Caffeine is one of the few things sold to gym-goers that has solid evidence behind it. It is also easy to overdo. The useful amount depends on your body weight, and a lot less is needed than most pre-workout tubs suggest.
Caffeine blocks a signal in your brain that says "you're tired". Effort feels a little easier, so you can keep going or lift a bit faster. A major sports nutrition review (the International Society of Sports Nutrition position stand) found that doses around 3–6 mg per kg of body weight reliably help performance. The biggest and most consistent gains are in endurance, such as running or cycling. For lifting the effect is smaller: a bit more strength, a few more reps, slightly faster bar speed.
Two more things from that review matter for beginners. The lowest dose that works might be as low as 2 mg per kg. And very high doses, around 9 mg per kg, bring more side effects without extra benefit. More is not better.
The calculator shows three numbers. Gentle is 2 mg per kg, the low end of what may work. Starter is 3 mg per kg, a sensible first try. Upper is 6 mg per kg, the top of the studied range. We cap every dose at 200 mg, which is the single-dose limit that the European Food Safety Authority (EFSA) considers safe for adults. A heavy person's "upper" number is therefore held at 200 mg.
The chart is the part people forget. Caffeine does not vanish after your workout. Roughly half is still in your body about five hours later, though some people clear it much faster or slower. Slide the training time to late evening and see how much is still around at bedtime. Poor sleep costs you more recovery than caffeine gives you.
As a rough guide, an espresso shot holds about 80 mg, a cup of filter coffee about 90 mg, and a can of energy drink about 80 mg. Labels and cafés vary, so treat these as ballparks.
This is general information, not medical advice. EFSA's advice is lower for pregnant or breastfeeding women (about 200 mg a day in total) and for children and teenagers (about 3 mg per kg in a single dose). If you have a heart condition, high blood pressure, an anxiety disorder, trouble sleeping or take regular medication, check with your doctor before using caffeine to train. The calculator is built for healthy adults.
Caffeine is a small nudge, not a plan. Eating enough, sleeping well and progressing your lifts will do far more than any cup of coffee. If a coffee helps you show up and train, great. If you feel you need one to get through every session, that is a sign to look at sleep and food first.