Free running calculators
Run the numbers.
Then run the race.
Upload a GPX course and get a hill-, heat-, and wind-aware pace plan with the Course Pace Planner, or reach for pace, race-time prediction, VDOT training paces, easy pace and Zone 2, splits, age-graded scoring, training load, training volume race prediction, heart rate training zones, goal time reality checks, heat pace adjustment, grade-adjusted pace, elevation gain time cost, Yasso 800s, and cadence and stride length: 19 free calculators, no login required. Connect Strava, Whoop, or Garmin later for a version personalised to your actual training.
19 free calculators
Every calculator runs entirely in your browser. The one exception is the Course Pace Planner's optional forecast fetch and map tiles — see the privacy page for details.
Pace Calculator
Enter any two of pace, time, and distance to solve for the third.
Race Time Predictor
Predict equivalent race times at other distances from one recent result.
VDOT / Training Pace Calculator
Get your Daniels & Gilbert VDOT and suggested training paces.
Easy Pace & Zone 2 Calculator
Find your real easy pace range and Zone 2 heart rate from one recent race.
Split Calculator
Build an even- or negative-split pacing table for a goal time.
Negative Split Planner
Plan a negative split race strategy: first- and second-half time and pace from one split difference.
Age-Graded Performance Calculator
Score a performance against WMA age-grading factors.
Training Load (ACWR) Calculator
Check your acute:chronic workload ratio against injury-risk research zones.
Training Volume Race Predictor
Predict your marathon time, plus 5K, 10K, and half marathon, from your weekly training volume and pace.
Heart Rate Training Zones Calculator
Get your 5-zone heart rate training breakdown from max and resting heart rate.
Goal Time Reality Check
Compare your current VDOT fitness to what your goal race time requires.
Heat Pace Adjustment Calculator
Adjust your target pace for temperature and dew point (or humidity).
Grade-Adjusted Pace Calculator
Convert a pace on a hill to its flat-equivalent effort, or the reverse.
Elevation Gain Time Cost Calculator
Estimate how much elevation gain and loss adds to a course's finish time.
Yasso 800s Calculator
Convert a goal marathon time to a target 800 m repeat pace, or the reverse.
Cadence & Stride Length Calculator
Solve for pace, cadence, or stride length from the other two.
Altitude Pace Adjustment Calculator
Adjust a race time or pace for altitude, or convert an altitude result back to sea level.
Course Pace Planner
Upload a GPX course and get a predicted or target-time pace plan from its grade, heat, and wind.
Run/Walk Calculator
Predict your run/walk finish time, or solve the run pace a goal time needs.
Beyond generic formulas
The free tools use standard published formulas. Connect a provider and Slow Run Fast swaps those generic estimates for numbers grounded in your own training history.
Strava
Sync runs automatically and see your real training load behind every number.
Whoop
Layer in recovery and strain so pacing advice accounts for how your body is actually doing.
Garmin
Import files directly for splits, HR zones, and power data the free tools can't see.