How much caffeine for your weight? The mg/kg rule

Is 400 mg of caffeine the same for everyone? No — because caffeine’s effect scales with your body weight, and the familiar limits are really per-kilogram figures rounded to a single number. The European Food Safety Authority (EFSA) points to about 3 mg per kilogram for a single dose and roughly 5.7 mg/kg per day — which happens to work out to 400 mg for a 70 kg adult. For a 55 kg person the same 200 mg coffee is a much bigger dose than it is for someone at 90 kg. Here’s how to work out your own reference, and why weight is only part of the picture.

Why caffeine is measured per kilogram

A stimulant’s effect depends on its concentration in your body, not just the raw milligrams you drank. A bigger body has more blood and tissue to distribute caffeine through, so the same dose ends up more dilute. That’s why dose guidelines for caffeine — and for many substances — are expressed per kilogram of body weight: it’s the unit that actually tracks the effect.

Seen that way, the “400 mg” ceiling isn’t a universal truth. It’s the 3–5.7 mg/kg guidance, calculated for an average 70 kg adult and rounded off.

Your caffeine reference by weight

Using EFSA’s ~3 mg/kg single-dose figure as a starting reference:

Body weight	~3 mg/kg single dose
50 kg	~150 mg
60 kg	~180 mg
70 kg	~210 mg
80 kg	~240 mg
90 kg	~270 mg

So a lighter person reaches the same relative dose with noticeably less caffeine — which is one reason the standard advice can quietly over-serve them.

The same coffee, two different doses

Take a 200 mg cold brew. For a 55 kg person that’s about 3.6 mg/kg — already past the single-dose reference. For a 90 kg person it’s about 2.2 mg/kg, comfortably under it. Same cup, same milligrams, meaningfully different dose. It’s a big part of why two friends can order the same drink and have completely different afternoons.

Do men and women need different references?

On average, yes — mostly because of body weight. Women tend to weigh less on average, so the same coffee is often a higher mg/kg dose, and the flat 400 mg figure fits the lighter end less well. There’s a second thread too: oral contraception and pregnancy slow caffeine clearance, so the same dose can linger longer — which is why the pregnancy reference drops to about 200 mg/day and is a conversation for a healthcare professional. These are averages, not rules — plenty of individual variation sits on top — but they’re part of why a shared cup can land so differently.

Weight isn’t the whole story

Here’s the honest caveat: mg/kg is a better starting point than a flat number, but it isn’t the finish line. How fast you clear caffeine (metabolism) and how strongly your receptors respond (sensitivity) matter just as much, and tolerance shifts things again. Two people of identical weight can still react very differently. And some references aren’t per-kg at all — the pregnancy figure of about 200 mg/day is a flat cap, a question for a healthcare professional. Use your weight-based number to set a sensible ceiling, then let your own reaction fine-tune it.

Muscle, fat, and the limits of mg/kg

One more nuance for the precise: mg/kg uses your total body weight, but caffeine spreads mainly through body water. Someone with more muscle carries more water and dilutes caffeine a touch more than body weight alone suggests; more body fat works slightly the other way. It’s a small effect next to metabolism and sensitivity, so there’s no need to overthink it — mg/kg is a reference, not a lab measurement. The point is simply that a number scaled to your body beats a flat 400, even if it’s still an approximation you refine by feel.

Use your number as a starting line

The practical move: multiply your weight in kilograms by about 3 for a single-dose reference, treat it as a ceiling rather than a goal, and adjust down from what you actually feel — exactly the process in finding your personal limit. For a 60 kg adult that’s roughly a 180 mg single-dose reference — about two espressos, or one strong mug — with plenty of room to sit below it. A number tuned to your body beats a generic 400.

Track it in real terms

CaffIQ counts actual milligrams and can weigh them against your body, so your daily total means something personal — not a one-size-fits-all figure that never quite fit you.

CaffIQ provides general estimates, not medical advice. For any health question, consult a qualified professional.