Rubbing Alcohol Dilution Calculator

Why 70% rubbing alcohol works better than the stronger stuff

A bottle of 91% or 99% isopropyl alcohol sounds stronger, but for disinfecting it is not the better choice. CDC describes the optimum bactericidal concentration of alcohol as 60% to 90% in water and notes that its activity drops sharply when diluted below about 50%, so 70% sits inside the best range and 91% to 99% sit at or beyond its top. To make 70% from 91%, mix about 77 parts alcohol with 23 parts water; from 99%, about 71 parts alcohol with 29 parts water.

Contact time is the part people miss. Alcohol evaporates quickly, and CDC notes that this makes extended exposure time difficult to achieve unless items are immersed. Wipe on enough to leave the surface visibly wet, and follow the contact time on any registered disinfectant's label for the germs you care about. A homemade 70% mix is not an EPA-registered product, so it should not replace one where a specific label claim matters. Use clean water and measure by volume.

For glass, far less alcohol is needed. A mix of about 25%, roughly 1 part alcohol to 3 parts water, is a common household recipe that dries quickly and helps prevent streaks, though it is below the disinfecting range, and this calculator also accepts a custom strength for other recipes. Keep alcohol away from unsealed wood, some plastics, acrylic and coated or anti-glare screens, and test a hidden spot first.

Alcohol is flammable, even at 70%. Keep sprays away from flames, stoves, pilot lights and static sparks, and do not soak a room in it. Store the bottle in a cool, ventilated spot and never combine it with bleach, because the mix creates chloroform, a toxic liquid. If fumes have already been released, leave the area for fresh air, open windows and call Poison Help at 1-800-222-1222 (US, America's Poison Centers); call 911 for trouble breathing, chest pain or collapse.

Where these numbers come from

The formulas and default values in this calculator are based on:

• CDC, Chemical Disinfectants (Disinfection and Sterilization guideline: alcohol 60-90% optimum, activity drops below 50%, flammable, evaporates rapidly)
• US EPA, What's the difference between products that disinfect, sanitize, and clean surfaces? (registered products and contact time)
• Transport Canada CANUTEC, Improper mixing of common household cleaning products (alcohol with bleach forms chloroform). Note: the 25% glass mix is a common household recipe, not a published standard

This is general household-cleaning guidance and planning math, not medical or professional advice. Product formulas change, so the label on your bottle always wins: follow its directions and warnings, test a hidden spot first, and never mix cleaning products. For a poisoning emergency call Poison Help at 1-800-222-1222 (US), or 911 if someone has trouble breathing.