Ultra Fueling Calculator

How the math works

Calorie target scales primarily with body weight and intensity — heavier athletes burn more, harder efforts burn faster. We use these baseline rates per hour:

• Easy / Z2: 1.5 cal/lb/hr
• Steady (race effort): 1.7 cal/lb/hr
• Moderate (strong race): 1.9 cal/lb/hr

The body can absorb roughly 50–80% of those calories under good conditions. We target the upper range of absorbable calories — the calculator output assumes a trained gut.

Sodium scales with weight and heat:

• Cool: 4 mg/lb/hr
• Mild: 5 mg/lb/hr
• Warm: 6 mg/lb/hr
• Hot: 8 mg/lb/hr (plus pre-load)

How to actually fuel by hour

The hourly target is a budget, not a prescription. Mix your sources:

1. Liquid base: a bottle of Tailwind (200 cal, 310 mg sodium per scoop) per hour covers your hydration + about a third of your calorie target.
2. Gels: 1 gel per 30 min in the first half (Maurten 100, SiS Beta Fuel, or Spring).
3. Real food after hour 4: boiled potatoes, PB&J quarters, broth at aid. Sweet flavors get nauseating; savory extends gut tolerance.
4. Salt backup: SaltStick chews (200 mg sodium each) in your vest. One per hour above your liquid sodium baseline if it's hot.

The pre-race load

In hot conditions (75°F+), drink a packet of Skratch Hyper Hydration (1,720 mg sodium) 60–90 minutes before the start. This expands plasma volume and buffers you for the first three hours. Single best race-day intervention available — costs $4 and adds maybe 30 minutes to your finish time.

The honest caveat

Sweat-rate and sodium concentration vary widely between athletes. The ranges above are average; you might be a heavy or light sweater. The only way to know is to weigh yourself before and after a 90-minute training run in race-day conditions. (1 lb lost = 16 oz of sweat = ~500 mg of sodium for an average sweater.)