What this page does
A carbs per hour calculator turns a rate into a shopping list and a schedule. You type the duration of the race and a carbohydrate rate in grams per hour; the page returns the total carbohydrate for the race, the number of sachets that total works out to, the volume of sports drink that carries the same carbohydrate, and an hour-by-hour card you can print and put in a pocket. Two more blocks take the numbers a label does not give you: a sweat rate worked out from two weighings, and a sodium figure converted from millimoles per litre into grams per hour.
No other page on this site counts carbohydrate. The running calorie calculator estimates what a run costs in kilocalories from body mass, distance and gradient; it has no intake side at all. The fat-burning heart rate page states the intensity at which fat oxidation peaks and explicitly prints no calorie or fat-gram estimate. The lactate testing page reads a threshold from two blood samples and converts it into a pace. This page is the one that answers “I am running for three hours and I want 60 grams an hour — how much do I carry, and in what”.
Everything runs in your browser. Nothing you type is sent to a server, there is no account and no upload, and the numbers never leave the page. The three rates the table prints, and the source each one comes from, are written up below.
How the numbers are calculated
The whole page is multiplication and division. Let T be the race duration in hours and r the carbohydrate rate in grams per hour. The total is T × r. Everything else is that total divided by a label figure:
total = T × r sachets = total ÷ ggel drink = total ÷ cdrink × 100
where ggel is the carbohydrate in one sachet and cdrink is the carbohydrate in 100 millilitres of drink. The volume that equals one gel is ggel ÷ cdrink × 100 millilitres, which is the same arithmetic read the other way. Nothing here is a model, and no coefficient is invented: the two label figures are yours, and the rate is the one you type.
The three rates in the table are not this page's opinion. Jeukendrup A.E. (2014), A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise, Sports Medicine 44 (Suppl 1):25–33, DOI 10.1007/s40279-014-0148-z, states all three in the text: “The most recent guidelines by the American College of Sports Medicine (ACSM) state that a carbohydrate intake of 30–60 g/h is recommended during exercise”; “A single carbohydrate source can be oxidized at rates up to approximately 60 g/h and this is the recommendation for exercise that is more prolonged (2–3 h)”; and “For ultra-endurance events, the recommendation is higher at approximately 90 g/h.” The table prints those three rates as three columns and multiplies each by the duration. Which column you plan against is your decision, and the page does not make it for you.
A worked example: three hours at 60 grams an hour
With T = 3.0 hours and r = 60 g/h, the total is 3.0 × 60 = 180 g. On a 25 g sachet that is 180 ÷ 25 = 7.2 sachets, so you carry 8. In a drink at 6 g per 100 mL the same carbohydrate is 180 ÷ 6 × 100 = 3,000 mL. Per hour that is 2.4 gels and 1,000 mL of that drink, and one 25 g sachet equals 25 ÷ 6 × 100 = 416.7 mL, printed 417. Change the sachet to 22 g and the same race needs 180 ÷ 22 = 8.18 sachets, which is 9 to carry.
The reverse block is the same equation solved for r: a total of 180 g over 3.0 hours is 180 ÷ 3.0 = 60 g/h. It exists because a lot of people arrive knowing what is in the bag rather than what rate they want.
Sweat rate and sodium
Sweat rate is measured, not modelled. Let m1 be the mass before the run in kilograms, m2 the mass after, and V the volume drunk in millilitres. Then:
loss (mL) = (m1 − m2) × 1000 + V rate (mL/h) = loss ÷ T
The 1000 is the assumption that one kilogram of mass change is one litre of fluid, which is the density of water to two significant figures and the convention the whole calculation rests on. With 70.0 kg before, 68.6 kg after, 500 mL drunk and three hours, the loss is 1,400 + 500 = 1,900 mL and the rate is 1,900 ÷ 3 = 633 mL/h, printed as 0.63 L/h. If you drank more than you lost, the loss can come out negative, and the page prints it as a negative number rather than hiding it — that is a weighing that says you finished heavier than you started.
Sodium is a unit conversion. The atomic weight of sodium is 22.98977 grams per mole, so one millimole of sodium is 22.98977 milligrams:
mg/L = mmol/L × 22.98977 g/h = mg/L × (L/h) ÷ 1000
At 40 mmol/L that is 40 × 22.98977 = 919.59 mg/L, and at the 0.633 L/h above it is 919.59 × 0.633 ÷ 1000 = 0.58 g/h, which is 582 mg/h. That is the whole of the sodium block: a concentration times a rate, with the atomic weight in between. What you do with the answer is not on this page.
