The math
Dilution is one equation, and it's the same one whether you're mixing peroxide, bleach or fertiliser. The amount of actual peroxide in the bottle doesn't change when you add water - only the volume it's spread through does.
C₁ × V₁ = C₂ × V₂
peroxide = finished volume × target ÷ stock
water = finished volume − peroxide
C₁ is what's in your bottle, V₁ is how much of it you pour, C₂ is the strength you want and V₂ is what you end up with. Everything on this page is that rearranged one way or the other. Because both sides are volumes, the units cancel - the answer is the same arithmetic in millilitres, ounces or gallons, which is why the unit selector just relabels the numbers.
Frequently asked
How do I make 3% hydrogen peroxide from 35%?
One part 35% to 10.67 parts water. The arithmetic is 35 divided by 3, which is 11.67 total parts, so the peroxide is one of them and water is the other 10.67. For a 32 fl oz spray bottle that's about 2.7 fl oz of 35% topped up with 29.3 fl oz of water. Rounding to 1 part peroxide to 11 parts water gets you 2.9%, which is close enough for most household use.
Does "volume" on a hair developer bottle mean the same thing as percent?
No, but they convert cleanly. Volume strength is how many litres of oxygen gas a litre of the solution can give off, and it works out to roughly 3.3 times the percentage. So 10 volume is about 3%, 20 volume is about 6%, 40 volume is about 12%. That's why 6% and 12% are such common strengths to find - they're the developer grades.
Is the percentage by weight or by volume?
Commercial hydrogen peroxide is labelled by weight, so 35% means 35 grams of peroxide in 100 grams of solution. This calculator mixes by volume, which is what everyone actually does with a measuring jug, and it introduces a small error because concentrated peroxide is denser than water - about 1.13 g/mL at 35%. Diluting 35% by volume to a nominal 3% really lands near 3.4% by weight. For cleaning, laundry and garden use that's inside the noise. If you need the weight percent to be right, weigh the two parts on a scale instead of measuring them.
Why did my diluted peroxide stop working?
It decomposed into water and oxygen, which is what peroxide does. Light, warmth, metal ions and stray organic matter all speed it up, and dilute solutions go off faster than concentrated ones. Mix with distilled water rather than tap, keep it in the original opaque bottle or another dark one, store it somewhere cool, and mix in batches you'll get through in a few weeks rather than making a gallon that sits.