The ratio that matters
The ratio that matters for green tea is about one gram of leaf to 50 millilitres of water. That is 3 grams in a 150 millilitre cup, or a heaped teaspoon if you have no scale.
Green tea is light and often finely rolled, so a given weight takes up less room than you expect. Measuring by the spoon misleads people here more than with any other category, because a teaspoon of a needle-shaped green weighs much less than a teaspoon of a flat, pressed one.
A scale costs very little and removes the guesswork entirely. If you brew the same tea every morning, weigh it once, note what it looked like, and you will not need the scale again for that tea.
Gongfu brewing uses far more leaf: 5 grams in a 100 to 120 millilitre gaiwan, which is roughly one gram to 20 millilitres. The cup is smaller and the steeps are much shorter, so the total drink is similar even though the ratio looks extreme.
Too little leaf does not make a weaker version of the same tea. It makes a thin, watery cup where the sweetness never arrives, because there is not enough material to build any body before the aromatics are gone.
Too much leaf at a high temperature produces bitterness and a drying finish almost immediately. With green tea the usual culprit is not the ratio on its own, it is a heavy ratio combined with water that is too hot.
Time by the clock
Time by the clock for green tea is short: one to two minutes in a Western-style brew, and 20 to 40 seconds for the first infusion if you are brewing gongfu style.
Temperature and time trade off against each other. At 80 degrees you have more room to move and can stretch to two or three minutes. At 90 degrees the same leaf will turn bitter inside a minute.
For multiple infusions, add roughly 15 to 20 seconds per steep. Green tea rarely gives more than three good infusions, and the third is usually a shadow of the first.
What comes out first is the sweet amino acid character, above all theanine, along with the volatile top notes. What comes out later, if you wait, are the catechins and caffeine that read as bitterness and astringency.
Under-brewing is the more common mistake among people who have been burned by bitterness. A green tea pulled after 30 seconds at 75 degrees tastes like scented water, and there is a lot of flavour sitting in the leaf that never made it into the cup.
The leaf tells you when it is ready. Rolled greens unfurl as they hydrate, and once the leaf is fully open and has sunk, the infusion is close to done. Watch it once and you will not need the clock again.
A simple starting recipe
A simple starting recipe: 3 grams of leaf, 150 millilitres of water at 80 degrees, steeped for 90 seconds. Strain completely and taste it before you adjust anything.
If it is thin, add leaf rather than time. Going from 3 grams to 4 will do more than going from 90 seconds to two minutes, and it will not bring bitterness with it.
If it is bitter, drop the temperature first, to 75 degrees, and only then shorten the time. Temperature is usually the real problem, and fixing it lets you keep the body you wanted.
For the second infusion, use the same leaf and add about 30 seconds. For the third, add another 30. By the fourth most green teas are finished, and there is no point pretending otherwise.
Two things to avoid. Do not use boiling water on green tea, which is the single most common error and the one that makes people think they dislike the category. And do not leave the leaf sitting in the water after pouring, which keeps extracting and ruins the next steep.
If you have no thermometer, boil the water and let it sit for five minutes with the lid off. That lands close to 80 degrees, and it is more reliable than guessing by eye.
Weigh the leaf once and you will never go back to spooning it. The difference between a good cup and a great one is usually half a gram.