Fuelling · Free tool
Running Caffeine Calculator
Enter your body weight, how long you expect to be out, a dose level, and how much caffeine each gel, tablet or coffee carries. We work out your pre-start dose, whether a long effort needs top-ups, the total, and how many servings that is.
How much caffeine helps
Caffeine is one of the few supplements with solid evidence behind it: doses of 3–6 mg per kilogram of body weight improve endurance performance by around 2–4% by lowering perceived effort and sparing glycogen. For a 70 kg runner that is 210–420 mg — two to four strong caffeinated gels, or roughly two to four espressos.
More is not better. Above 6 mg/kg the side effects — racing heart, jitters, stomach trouble — grow faster than the benefit, and studies show doses as low as 2 mg/kg still help. Start at the low end and only move up if you have tested it in training.
When to take it
Caffeine peaks in the blood about an hour after you take it and has a half-life of around five hours, so the classic timing is 45–60 minutes before the start. Caffeinated gels absorb a little faster than tablets or coffee.
On efforts longer than two hours, a small top-up — about 1 mg/kg every two hours — keeps the level up as the first dose fades, and a late-race top-up in the final hour is well supported. The calculator schedules top-ups every two hours and drops any that would land in the last half hour, when there is no time left for it to work.
Limits and habits
Around 400 mg a day is the general safe limit for healthy adults. The calculator flags any plan above it. Pregnant runners, anyone with a heart condition or high blood pressure, and those on medication that interacts with caffeine should talk to a doctor before racing on it.
You do not need to give up coffee in the weeks before a race: the evidence for a withdrawal period is weak and habitual users still get the benefit. What does matter is rehearsal — the same dose, product and timing on a long run — because caffeine at race intensity affects the stomach and the nerves differently from a coffee at your desk.
Frequently asked questions
How much caffeine should I take before a marathon?
Around 3 mg per kilogram of body weight, 45–60 minutes before the start — about 210 mg for a 70 kg runner — with a 70 mg top-up at the two-hour mark for a four-hour race. Up to 6 mg/kg is supported by the research but the extra benefit is small and side effects rise.
Should I stop drinking coffee before a race?
No. Studies find that habitual coffee drinkers still get the performance benefit from a race-day dose, and withdrawal headaches in race week cost more than any small boost. Keep your usual routine and add the planned dose on the day.
Is coffee as good as caffeine gels or tablets?
Caffeine is caffeine — the difference is dose control. Coffee varies from 60 to 150 mg a cup depending on the beans and the brew, so it is hard to hit a target; gels and tablets state their content. Coffee before the start plus gels during the race is a common, well-tested combination.