- T — the race duration in hours, from the box at the top. 0.5 to 12.
- r — the carbohydrate rate in grams per hour, 1 to 200. Typed, not recommended.
- ggel — carbohydrate in one sachet, in grams, from your own label.
- cdrink — carbohydrate per 100 mL of drink, in grams, from your own bottle.
- 1 mmol sodium = 22.98977 mg — the atomic weight of sodium, a definition.
- 1 kg mass change = 1 L fluid — the density assumption in the sweat-rate block.
The rate table, and why it has three columns
The table above holds one row for each half hour from one hour to six, and one column for each of the three rates the source states. Read across a row and you see what the same race costs at 30, 60 and 90 grams an hour; read down a column and you see the total grow with the duration. The row nearest your own duration is highlighted, so the table doubles as a way of reading the page at a glance.
Three columns rather than one is deliberate. A single column would have to be labelled with a rate this page picked, and picking one is exactly the thing the page is trying not to do. The three figures are in the source with their own conditions attached — 30 to 60 as the general guideline, 60 as the ceiling for a single carbohydrate source over two to three hours, 90 for ultra-endurance — and printing them side by side keeps the condition visible instead of collapsing it into one number. The page multiplies; it does not choose.
The same arithmetic drives the hour-by-hour card. Each row is the rate you typed, unchanged, because a constant rate over a constant hour is a constant. If your duration is not a whole number of hours the last row is the part hour, and its carbohydrate is proportionally smaller: three hours thirty at 60 g/h is four rows, the last one 30 grams instead of 60.
Sweat rate: the one number you have to measure
Nothing on this page estimates how much you sweat, and that is on purpose. Sweat rate varies with the person, the pace, the clothing and the weather, and a published average would be a number about somebody else. The measurement is two weighings and a bottle: weigh in dry, run, weigh again the same way, and count what you drank. The difference plus the intake is your loss.
Three practical notes on getting the weighing right. Weigh with no clothes on, or the same clothes both times, because a soaked shirt is most of a kilogram. Weigh straight after, before you drink anything, or the intake you are adding back is already in the number. And do it on a run that looks like the race — a rate measured over forty easy minutes on a cold morning is a rate for forty easy minutes on a cold morning, and the page applies no correction of any kind to move it anywhere else.
The rate the page returns is a loss rate. It is not an instruction to drink that much: what you replace, and how much of the loss you leave unreplaced, is a decision about your race and not arithmetic. This page converts; it does not prescribe.
Sodium: grams from a concentration
A sweat sodium figure usually arrives in millimoles per litre, and a race plan is usually written in grams, so the useful thing a calculator can do is stand between the two. The conversion is the atomic weight of sodium and nothing else: 22.98977 grams per mole, so one millimole is 22.98977 milligrams. Forty millimoles per litre is 919.59 milligrams per litre. Multiply by a sweat rate in litres per hour and you have milligrams per hour, which is grams per hour divided by a thousand.
This block exists because the multiplication is easy to get wrong by a factor of a thousand, and because the two units are never printed next to each other on the same label. It stops at the conversion. There is no target concentration, no replacement rate, no advice about what to take, and no discussion of what happens when intake and loss disagree — those are questions for a clinician or a sports dietitian, and this site is a calculator.
If you do not have a concentration, the block will still convert one you type from a lab report or a patch test. It has no default that means anything: the 40 in the box is an example to show the arithmetic, not a value to plan against.
Supported range and limits
- Duration: 30:00 to 12:00:00, entered as hh:mm:ss or mm:ss. Under half an hour the page refuses, because a part hour of fuel arithmetic is not a plan.
- Rate: 1 to 200 grams per hour. The upper end is a guard against a typo, not a recommendation.
- Gel: 1 to 100 grams per sachet. Drink: 0.1 to 30 grams per 100 mL.
- Body mass: 20 to 250 kg. Volume drunk: 0 to 20,000 mL. Sweat sodium: 0 to 300 mmol/L.
- Rounding: grams to one decimal, millilitres to the nearest whole millilitre, sachets to one decimal with the carry count rounded up. One rounding, at the end, never halfway through a chain.
- Cost: none. No account, no email, no paywall, no daily limit, no request to a server.
- Privacy: every number is computed in your browser. Nothing you type is sent anywhere or stored — reload the page and it is gone.
- Works offline once the page has loaded, and on a phone.
What this page deliberately does not do
- No recommended intake. The page prints the three rates the source states, with the conditions attached to each, and multiplies by whichever you type. It does not tell you which one to use, and it prints no target of its own.
