How this instrument works
Household bleach is a solution of sodium hypochlorite (NaOCl) in water, typically sold at a concentration printed on the label — commonly around 6% to 8.25% in the US, though it varies by brand and has changed over the years. Most disinfecting and sanitizing uses call for a much weaker solution, often around 0.5% or lower, so the bleach needs to be diluted with water before use. This instrument applies the standard dilution relationship, C1V1 = C2V2, rearranged to solve for how much total volume — and therefore how much added water — gets you from your starting strength to your target strength.
The underlying idea is that diluting a solution doesn't change the total amount of dissolved substance, only the volume it's spread through: the concentration times the volume before dilution equals the concentration times the volume after, because both sides equal the same fixed quantity of sodium hypochlorite. Rearranging that relationship gives the total final volume needed for any target concentration, and subtracting the starting volume from that total tells you exactly how much water to add.
Bleach concentration and target disinfecting strength both matter here, and neither is universal: bottle strength varies by brand, region, and how long the bottle has sat on a shelf (sodium hypochlorite slowly decomposes over time, especially in heat or light), and target strength depends entirely on the application — a strong solution for blood or body-fluid cleanup calls for a higher percentage than routine surface sanitizing. This instrument does the arithmetic once you supply both figures; confirming the correct figures for your bottle and your task is a separate step.
- Enter Starting bleach strength (% NaOCl) — read this from your bottle's label, since it varies by brand and region.
- Enter Bleach volume to use — how much of the concentrated bleach you're starting with.
- Enter Desired final strength (%) — your target disinfecting concentration.
- Read Total final volume — the total volume your diluted solution should measure once water is added.
- Read Water to add (v2 - v1) — the total final volume minus your starting bleach volume, i.e. how much water to pour in.
- Desired final strength must be greater than zero and cannot exceed your starting strength — you can only dilute a solution, not concentrate it by adding water.
Worked example — 8.25% bleach diluted to 0.5%
A bottle of household bleach is labeled 8.25% sodium hypochlorite, and 100 mL of it is being diluted down to a 0.5% working strength. Enter 8.25 into Starting bleach strength (% NaOCl), 100 into Bleach volume to use, and 0.5 into Desired final strength (%); Total final volume reads 1650.00 and Water to add (v2 - v1) reads 1550.00.
In other words, that 100 mL of concentrated bleach needs 1,550 mL of water added to it to reach a total of 1,650 mL at 0.5% strength — the same disinfecting concentration commonly cited by public-health guidance for strong chlorine solutions made from concentrated household bleach.
Questions
Why does 0.5% keep coming up as a target bleach strength?
Because it's a commonly cited strong disinfecting concentration in public-health guidance for cleaning surfaces and handling body-fluid spills, distinct from the much weaker concentrations (often well under 0.1%) used for routine food-contact sanitizing. The right target strength depends entirely on the task, so it's worth confirming against current guidance for your specific situation rather than assuming one figure fits every use.
Why can't the desired final strength be higher than the starting strength?
Because dilution only ever adds water, which can only lower a concentration, never raise it. Reaching a higher percentage than the bottle already has would require removing water or adding more sodium hypochlorite, not diluting — this instrument will not compute a result for a target strength above your starting strength, since no amount of water addition could reach it.
Where do I find my bleach's starting strength?
It's printed on the product label, usually as a percentage of sodium hypochlorite (NaOCl) — commonly around 6% for older or lower-concentration formulations and up to about 8.25% for many current US household bleach products, though this varies by brand, region, and has shifted over time. Always use the figure from your specific bottle rather than assuming a standard value, since using the wrong starting percentage will make every downstream figure wrong.
Does bleach concentration change while the bottle sits on a shelf?
Yes — sodium hypochlorite decomposes gradually over time, and that decomposition speeds up with heat and light exposure, so an old or poorly stored bottle may be meaningfully weaker than its label states. This instrument has no way to account for that decomposition; it works from whatever starting strength you enter, so a very old bottle may need a different figure than a fresh one to reach the same effective disinfecting strength.
Is the C1V1 = C2V2 relationship used here specific to bleach?
No — it's the general dilution equation used throughout chemistry for diluting any solution: concentration times volume before dilution equals concentration times volume after, because the total dissolved amount doesn't change, only the volume it's spread through. This instrument simply applies it with bleach-specific field labels, since that's one of the most common places people encounter it outside a chemistry classroom.
What if I want to know the dilution ratio instead of a total volume?
You can derive it from this instrument's output: the total-volume ratio — final volume to starting bleach volume — equals your starting strength divided by your target strength, since V2/V1 = C1/C2. In the worked example that's 8.25/0.5 = 16.5, so the final solution measures 16.5 times the starting bleach volume. The bleach-to-water ratio is a related but slightly different figure, since water added is total volume minus the original bleach volume: 100 mL of bleach to 1,550 mL of water works out to roughly 1-to-15.5 — one less than the total-volume ratio, because it excludes the original bleach volume from the comparison.