“Take a gel every 30 minutes” is the advice every first-time marathoner hears. It hides the actual math, encourages over-fueling for some and under-fueling for others, and tells you nothing about when to take the first one or what to take it with. Here is the exact count, the timing, and the trade-offs.

The exact number for a 3, 4, or 5-hour marathon

Most modern energy gels deliver 20–25 grams of carbohydrate per packet. At a target of 60–90 grams per hour, you need 2–4 gels every hour of racing. Multiplied across a full marathon:

  • Sub-3-hour marathon: 6–9 gels (start around mile 4)
  • 3:00–3:30 marathon: 7–10 gels
  • 3:30–4:00 marathon: 8–11 gels
  • 4:00–4:30 marathon: 9–12 gels
  • 4:30–5:00+ marathon: 10–14 gels

Faster runners need fewer because they spend less time on course, not because they need less per hour. A 2:45 elite still aims for 80+ g/hr — they just finish before total intake stacks up.

The math behind it

Your muscle and liver glycogen together store roughly 2,000 kilocalories of fuel. A marathon at race pace burns 2,400–3,000 kcal. The deficit is what causes the wall at mile 20. Exogenous carbs from gels close that deficit by sparing glycogen and keeping blood glucose stable.

See our full breakdown in how many carbs per hour you actually need for the science behind the 60–90 g/hr target and why the old 60 g/hr ceiling no longer applies.

When to take the first gel

Mile 3–4, or 25–35 minutes in. Waiting for the urge to fuel is too late — blood glucose has already dipped and you are 15–20 minutes behind on absorption. The pre-race gel taken 10–15 minutes before the gun is not optional for marathoners; treat it as the zero-mile gel.

Spacing — every 30 versus every 45 minutes

Every 30 minutes maintains the steadiest blood-glucose curve but requires gut tolerance most untrained athletes do not have. Every 45 minutes is the conservative default and works for most recreational marathoners. The difference between the two is roughly one gel across the full race.

The fix is gut training — practising your race-day fueling protocol on long runs for 4–8 weeks so your gut can absorb at the upper end of the range on race day.

What to take it with

Plain water. Always. Gels arrive at the intestine as concentrated sugar syrup; the gut needs free water to dilute the bolus before absorption starts. Taking a gel with a sports drink doubles the osmotic load and slows everything down. Take the gel, sip 6–8 ounces of water in the next two minutes, then return to your normal fluid pattern.

When more gels backfire

Past roughly 90 grams per hour, the limiting factor stops being gut transporters and starts being osmotic pressure. Extra sugar the small intestine cannot absorb pulls water in, causes cramps and diarrhea, and ends your race in a porta-potty. More is not better. Hit the target, then stop.

Caffeinated or plain

Mix. Use plain gels for the first 60–90 minutes and caffeinated gels in the back half when you need a perceived-effort lift. Total caffeine across a marathon should land at 3–6 mg per kg of bodyweight — see our caffeine strategy for endurance athletes for the dose chart and timing.

Hot-weather adjustment

In heat, gut absorption drops. Cut each dose in half — take a gel, chase with more water, wait 5 minutes, take the second half. The total carb count for the race stays the same; the delivery slows so the gut can keep up. Full protocol in our hot-weather marathon fueling guide.

Personalisation matters more than the number

Body weight, sweat rate, GI history, race pace, and weather all shift the target. A 60 kg female running a 4-hour marathon in cool weather needs a different protocol than an 85 kg male targeting 3:15 in the heat. Generic gel counts are starting points, not finish lines.

Frequently asked questions

Can I take too many gels? Yes — past 90 g/hr the gut rebels. Stick to your race-pace target.

What if I miss a gel? Take it the next time you remember, then stay on schedule. Do not double up.

Are chews equivalent? Roughly — match by carb grams, not by packet. One gel ≈ one half-pack of chews.

What if I bonk anyway? Follow the bonk rescue protocol — slow, dose 30–60 g of fast carbs, sodium, walk 5–10 minutes.