Most fueling advice for runners comes in two flavours. The first is too vague: “eat carbs, drink water, take salt.” The second is too prescriptive: “eat exactly two gels per hour and one electrolyte tab per 500ml”. Neither is wrong. Both are useless on the morning of a race that doesn’t match the template.
The Running Momma fueling calculator exists for the gap between those two. You give it the inputs that actually matter, it gives you a plan that maps to your body, your race, and the weather. This post walks through what’s under the hood, what you control, and how the calculator’s outputs translate into “eat this at this point in the race”.
If you want the underlying research, the science page has the citations and the framework. This post is about how it works in practice — and where you, the runner, still have to think.
What the calculator actually does
Three independent models, then a combined timing plan:
- Carbohydrates — how many grams per hour your gut should aim to absorb.
- Fluid — how many millilitres per hour you should aim to drink.
- Sodium — how much salt you need to replace what you’re losing in sweat.
Each model takes a small set of inputs, applies the published ranges from sports-science research, and produces a number in a sensible range. Then the calculator combines them: how often you take a serving, what each serving should contain, and how the plan should bend if the temperature shifts on race morning.
The calculator is not a black box. The numbers come from a fixed framework — same inputs, same plan. There’s no machine-learning fudge, no random sampling, no marketing-driven multipliers. The whole system is auditable from the inputs.
The three inputs that actually matter
Most fueling calculators online ask for a dozen fields and produce one number. Ours asks for the inputs that meaningfully change the output, and ignores the rest.
1. Body weight
Carbohydrate and fluid recommendations both scale from body weight. A 55kg runner doesn’t need the same hourly intake as a 90kg runner — that’s obvious — but most generic “60g/h” advice ignores this entirely. The calculator scales the carb target with body weight (in g/kg/hr) and adds about 6 ml/h of fluid per kg above the baseline.
This matters most at the extremes. A 50kg woman following standard “take a gel every 30 minutes” advice is overshooting her gut’s absorption ceiling. A 90kg man following the same advice is undershooting and will hit the wall.
2. Effort level (heart-rate intensity)
Carb oxidation rises with effort. Gastric emptying — how fast your stomach actually passes food through to your gut — slows with effort. These two pull in opposite directions, and the calculator weights them based on the heart-rate zone you’ll be racing in.
You don’t have to guess HR zones. The app derives them from your VDOT, which itself comes from your training paces. The race effort you select (easy / steady / threshold / above threshold) maps to a multiplier on the base carb rate — about 0.7 at the easy end, 1.15 at the hardest end.
3. Gut training
This is the input most fueling tools ignore, and it’s the one that moves the calculator’s output the most. Your gut adapts to in-race carbohydrate the same way your legs adapt to mileage: gradually, with practice, over weeks.
The calculator asks how trained your gut is — not whether you “feel like” 60g/h, but whether you’ve practiced it on long runs:
- Not trained — 35–45 g/h ceiling. If you’ve never deliberately practiced eating during long runs, you live here. Pushing higher will probably make you nauseous.
- Moderately trained — 45–60 g/h. You’ve done 5–10 long runs eating during them, you can comfortably take a gel every 25–30 minutes.
- Well trained — 60–90 g/h. Dual-transporter formulations (glucose + fructose) help here.
- High-carb specialist — 90–120 g/h. You’ve trained your gut to handle elite-level fueling. Most runners don’t need to live here.
This is the input that explains why the calculator might give you 50 g/h when your training partner gets 80 g/h on the same race. They’ve spent six months practicing it. You haven’t.
What you get out
Three numbers per hour, and then a timing plan:
Carbs: ~60 g/h
Fluid: ~600 ml/h (60% sports drink, 40% water)
Sodium: ~700 mg/h
Interval: take 25g of carbs every 25 minutesThe interval is the practical bit. The hourly numbers don’t directly translate to “what do I do on the watch beep”. The calculator converts your hourly target into a real serving cadence: how many minutes between gels, how often to sip from your bottle, when to layer in a sodium tab.
Real fueling is what you do when your watch buzzes. The calculator is built around that — round numbers to the nearest 5 minutes, gel-sized servings, intervals between 15 and 45 minutes. Nothing in the plan ever asks you to “take 0.7 of a gel” or “sip exactly 213ml every 9 minutes”.
What the calculator gets honestly wrong
A few things the framework doesn’t model, that you still need to think about on race morning:
- Heat acclimation pace. The fueling side scales with temperature — sodium needs go up, fluid needs go up. But the calculator doesn’t slow down your pace recommendation. A hot race typically costs 3–7% per 5°C above mild conditions. Plan to run that slower.
- Altitude. No automatic correction above ~1500m. If you’re racing at elevation, drop your expected pace by 3–5% per 1000m.
- Sweat rate as personal. The calculator estimates sweat rate from intensity and temperature, then asks whether you’re a low / average / high salter. It can’t measure your actual sweat. If you’ve done a sweat test (weigh in, run an hour, weigh out), your numbers are better than the calculator’s estimate. Override accordingly.
- GI distress from new fuels. The calculator gives you grams of carbs, not brands. If you switch to a new gel formula on race day, all bets are off. Always practice race-day fueling with the exact products you’ll use.
Honesty about where the framework stops is more useful than pretending it doesn’t.
Picking the fuel itself
The calculator tells you grams per hour. It doesn’t tell you what bottle to grab off the aid-station table. For that, you pick the fuel you’ve already trained with.
A few common pairings:
Maurten Gel 100 → SiS GO Isotonic → Neversecond C30 → Precision PF30 →
If you want to read about which one fits which runner, the full gel comparison post goes brand by brand. If you want to back into a count first (“how many of these do I need?”), the marathon gel math post does the arithmetic.
Where you still have to think
The calculator does the maths. It can’t do these things:
- Tell you what to put in your bottle. Mix what you’ve trained with. If you’ve been using a sports drink in workouts, use that in the race.
- Tell you what to skip. If your stomach is rebelling at km 25 of a marathon, the right answer is sometimes to drop a planned gel, not force it. Trust your gut. The plan is a target, not a prescription.
- Tell you when you’re under-fueling. It can prescribe the right intake, but it can’t make you eat. Most runners under-fuel because they get distracted, not because the plan is wrong. Set the watch beeps. Eat on the beep.
- Tell you when to override it. A hot day, an upset stomach the morning of, a sleepless night before — these are signals to adjust. The plan is built for an average healthy race-day version of you, not the actual you on race morning. Use judgment.
The honest one-liner
A calculator can give you a plan. It can’t give you a race. The number on the screen is a starting point — it gets you about 85% of the way to the right answer. The last 15% comes from practicing on long runs, paying attention on race morning, and being willing to ditch the plan when your body says no.
This page references the framework documented on the science page. The carbohydrate, fluid, and sodium guidance is informational and based on published averages for healthy adult endurance athletes. If you have cardiovascular, kidney, or electrolyte conditions, are pregnant, or take medications that affect fluid balance, talk to a clinician before following any race-day fueling plan.