Guide · 70% ethanol
Formula
One dilution step uses C1 × V1 = C2 × V2. Solve for the stock volume:
V1 = C2 × V2 ÷ C1
C1 is stock ethanol (% v/v), C2 is the target (70% v/v), and V2 is the final volume. Measure that ethanol volume, then add water to the final volume in a graduated cylinder or volumetric flask. Do not assume you can add a fixed water volume calculated as V2 − V1 without checking the mark, because ethanol and water contract on mixing.
Steps
- Choose stock strength (often 95% or 100% v/v) and final volume.
- Calculate ethanol volume V1 = C2 × V2 ÷ C1 (use the calculator above).
- Pour or pipette that ethanol into a cylinder or flask sized for V2.
- Add water slowly and mix until the meniscus reads V2.
- Label with concentration, date, and your initials if required.
Recipe table
Ethanol volumes for 70% v/v; add water to the final volume.
| Stock | Final volume | Ethanol | Water |
|---|---|---|---|
| 95% v/v | 100 mL | 73.7 mL | to 100 mL |
| 95% v/v | 500 mL | 368.4 mL | to 500 mL |
| 95% v/v | 1000 mL | 736.8 mL | to 1000 mL |
| 100% v/v | 100 mL | 70 mL | to 100 mL |
| 100% v/v | 500 mL | 350 mL | to 500 mL |
| 100% v/v | 1000 mL | 700 mL | to 1000 mL |
Other concentrations
For other targets or units, use the C1V1 calculator with the same v/v percentages entered as C1 and C2.
FAQ
Why do I add water to volume instead of a fixed water volume?
Ethanol and water mix with volume contraction. The diluent line on this page is V2 minus V1 for planning, but in the lab you measure the ethanol volume and bring the solution to the final mark with water in a graduated cylinder or volumetric flask.
Are the percentages volume or weight?
They are volume/volume (v/v), as labeled on most laboratory ethanol bottles.
Why is 70% ethanol used for surface disinfection?
Many labs use about 70% v/v ethanol as a general-purpose disinfectant for bench surfaces and some equipment. Effectiveness depends on contact time, what you are treating, and whether the surface is clean and dry. Follow your facility’s procedure.