- No sodium advice. The sodium block converts a concentration into grams per hour. It gives no replacement rate, no target, and says nothing about what happens when intake and loss disagree — that is a medical question, and this is not medical advice.
- No training plan. No sessions, no gut-training protocol, no practice schedule, no weekly volume, no race-day strategy. What to practise and when to practise it is a plan; this is a calculator.
- No weather correction. Heat, humidity and wind move sweat rate, and this page models none of them. The rate it prints is the rate you measured, in the conditions you measured it in.
- No energy figure. Carbohydrate in grams only, no kilocalories and no fat grams. What a run costs is the running calorie calculator; what a rate of fat oxidation peaks at is the fat-burning heart rate page.
- No product database. No brands, no sachet list, no drink list. The two label figures are typed by you, which is the only way the answer can match what is actually in your bag.
- No aid-station schedule. The card is hours from the start, not kilometres along a course. Where the stations are on your race is not something this page knows.
Questions
How many grams of carbohydrate per hour should I take during a race?
This page does not pick a rate for you, and it prints no target. What it does is hold the three rates the literature states, next to their sources, and do the arithmetic from whichever one you type. Jeukendrup (2014) reports that the American College of Sports Medicine guideline is 30 to 60 grams per hour during exercise, that a single carbohydrate source can be oxidised at up to about 60 grams per hour and that 60 is the figure recommended for exercise lasting two to three hours, and that ultra-endurance events carry a higher recommendation of about 90 grams per hour. Type the rate you have decided on and the page returns the total, the gels and the drink that rate implies.
How many gels is 60 grams of carbohydrate an hour?
Divide 60 by the carbohydrate in one sachet. On a 25 gram sachet that is 2.4 sachets an hour, and over a three hour race it is 180 grams, which is 7.2 sachets, so you carry 8. The sachet size is the number printed on your own label and nothing else; the page uses whatever you type in the gel box. A 22 gram sachet at the same rate is 2.73 an hour and 8.18 over three hours, which is 9 to carry.
How much sports drink equals one gel?
Divide the carbohydrate in the gel by the carbohydrate per 100 millilitres and multiply by 100. A 25 gram gel in a drink at 6 grams per 100 millilitres is 25 divided by 6 times 100, which is 416.7 millilitres, printed 417. The same three hour race at 60 grams an hour is 180 grams, which is 3,000 millilitres of that drink. The page does the same conversion in both directions, so you can ask it how many millilitres replace a gel or how many gels replace a bottle.
How do I work out my sweat rate?
Weigh yourself with no clothes on before the run, weigh again the same way straight after it, and add the volume you drank during the run. The mass you lost plus the fluid you took in is the fluid you lost, and dividing by the hours gives the rate. A runner who goes from 70.0 to 68.6 kilograms over three hours while drinking 500 millilitres has lost 1,400 grams plus 500 millilitres, which is 1,900 millilitres in three hours, or 633 millilitres an hour, printed as 0.63 litres per hour. The page treats one kilogram of mass change as one litre of fluid, which is the density assumption behind the whole calculation.
How much sodium is 40 mmol/L of sweat?
Multiply the concentration by the atomic weight of sodium, 22.98977 grams per mole, so 40 millimoles per litre is 40 times 22.98977, which is 919.59 milligrams per litre. Multiply that by the sweat rate to get sodium per hour: at 0.633 litres per hour it is 582 milligrams an hour, which is 0.58 grams. That is a unit conversion and nothing more. This page does not say how much sodium to take, and it does not discuss what happens when intake and loss disagree.
Is carbohydrate per hour the same as calories per hour?
No. This page counts grams of carbohydrate only and prints no energy figure, because the number a race plan needs is the grams on the label, not a calorie total. The energy a run costs is a different question with a different model behind it, and the page that answers it is the running calorie calculator. The two pages do not overlap: one counts what you take in, the other estimates what the running costs.
Does heat change these numbers?
Heat changes the sweat rate, and the sweat rate is measured rather than modelled here, so the answer is that you have to measure it in the conditions you care about. This page applies no weather correction of any kind: no temperature, no humidity, no wind. A rate you measured on a cool morning is the rate for a cool morning. Nothing in the arithmetic here moves when the weather does.
Can I use this page for a marathon?
Yes, as arithmetic. Type the marathon duration and the rate, and the page returns the total, the gels, the drink and the hour-by-hour plan for that duration. What it does not do is tell you what to eat in a marathon, when to take it, or how to practise it: there is no distance table, no aid-station schedule, no session plan and no race-day strategy. Those are decisions about your race, and this is a calculator.