Ironman Nutrition Calculator

Total carbs = race duration × grams of carbohydrate per hour.

Bike carbs = bike duration × bike carb target per hour.

Run carbs = run duration × run carb target per hour.

Total fluid = race duration × fluid per hour.

Total sodium = race duration × sodium per hour.

Product count = total needed ÷ amount per product.

Most athletes fuel more consistently on the bike than on the run because eating is usually easier while cycling.

This trips people up constantly, and at least one competing calculator has it backwards. The hourly carbohydrate target for a 70.3 is higher than for a full Ironman, not lower, because you race a half at a greater intensity and burn a larger share of carbohydrate rather than fat.

The total is of course far larger over a full distance - eight or ten hours of intake adds up - but the rate per hour is what your gut has to keep up with, and that rate goes up as the race gets shorter and harder. A 70.3 ridden at 78% of FTP demands a higher hourly intake than an Ironman ridden at 70%.

Professional fuelling has moved a long way in the last few seasons. Reported intakes at the front of the 2026 world championship field ran from roughly 145 up to 180 grams per hour on the bike - numbers that would have been considered impossible a decade ago.

That is genuine, and it is also the wrong target for almost everyone reading this. Those athletes have spent years training their gut to it, they are racing for four hours less than an age-grouper, and they have staff working out the mix for them. Age-group guidance in the same articles that report those figures stays at 70 to 90 grams.

The practical rule: do not exceed what you have actually rehearsed. Athletes who read a pro number and try it cold report exactly what you would expect - bloating, cramping and a stomach that stops absorbing anything at all, usually somewhere around the 120 gram mark. If you want to raise your ceiling, raise it in training over a block of weeks, ten grams at a time, on rides at race intensity rather than easy ones.

Above roughly 60 grams per hour you also need more than one type of sugar. Glucose transport saturates around that point, so the extra carbohydrate has to arrive as fructose, which uses a different transporter. This is why sports products advertise glucose-to-fructose ratios of about 1:0.8 - it is the mechanism that makes high intakes absorbable at all.

Published sodium guidance ranges from about 300 to over 1,500 milligrams per hour, which understandably reads as though nobody knows. The range is wide because the underlying variation between athletes is genuinely enormous: sweat sodium concentration differs by several-fold between individuals, and it is largely a fixed personal trait rather than something training changes much.

A salty sweater racing in heat can legitimately need three or four times what a light sweater in cool conditions needs. Any single number published as “the” recommendation is therefore wrong for most of the people reading it, which is why this calculator asks you for a target rather than assuming one.

The two practical signals: visible white salt marks on your kit and hat after long sessions suggest you are at the higher end, and cramping late in races despite adequate fluid points the same way. If neither applies to you, the middle of the range is a reasonable starting assumption.

The failure mode that actually hurts people is not too little sodium - it is drinking large volumes of plain water over many hours, which dilutes blood sodium and causes hyponatremia. That is a medical emergency and it is more common in slower finishers who are on course longest. Drink to a plan rather than to thirst alone over a full distance, and get electrolytes in alongside the fluid.

Fluid targets are the easiest part of this to personalise, and it takes a single session. Weigh yourself naked before a one-hour ride or run at race intensity, record everything you drink during it, then weigh yourself naked again afterwards.

Your sweat rate in litres per hour is the weight you lost in kilograms plus the fluid you drank in litres. A rider who finishes 0.8 kg lighter having drunk 500 ml lost 1.3 litres in that hour. Repeat it once in cool conditions and once in heat, because the difference between the two is usually larger than athletes expect and it is exactly the number you need when planning a hot race.

Aim to replace roughly 70 to 80 percent of what you lose rather than all of it. Fully replacing sweat losses hour after hour is more than most stomachs will absorb while racing, and chasing it is how people end up over-drinking.

This calculator provides planning estimates only. Actual nutrition needs vary by body size, sweat rate, heat, humidity, intensity, gut tolerance, medical conditions, product choice, and race logistics.

Athletes with medical concerns, history of hyponatremia, GI issues, diabetes, kidney problems, or other health conditions should consult a qualified sports dietitian or medical professional.

Always test your nutrition plan in training before race day. Do not try new products, doses, or timing on race day without prior practice.