Calculate Your Half Marathon Carb & Fluid Needs

Every hydration guide gives you a wide sweat-rate range and leaves you to guess where you fall in it. Stop guessing. Weigh yourself, in minimal clothing, immediately before and after a 60-minute run in conditions similar to your race — without using the bathroom in between — and account for any fluid you drank during the run. You'll lose weight; convert it to millilitres (1g = 1ml) and that's your hourly sweat rate. Most runners lose 500-1,500ml/hour, but the only number that matters is yours. This page walks through testing it properly, a full worked example, your carb and sodium targets, and converting all of it into actual gels, cups, and bottles for race day.

Fifteen minutes of admin now is what stands between a plan built on your own numbers and one built on a guess you'll be paying for at mile 10, when a fluid or carb shortfall stops being theoretical.

How to Measure Your Own Sweat Rate Properly

One test gives you a data point. Several tests give you a usable plan. I'd recommend repeating the weigh-test 2-3 times across different conditions — a cool day and a warm day — since sweat rate varies meaningfully with temperature and humidity.

Use the warm-condition result to plan for a hot race and the cool-condition result for a cool one. Don't rely on a single average that might not match your actual race-day weather.

Worked Example: One Runner's Full Race-Day Numbers

Numbers mean little without an example attached, so here's a complete one. Take a 70kg runner targeting a 1:45 half marathon with a tested sweat rate of 900ml/hour in warm conditions.

Grams-per-hour means nothing to you standing at an aid station. I always convert to "this many gels, this many cups" before race week — because that's the decision you'll actually be making out there.

Calculating Your Carb-Per-Hour Target From Body Weight

Aim for 30-60g of carbohydrate per hour for most half marathon efforts. Scale toward the higher end for larger body mass or slower, longer-duration finish times, and toward the lower end for sub-90-minute efforts — where you'll deplete less total glycogen.

Converting Your Targets Into Real Gels, Chews, and Bottles

A standard gel carries roughly 22-25g of carbs, so a 45g/hour target means roughly 2 gels per hour, spaced 25-30 minutes apart. A standard race aid-station cup holds roughly 150-200ml — so a 900ml/hour fluid target means roughly 4-5 cups per hour, or one full bottle from a hydration belt.

Estimating Your Sodium Loss Without Expensive Testing

Sweat sodium varies hugely between individuals — from roughly 200mg/L to over 2,300mg/L — driven mainly by genetics rather than diet. There's no single "typical" figure. My advice is to start at 500-800mg/L if you're a moderate sweater. If you're a "salty sweater" — you see visible white salt crusting on your skin or dark clothing after a run — estimate substantially higher, toward 900-1,200mg/L or more, and consider an electrolyte tablet with 300-400mg sodium per aid station rather than a standard low-sodium drink mix alone.

I worked with a visibly salty sweater — white salt rings on his running shirt after every long run — who doubled his standard electrolyte intake based on this self-check. It resolved a recurring late-race cramping problem that had followed him for two cycles.

Adjusting Your Plan for Race-Day Heat, Humidity, or Altitude

If the race-day forecast is meaningfully hotter or more humid than your test conditions, scale your fluid target up by roughly 20-30% and prioritise sodium intake. Sweat rate rises faster than carb-burning demand in heat — so the ratio between the two shifts.

At altitude, your fluid needs rise too, from increased respiratory water loss — even in cooler temperatures. Don't assume altitude only affects your pacing.

Building Your Final Personalised Race-Day Plan

Write your final plan as a simple timeline — "gel at 20 min, gel at 45 min, sip at every aid station from mile 3 onward" — and rehearse it exactly on 2-3 long runs before race day. Race morning is not the moment to test a plan for the first time.

Frequently Asked Questions

Do I need a lab test for my sweat rate, or can I test it myself?

You can get a genuinely useful number from a simple self-test — no lab required. Weigh yourself before and after a 60-minute run in race-like conditions. You'll lose weight; convert it to millilitres and that's your hourly sweat rate.

How many times should I test my sweat rate?

Repeat the test 2-3 times across different conditions — a cool day and a warm day. Sweat rate varies meaningfully with temperature and humidity, so one test alone isn't enough.

Does sweat rate change on race day compared to training?

Yes, primarily due to conditions. If the race-day forecast is meaningfully hotter or more humid than your test conditions, scale your fluid target up by roughly 20-30%.

How do I convert my targets into bottles or cups at aid stations?

A standard cup holds roughly 150-200ml, so a 900ml/hour target needs roughly 4-5 cups per hour. A standard gel carries roughly 22-25g carbs, so a 45g/hour target needs roughly 2 gels per hour.

What's a "normal" sweat rate?

Most runners lose 500-1,500ml per hour, but the range is wide enough that the only number that matters is your own — tested directly, not assumed from an average.

How do I estimate sodium loss without expensive testing?

Sweat sodium varies hugely between individuals — from roughly 200mg/L to over 2,300mg/L — mostly down to genetics. I'd start at 500-800mg/L for a moderate sweater. If you're a "salty sweater" — visible white salt crusting on your skin or clothing after a run — estimate substantially higher, toward 900-1,200mg/L or more.

Does my carb target change based on my body weight?

Yes. Aim for 30-60g of carbohydrate per hour, scaling toward the higher end for larger body mass or slower finish times, and toward the lower end for sub-90-minute efforts.

What if my calculated fluid need is more than my stomach can handle?

Don't force the full calculated volume if it causes GI discomfort. Practise on your training runs first — a slightly lower intake you can actually absorb beats a theoretically perfect number you can't keep down.

How do I adjust my plan for a race day hotter than I trained in?

Scale your fluid target up by roughly 20-30% and prioritise sodium, since sweat rate rises faster than carb-burning demand in heat. At altitude, your fluid needs rise too — even in cooler temperatures.

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