How to Make 70% Isopropyl Alcohol (From 91% or 99%)

If you’re diluting isopropyl alcohol, the goal is consistency: the same strength, the same bottle, the same measured result—without guessing.

12 min read Alcohol dilution

Part of the main guide

This article belongs to the Surface Cleaning Guide, where readers can compare alcohol ratios, peroxide mixing, branded cleaner dilution, and other practical surface-cleaning workflows.

Quick answer

To make 70% isopropyl alcohol from a stronger bottle (like 91% or 99%), use this rule: final volume × (target % ÷ starting %) = amount of alcohol. The rest is water. If you want it instant (and accurate for any bottle size), use the Cleaning Dilution Calculator.

Safety baseline: follow product label instructions. Don’t combine alcohol with other cleaners (especially bleach). Keep away from flames and heat.

Exact amounts for common bottle sizes

These are practical, “grab-a-measuring-cup” amounts. Small rounding is fine—what matters is staying close and being consistent.

Make 70% from 91% isopropyl alcohol

  • 500 mL final: 385 mL of 91% + 115 mL water
  • 1 liter final: 769 mL of 91% + 231 mL water
  • 32 oz final: 24.6 oz of 91% + 7.4 oz water

Easy ratio shortcut (91% → 70%): it simplifies to about 10 parts alcohol : 3 parts water. If you’re doing a quick batch, that ratio gets you very close without doing fresh math each time.

Make 70% from 99% isopropyl alcohol

  • 500 mL final: 354 mL of 99% + 146 mL water
  • 1 liter final: 707 mL of 99% + 293 mL water
  • 32 oz final: 22.6 oz of 99% + 9.4 oz water

If your bottle isn’t one of these sizes, don’t “eyeball.” Use the Cleaning Dilution Calculator and match your exact final volume.

The cleanest method (no math): use the calculator

This is the most repeatable workflow when you’re mixing for different containers (sprayers, refill bottles, wipe-down bottles).

  • Open: Cleaning Dilution Calculator
  • Set Cleaner concentration (%) to your bottle (91 or 99).
  • Set Desired final strength (%) to 70.
  • Enter your final solution volume (500 mL, 1 L, 32 oz, etc.).
  • Measure the alcohol amount it gives you, then top up with water.

Measuring tip: dedicate one measuring cup/spoon to cleaning mixes. Don’t reuse kitchen tools that will touch food later.

When not to DIY a mix

If a product label already gives instructions for your use-case, follow it. If you need a specific disinfecting product for a regulated setting (workplace protocols, medical/clinical environments), use an approved product and its label directions rather than “home-mixing.”

Also avoid DIY dilution if you can’t store the solution safely: alcohol is flammable, and casual storage near heat sources is a real risk.

Common mistakes (the ones that actually cause problems)

  • Guessing the amounts: “a splash of water” is how people drift into inconsistent strength.
  • Reusing a bottle that had other cleaners: rinse thoroughly first and don’t mix products in the same container.
  • Using alcohol on sensitive finishes: test a small hidden area first; alcohol can dull some finishes and damage some plastics.
  • Ignoring ventilation and flames: open windows where possible; keep away from sparks, cigarettes, and heat.

If you want a solid “measurement mindset” for all concentrates (not just alcohol), read: How to Read Cleaning Dilution Instructions on Labels .

FAQs

Should I use 91% or 99% straight instead of making 70%?

For many cleaning tasks, higher isn’t automatically better. Follow the product label for intended use. If you’re targeting a specific percent because a protocol calls for it, hit that number consistently rather than guessing “stronger is safer.”

What water should I use?

Regular tap water is usually fine for household mixes. If your water is very hard or leaves visible residue, distilled water can be a cleaner option for spray bottles.

Can I mix alcohol with bleach or other cleaners?

Don’t mix products unless the label explicitly allows it. Keep alcohol separate from bleach and other cleaners to avoid unpredictable fumes, reduced performance, or surface damage.

How do I scale this to any bottle size?

Use the Cleaning Dilution Calculator and enter your starting %, target %, and final volume. It will give you exact amounts without manual math.