How Much Laundry Detergent to Use: Load Size First, Water Hardness Second

The four levers, ranked by how far they actually move the dose

The model is a single multiplication: a 37 ml reference dose times four coefficients. Load size contributes 0.6, 1.0, 1.4 or 1.8. Soil level contributes 0.8, 1.0 or 1.3. Water hardness contributes 0.9, 1.0 or 1.15. Machine type contributes 1.0 for a high-efficiency drum or 1.5 for a standard top-loader. Ranked by the ratio between each lever's top and bottom, that is load size at 3.00, soil at 1.63, machine at 1.50 and hardness at 1.28. In other words, moving from a small load to a full one changes the dose more than every other setting on the page put together.

That ranking is useful because it is the opposite of how most people dose. The cap goes in full regardless of the load, and then the wash is judged by how it smells. If you want one habit to change, make it this: dose the load you actually put in, not the machine you own. A drum a quarter full is a small load even in a nine-kilogram machine, and the calculator says it takes 22 ml where a full drum takes 67. Weigh a typical load once, on a bathroom scale, holding the basket — you only need to do it once — and you will know for the rest of the machine's life which of the four buttons you are.

The hardness input with no units, and the scales it is missing

The calculator's three hardness buttons are unlabelled. They are almost certainly meant to be the three bands that a detergent pack sold in the European Union has to carry: the detergents regulation requires laundry products sold to the general public to state recommended quantities or dosage instructions, in millilitres or grams, appropriate to a standard washing-machine load, for soft, medium and hard water hardness classes. In Germany those classes have exact statutory thresholds — under 1.5 millimoles of calcium carbonate per litre is soft, 1.5 to 2.5 is medium, above 2.5 is hard, and the water supplier has to tell customers which band they are in. Nothing of that appears in the tool.

The reason it matters is that the scales genuinely disagree, and not by a rounding error. Germany reports hardness in degrees of German hardness; France and much of southern Europe in French degrees; the United States and the United Kingdom in parts per million of calcium carbonate or in grains per US gallon. One German degree is 17.85 mg/L; one French degree is 10 mg/L; one grain per gallon is 17.12 mg/L. Now apply two classifications to the same water. At 200 mg/L — 11.2 German degrees, 20 French degrees, 11.7 grains — the United States Geological Survey calls it very hard, because its top band starts at 180 mg/L. The European detergent pack calls it medium, because its hard band does not start until 250. Same water, two labels a whole band apart.

Why hard water eats part of the dose before it cleans anything

Hardness is dissolved calcium and magnesium. Both react with the anionic surfactants in a detergent and with the builders whose job is to sequester them, so in hard water a share of every dose is spent taking the calcium out of circulation before any of it reaches the dirt. That is the whole reason a dosing table has three columns instead of one, and it is why the figure printed largest on a bottle is usually the hard-water figure: it is the safe one to print, because a dose that is right for hard water merely wastes product in soft water, whereas a dose that is right for soft water underperforms in hard.

So the practical asymmetry is real even though this tool understates it. If your water is genuinely soft, the number on the bottle is not your number — it is somebody else's, and you can take it down. The calculator's own soft setting only cuts the dose by a tenth, which is conservative next to what a three-column table on a bottle typically shows. Over a year at five washes a week, a large load in hard water comes to about 15.5 litres of detergent on the calculator's figures and about 12.1 litres in soft water. That gap is real money and a real quantity of surfactant going down the drain, and it is entirely decided by a number your water company publishes and most people have never looked up.

More is not cleaner: suds, plateaus and the towel that stopped absorbing

Detergency does not scale with dose. Past the point where there is enough surfactant to lift and hold the soil in the wash water, adding more buys very little, and the curve flattens well below the amount a generous hand pours. What extra dose does buy is foam, and foam is not free in a front-loading drum: the washing action there is mechanical, clothes lifted and dropped against each other, and a thick cushion of suds damps exactly that. This is why high-efficiency detergents are formulated to be low-sudsing in the first place, and why a top-loader — which washes by agitating in a much larger volume of water — carries the 1.5 multiplier in this tool rather than a smaller one.

