Pace Calculator: One race time in,
every pace out.

1. Convert your input race to a 5K-equivalent time using Riegel's 1981 equation: t5K = trace × (5000 / drace)1.06 for distances at or above 1500m, exponent 1.08 for shorter sprints.
2. Compute VDOT from the 5K time using Daniels' VO₂max formula. The function uses an empirical estimate of what fraction of VO₂max a runner can sustain for a 5K, which depends on the 5K time itself.
3. Map VDOT to training paces by inverting the same VO₂max-to-velocity equation at specific intensity fractions: 59% for recovery, 67% for easy, 75% for steady, 71% for marathon, 88% for threshold, 100% for interval/VO₂max, ~103% for repeats, and a 1.08-exponent Riegel mapping for 800m and 400m sprint pace.
4. Predict equivalent race times by running Riegel forward from the 5K time to each target distance.

This is the same engine used by the NXT RUN training app to set workout paces - ported directly from lib/util/pace_calculator.dart.

VDOT is Jack Daniels' running fitness score. It is essentially your VO₂ max, but derived from a recent race performance rather than a lab test. Two runners with the same VDOT are predicted to run roughly the same race times across all common distances and should train at very similar paces. VDOT typically lands in the 30s for new runners, 50s for committed amateur racers, and 70s+ for elites.

Riegel's equation is most accurate when the input race is reasonably close to the target distance. Predicting a marathon from a recent half marathon is dependable; predicting a marathon from an 800m time is not. Daniels has noted that Riegel slightly overpredicts the marathon for runners who haven't done specific marathon-pace work - if you've never run more than 10K, treat the predicted marathon time as a ceiling, not a forecast. Use our BQ predictor to see if a predicted marathon would qualify.

Most runners run easy days too hard. Daniels sets easy pace at roughly 67% of VO₂max - usually 60-90 seconds slower per mile than threshold pace. The point of easy running is to build aerobic capacity and recover, not add stress. If easy feels too slow, that almost always means you've been running easy days at steady or marathon effort, which is one of the most common reasons fitness plateaus. If the prescribed easy pace surprises you, trust the math and slow down.

A standard week for an intermediate distance runner:

• 4-5 easy runs at easy pace
• One threshold session (4-5 miles continuous, or 4-6 x 1 mile with short rest)
• One interval session (5-8 x 800m to 1000m at interval pace, equal-time rest)
• Long run at easy, fading toward steady in the final third
• Repeat and sprint paces show up sparingly - usually as 200-400m strides or short reps with full rest, used to maintain leg speed

NXT RUN takes this further by automatically adjusting which workout falls on which day based on how you're feeling and how prior runs went.

Pace is the cleanest target on flat ground in moderate weather. On hot days, in heavy wind, on hills, or at altitude, heart rate and RPE (rating of perceived exertion) become more reliable than the raw pace number. The NXT RUN app cross-references all three. For this calculator, treat the paces as targets for ideal conditions and shift effort by 10-30 seconds per mile slower in heat, headwind, or hilly terrain.