The dilution formula, and what stays constant
The amount of active chlorine does not change when you add water, so the concentration times volume before equals concentration times volume after. Rearranged, it tells you how much stock bleach to pour.
To get parts per million from a percent, multiply by 10,000: a 0.05% working solution is 500 ppm. Add the bleach to the water, never water to bleach, and never mix bleach with ammonia or acids.
Common targets from 5.25% household bleach
These are the CDC and WHO reference strengths. The dilution ratio is parts water per part bleach; the last column scales it to 1 gallon (about 3.8 L) of finished solution.
| Use | Target | Dilution | Per gallon |
|---|---|---|---|
| General surface cleaning | 500 ppm (0.05%) | 1:100 | ~2.5 tbsp bleach |
| Surface disinfection | 1,000 ppm (0.1%) | 1:50 | ~5 tbsp bleach |
| Blood spills, C. diff spores | 5,000 ppm (0.5%) | 1:10 | ~1.5 cups bleach |
| Emergency drinking water | ~2 ppm free chlorine | very high | ~8 drops per gallon |
A "1:100" dilution means 1 part bleach to 99 parts water, giving 100 parts total. Emergency water treatment uses far less — measured in drops — and is a last resort only.
The gotchas that change your numbers
- Stock strength varies. Household bleach runs 5.25% to 6%; concentrated bottles hit 8.25%. Enter the actual figure or your solution will be off by that ratio.
- Diluted bleach decays. A working solution loses 5–10% of its strength per day. Mix fresh, and store any leftover no more than a week in a cool, dark, closed container.
- Not for skin. The 0.1% (1,000 ppm) strength is for surfaces, not hands. Keep skin contact well below 0.05%.
Common questions
How much bleach to water for general disinfecting?
For everyday surface cleaning the CDC uses about 500 ppm, which is roughly 1 part bleach to 100 parts water when starting from 5.25% household bleach. That is about 1 tablespoon per quart, or a third of a cup per gallon.
What is the difference between ppm and percent?
They measure the same thing at different scales: 1% equals 10,000 ppm. Percent is convenient for strong stock bleach, ppm for the dilute working solution. A 0.05% solution is 500 ppm.
Does my bottle strength change the recipe?
Yes. An 8.25% bottle is stronger than a 5.25% one, so you need less of it for the same target. The dilution formula C1 x V1 = C2 x V2 handles any starting strength; just enter your bottle percentage.
