30+ endurance calculators, built over the years for triathletes, runners, cyclists, and swimmers. Free to use, free to share. The reason is simple: triathlon and endurance sports are hard enough, this is one of the ways we try to help.
These tools give estimates, useful inputs, not the whole picture. Pair the numbers with what your body tells you and, when it matters, real testing at the Performance Lab. Want results saved to your athlete profile across sessions, plus the race-planning wizard, race-day fueling planner, and FIT-file analytics? The same calculators (and more) live inside Grepiac with per-athlete storage. Found a mistake or want a calculator added? Tell us.
Plug in one effort, get estimated finish times across 1.6 km to a marathon. Three columns (Perfect / Normal / Poor) for the realistic range.
Open → Advanced running predictorTighter prediction using three race points (best 400 m, 1600 m, 5000 m). Recommended alongside the single-point version above.
Open → Triathlon finish timePick a race distance, enter swim/bike/run paces and transition times. Get cumulative splits and check if you finish under cutoffs.
Open → Power to finish (cycling)Pick a race, 130+ courses including Kona, Lanzarote, Roth, every IM 70.3 worldwide. Enter power strategy or target time. Get finish time, splits, route map, elevation profile.
Open → VO2 → cycling powerEstimate target power from VO2max + efficiency + RER + duration. Includes VO2 kinetics, FTP calibration, and Coggan profile lookup.
Open → VO2 → run finish timePredict run finish time, pace, and running power from VO2max + economy (Cr) + fraction. Per-distance plausibility flags.
Open → VO2 → swim timePredict swim time + pace per 100. Pool / OW / wetsuit / drafting / chop modifiers. Cr calibration from a known race time.
Open → Boston qualifier (BQ)Enter your age group, see exactly how fast you have to run to make it to Boston (without donating a gazillion dollars).
Open → IM & IM 70.3 Worlds qualifierQuick check on whether you’re competitive enough for Worlds and podiums. Uses the 2026 maths.
Open →Enter duration and fueling mode (minimal, recommended, max). Get suggested intake by hour. Useful reminder for long sessions.
Open → Triathlon race-day fueling plannerFull race-day fueling and hydration plan for Sprint, Olympic, 70.3, and Ironman. 120+ products, transitions, aid stations, glycogen trajectory, race scorecard, and a printable cheat sheet. Strongly recommended for long events.
Open → Sweat rateEnter duration, weight before, weight after, and liquid consumed, get sweat rate and approximate sodium loss.
Open → Triathlon calorie burnQuick way to estimate how many calories you’re burning when you train across swim, bike, and run.
Open → Resting Metabolic Rate (RMR)Estimate the energy your body needs at rest. Eating below RMR can lead to health and performance issues. Pair with an actual RMR test when it matters.
Open → Macros & TDEETotal Daily Energy Expenditure based on your training, RMR, and more, with macro split suggestions.
Open → RPE / RERA loose link between rate of perceived exertion and respiratory exchange ratio, rough way to gauge fat vs. carb burn.
Open →Simple table showing how chainring + cassette combinations turn the wheels. Useful for understanding what you have.
Open → Gear × Speed × PowerAdvanced version of the gear table, speed and power output included. Useful for checking gears for climbs.
Open → Tire pressure guideImprove safety, speed, and comfort with correct tire pressure for road, MTB, recumbent, tandem, and more.
Open → Critical power (CP)Go beyond FTP. Estimate critical power using three maximal efforts to fully appreciate aerobic and anaerobic capacity.
Open → CdA estimateManual entry or load a FIT file to estimate your aerodynamic drag (CdA). Strongly recommended if you like going faster at the same power. Pair with field aero testing.
Open → Crr calculatorReverse calculator: if you know power, speed, and CdA, this confirms your tire rolling resistance. Recommended with aero sensor data.
Open →Enter two distances (100–400 / 200–400 / whatever you have) to get CSS, predicted finish times by length, and training zones.
Open → EBT I, Stroke efficiencyLooks at your height, stroke rate, and distance per stroke to tell you whether to increase rate or improve DPS.
Open → EBT II, Stroke tipping pointsDifferent sets of 50 m at varying stroke rates. Highlights tipping points where efficiency plummets and shows normal DPS/SR ranges by speed.
Open →LTHR-based heart rate zones for bike and run, including a dedicated Zone 2 band. Pair with VO₂max testing for real metabolic zones.
Open → Reactive strength index (RSI)Measured via a simple jump test, estimates how much bounce and free speed you can get in running. Strength & plyo are your friends.
Open →Calculate body fat % and lean body mass from caliper measurements, so weight loss isn’t actually muscle loss.
Open → Strength training intensityHow much to train across each phase, early base to race. Intensities, sets, rest intervals, and example exercises.
Open →Compare up to 3 FIT files from different devices on the same activity. Auto-aligned, with synced-zoom charts for power / HR / cadence / speed / GPS / elevation, side-by-side power-duration curves, and an automated diagnosis (offset, drift, dropouts).
Open → Multisport FIT splitterSplits a multisport FIT file into individual leg files (swim / T1 / bike / T2 / run). Auto-detected legs with confidence scores, drag-to-adjust boundaries, sport-type override, route map + sanity-check chart, export per-leg TCX or FIT for WKO5 / TrainingPeaks / intervals.icu.
Open → FIT file merger / stitcherStitch a single intended activity back into one continuous .fit file when the device split it into two or more (battery died, accidental stop/start, coffee-stop got recorded as its own ride). Two modes: stitch (treat as continuous) or preserve (keep real-time gaps). Multi-device parallel merge with developer-field preservation lives on Grepiac.
Open → All-in-one converterPace, speed, temperature, weight, height, fluid, RPE, carb concentration, nine converters in one tool, with apparel guidance and bottle-mix sanity checks.
Open →Calculators give estimates from a few inputs. The Performance Lab measures, VO₂max, RMR, CdA, gait, swim mechanics. Pair the two and you stop guessing.
A coach is in here too, mistakes happen. Spot one? Drop a note and we’ll fix it.
