Run Your Next Marathon.

Enter a finish time and distance to get per-mile and per-kilometer splits. Generates cumulative split tables at every mile marker — useful for programming your watch or printing a pacing band. A 4-hour marathon works out to roughly 9:09/mi or 5:41/km.

Uses the Riegel formula (T2 = T1 × (D2/D1)^1.06) to extrapolate finish times across 5K, 10K, half marathon, marathon, and 50K. The 1.06 fatigue factor means doubling the distance costs more than double the time. Accurate within 2–3% for trained runners between 5K and marathon.

Uses the BAA qualifying standards for 2026 and 2027 (e.g. 2:55:00 for men 18–34, 3:25:00 for women 18–34) with age-group adjustments in 5-year increments. Factors in the cutoff buffer — recent cutoffs have fallen 4–7 minutes under the standard. Also applies the BAA's time penalty for steep downhill courses.

VDOT is Jack Daniels' metric that maps race performance to aerobic fitness while accounting for running economy. Calculates five zones: Easy, Marathon pace, Threshold (lactate turnpoint, ~60-minute sustain pace), Interval (VO2max efforts, ~12-minute sustain pace), and Repetition (speed/economy work). Recalculate after PRs or every 4–6 weeks.

Targets 60–90g carbs/hour for races longer than 2.5 hours, starting near 60g if your gut is not trained for it. Plans hydration at 400–800ml/hour adjusted for heat. Your body stores roughly 2,000 calories of glycogen — about 90–120 minutes of hard running — so anything longer needs fueling. Includes carb-loading guidance at 8–12g per kg bodyweight for the 2–3 days before.

Scores six weighted factors: elevation gain per mile, max altitude (impact starts above 4,000ft), average grade, heat index, surface type, and course profile. Scale runs 10–30 (PR friendly), 31–55 (moderate), 56–75 (serious grind), 76–100 (brutal). Rule of thumb: every 100ft of elevation gain per mile costs 12–15 seconds per mile.