to take while making it.
Anhydrous ethanol vs disinfecting ethanol
Pharmacies in Japan mainly sell two kinds of ethanol: “anhydrous ethanol” and “disinfecting ethanol”. Anhydrous ethanol contains almost no water, at about 99.5 vol% or more, and is used for things like cleaning precision equipment. Disinfecting ethanol is diluted with water to around 80 vol% (76.9 to 81.4 vol% under the Japanese Pharmacopoeia).
Ethanol is considered better for disinfection when it contains a certain amount of water, and concentrations around 70 to 80% are commonly used. Anhydrous ethanol used neat evaporates almost immediately and is not well suited to disinfection, so one option is to dilute it with water to 70 to 80%.
Amounts for 70%, 75% and 80%
Taking anhydrous ethanol as 99.5%, here are the amounts of ethanol and water needed for each finished volume. Ethanol = “finished volume × target concentration ÷ 99.5”, and water makes up the rest.
| Target concentration | 100 mL | 300 mL | 500 mL |
|---|---|---|---|
| 70% | Ethanol about 70.4 mL + water about 29.6 mL | about 211.1 mL + about 88.9 mL | about 351.8 mL + about 148.2 mL |
| 75% | about 75.4 mL + about 24.6 mL | about 226.1 mL + about 73.9 mL | about 376.9 mL + about 123.1 mL |
| 80% | about 80.4 mL + about 19.6 mL | about 241.2 mL + about 58.8 mL | about 402.0 mL + about 98.0 mL |
If you measure “70 mL ethanol + 30 mL water” for 70%, the calculated result is about 69.7%, a tiny difference. For the quantities you make at home, remembering 7 parts ethanol to 3 parts water, or 8 to 2, is accurate enough.
Using up a whole 500 mL bottle of anhydrous ethanol
If you want to dilute an entire bottle of anhydrous ethanol, fix the amount of ethanol first and calculate how much water to add.
| Using up 500 mL | Finished volume | Water to add |
|---|---|---|
| To make 70% | about 710.7 mL | about 210.7 mL |
| To make 80% | about 621.9 mL | about 121.9 mL |
If your spray bottle holds a fixed amount, calculate from the finished volume; if you want to use up a whole bottle, calculate from the amount of ethanol. Choose anhydrous ethanol in the tool’s “Dilute by concentration” and it gives the amounts either way. The idea is the same as in How to Calculate Dilution Ratios.
Why ethanol and water shrink in volume when mixed
When you mix ethanol and water, the result is a few percent less than the two volumes added together. Mixing 50 mL of each, for example, gives about 96 to 97 mL rather than 100 mL. This is because water and ethanol molecules attract each other and pack together, filling in each other’s gaps.
- Because the volume shrinks, if you measure and mix exactly as calculated, the actual concentration is slightly higher than calculated
- Even if you aim for 70%, the result stays within the roughly 70 to 80% range used for disinfection, so you don’t need to worry about it at home
- If you want a finished 500 mL and top up with water to the line, the concentration drops. For an accurate amount, measure the ethanol and the water separately and then mix them
The mixture may also warm up slightly just after mixing. Let it stand until it returns to room temperature before use. Ethanol’s volume also changes with temperature, so bringing the liquids to room temperature before measuring keeps the gap between calculation and reality smaller.
| Amounts measured and mixed | Simple sum | Actual result (approx.) |
|---|---|---|
| Ethanol 50 mL + water 50 mL | 100 mL | about 96 to 97 mL |
| Ethanol about 70.4 mL + water about 29.6 mL | 100 mL | a few mL less than 100 mL |
If you want to fill a spray bottle right to capacity, allow for this loss and make slightly more than calculated. As long as you scale up the whole thing without changing the ethanol-to-water ratio, the concentration stays the same.
Which water to use, and precautions when making it
- Purified water is ideal. Tap water is fine if you will use it up quickly, but left for a long time, the minerals in tap water can let bacteria grow
- Ethanol catches fire easily. Make it away from any flame, such as a stove, cigarettes or a heater, and keep the room ventilated
- Use a container suitable for alcohol (PET, PP, glass and so on; check the container label) and close the lid tightly
- Don’t use fuel alcohol (which contains methanol) to disinfect hands or similar
- Write the contents, concentration and date made on the bottle
For dirt and viruses that alcohol works poorly against, switch to a chlorine-based disinfectant; see How to Dilute Sodium Hypochlorite for how to make it. Mixing two liquids of different strengths, such as 80% and 40%, to reach a target concentration is explained in Mixing Two Concentrations.
FAQ
- How much water do I add to anhydrous ethanol to make 70%?
- With 99.5% anhydrous ethanol, use about 70.4 mL of ethanol and about 29.6 mL of water per 100 mL. For 500 mL, about 351.8 mL of ethanol and 148.2 mL of water. “7 parts ethanol to 3 parts water” also gives almost exactly 70%.
- If I use a whole 500 mL of anhydrous ethanol, how much 70% do I get?
- 500 × 99.5 ÷ 70 = about 710.7 mL, adding about 210.7 mL of water. For 80%, you get about 621.9 mL, adding about 121.9 mL of water.
- Why does the volume drop when I mix ethanol and water?
- Water and ethanol molecules attract each other and pack together, filling in the gaps. Even mixing 50 mL of each gives only about 96 to 97 mL. The concentration ends up slightly higher as a result, but that’s not a problem for home use.
- Can I dilute it with tap water?
- Tap water is fine if you will use it up soon, but purified water is recommended if you plan to store it. Tap water left for a long time can let bacteria grow.
- What strength is disinfecting ethanol?
- Disinfecting ethanol under the Japanese Pharmacopoeia is 76.9 to 81.4 vol%. In general, concentrations around 70 to 80% are used for disinfection.
Tools in this article
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