What “strength” means

In coffee, strength means how much coffee is dissolved in the cup, measured as total dissolved solids (TDS). A cup at 1.25 % TDS is 98.75 % water and 1.25 % dissolved coffee. It is not the same as bitterness, roast level or caffeine.

Two things set it:

The brew ratio: how much coffee you use for the water. This is what you control most directly, and what the calculator works from.
Extraction: how much of the coffee actually dissolves, usually around a fifth of its weight. Grind size, water temperature and brew time move it up or down.

How the estimate works

The calculator assumes the grounds hold back about twice their weight in water, and that between 18 % and 22 % of the coffee dissolves. Strength is then the dissolved coffee divided by the coffee that ends up in the cup:

estimated TDS ≈ coffee × extraction ÷ (water − 2 × coffee)

For 20 g of coffee and 320 ml of water, that gives roughly 1.3–1.6 %. The result is a range because the calculator can't know how well your coffee extracted.

The reference range

The Specialty Coffee Association's Golden Cup guidance puts ideal filter coffee at about 1.15–1.35 % TDS, with 18–22 % extraction, and suggests about 55 g of coffee per litre of water (plus or minus 10 %), roughly 1 : 18. It is a well-known reference, not a rule: many home brewers and cafés prefer stronger coffee, at 1 : 15 to 1 : 17. Brew to taste.

What it can't tell you

The calculator does not measure your coffee. Only a refractometer can measure TDS. It is also meant for filter and immersion brewing; espresso is several times stronger and is brewed in a completely different way. Use it to compare recipes and to see what changing the dose will do.

Sources

Want quantities for a particular method? Use the coffee-to-water ratio calculator.