The four variables, in the order they matter
LEAF QUANTITY sets the ceiling on strength — no amount of steeping extracts more than the leaf contains, and the commonest cause of weak tea is simply not enough leaf. TEMPERATURE decides what comes out first: the compounds carrying bitterness and astringency extract far more readily in hot water than those carrying sweetness and savoury depth, which is why boiling water on a delicate green tea gives a cup that is thin and harsh at once. TIME decides how much of it comes out. And SEPARATION — removing the leaf when the time is up — decides whether the cup you are drinking stays the cup you made. Get the first two roughly right and the rest is adjustment.
A starting point you can adjust from
For a western-style mug or pot, 2 to 3 g of leaf per 200 ml is a reasonable default. Temperature runs from about 70 °C for ordinary Japanese green tea up to a full boil for black, roasted oolong and dark tea, with Chinese greens and light oolongs in the 80 to 90 °C middle. Time runs from around 45 seconds for Japanese green tea to three or four minutes for black. These are starting points, not rules — each tea’s own page carries its parameters, and those are always the better guide.
Give the leaf room
Leaf expands to several times its dry volume. A tightly rolled oolong pellet can open to twenty times its packed size. If it is confined in a small infuser basket it cannot circulate, water cannot reach most of its surface, and the result is weak however long it steeps. This is the single most common equipment mistake in loose-leaf brewing, and the fix costs nothing: a bigger basket, or no basket at all.
Adjust one thing at a time
When a cup is wrong, the instinct is to change everything. Change one variable and taste again. If it is bitter, drop the temperature before you shorten the time — bitterness is more temperature-driven than time-driven. If it is weak, add leaf before you extend the steep, because a longer steep on too little leaf extracts the harsh fraction without adding body. If it is both weak and bitter at once, the water was almost certainly too hot for the quantity of leaf.
What does not matter as much as people think
Precision to the degree. Warming the pot. Whether the water is filtered, unless yours is very hard or heavily chlorinated. Expensive equipment of any kind. A drinker with cheap leaf, correct temperature and enough of it will make a better cup than one with fine leaf, a beautiful pot and boiling water.