The visible symptom of chronic overdosing is towels that stop absorbing, and the usual explanation is a residue of detergent and softener left on the fibres that a rinse never fully removes. It is worth saying that this is the common account rather than something this article measured — but it is easy to test at home and costs nothing. Run the towels through a hot wash with no detergent at all and no softener, and dry them as usual. If they come out noticeably thirstier, you were dosing above what your rinses could clear. If they do not, the problem is fabric softener, hard-water scale on the fibres, or the tumble dryer, in roughly that order of likelihood.

What the calculator assumes, and where it stops

Three assumptions are baked in and none of them is stated. It answers in millilitres, so it is a liquid-detergent tool: powder is dosed by weight and behaves differently in hard water because much of the water-softening capacity in a powder sits in the builder system, and pods cannot be part-dosed at all, which makes them a poor fit for a small load and a worse one for soft water. It converts to cap-fulls at exactly 35 ml, a figure that is nowhere near universal — caps on concentrated liquids commonly run from 25 to 75 ml, and the fill lines inside them are not evenly spaced. And there is no validation: the load, soil, hardness and machine fields all fall back to a coefficient of 1 if they receive anything the tool does not recognise, so an unexpected value silently returns the reference dose rather than an error.

A dosing routine that survives contact with a laundry basket

Do three things once and one thing every wash. Once: look up your water hardness on your supplier's website and write the figure and its unit on a sticker on the machine — the tool will not do the conversion for you, but the table below will. Once: check the actual capacity of your detergent cap by filling it to each line with water and pouring it into a measuring jug, because the tool's 35 ml is a guess about your bottle. Once: weigh a typical load so you know which of the four load buttons you are. Then, every wash, dose the load rather than the drum, and only bump the dose for a genuinely dirty wash rather than as a default.

Frequently asked questions

How much liquid detergent should I use for a normal load?

The calculator's reference case — a medium load, normally soiled, medium-hardness water, high-efficiency machine — is 37 ml. Scale from there: 22 ml for a quarter-full drum, 52 ml for three-quarters, 67 ml for a full one. Add half again if you have a standard top-loader rather than a front-loading or high-efficiency drum. Then check that figure against the three-column table on your own bottle, which is set for that specific product's concentration, and follow the bottle where the two disagree.

How do I find out how hard my water is?

Your water supplier publishes it, usually by postcode or by supply zone, and in several countries is required to. The catch is the unit, which depends on where you are: parts per million of calcium carbonate, grains per US gallon, degrees of German hardness or French degrees. Convert with the table above before you touch a dosing chart. A test strip is a reasonable fallback if the supplier's figure is hard to find, but it gives a band rather than a number, and it will be reading whatever the tap gives today rather than the annual average.

Does more detergent get clothes cleaner?

No, past a fairly low threshold. Once there is enough surfactant to lift the soil and hold it in suspension, extra dose adds very little cleaning and a lot of foam — and foam is a problem in a front-loading drum, where the wash is done by clothes tumbling against each other and a thick layer of suds cushions exactly that motion. Excess also has to be rinsed out, and what is not rinsed out stays on the fibres. If your machine is running longer rinse cycles or your towels have gone stiff and water-repellent, dose is the first thing to look at.

Should I use less detergent if my water is soft?

Yes. In hard water a share of every dose is consumed neutralising dissolved calcium and magnesium before any of it works on the soil, which is why a dosing table has a separate column for each hardness band and why the largest figure printed on a bottle is usually the hard-water one. This calculator is conservative about it — its soft setting only takes 10 percent off — so if your water is genuinely soft, use the soft column on your own bottle rather than this tool's figure, and expect it to be a bigger cut than a tenth.

Does the calculator work for powder or pods?

No. It answers in millilitres and converts to cap-fulls, so it is built around liquid. Powder is dosed by weight and a good deal of its water-softening capacity sits in the builder system rather than in the surfactant, which changes how it responds to hardness. Pods cannot be part-dosed at all: one is one, which makes them a poor fit for a small load and for soft water, the two cases where the right answer is less. If you use pods, the lever left to you is load size — fill the drum properly so that one pod is the right dose rather than a rounding-up.