One engine for every page
All calculators and printed charts use one calculation engine. Distances are the official ones: a mile is exactly 1,609.344 m (the international mile), a marathon 42.195 km and a half marathon 21.0975 km. Times are kept to the second and rounded only when shown. Before each release, automated tests compare the engine with hand-worked values.
Pace, splits and charts
Pace = time / distance; speed = distance / time. Negative and positive splits change pace in a straight line from start to finish so that the first half minus the second half equals the chosen share of the goal time; the finish time does not change. Elevation in the course pace planner is resampled every 10 m and smoothed before slopes are measured.
Race prediction
Riegel (1981), American Scientist 69(3): T2 = T1 x (D2 / D1)^k with the popular exponent k = 1.06 (and 1.08 as a cautious option). Daniels and Gilbert (1979), Oxygen Power: oxygen cost VO2 = -4.60 + 0.182258 v + 0.000104 v^2 (v in metres per minute) and the share of VO2 max sustainable for t minutes, 0.8 + 0.1894393 e^(-0.012778 t) + 0.2989558 e^(-0.1932605 t). VDOT and race equivalents are computed from these two equations, not copied from any book's tables. Your own exponent from two races is k = ln(T2 / T1) / ln(D2 / D1).
Training paces and heart rate
Training paces use Daniels' published intensity bands as a share of VO2 max (easy 59 to 74%, threshold 83 to 88%, interval 95 to 100%); marathon pace is the model's marathon prediction; repetition pace is our choice of your predicted mile race pace. Maximum heart rate: Tanaka, Monahan and Seals (2001), 208 - 0.7 x age, or 220 - age; heart rate reserve by the Karvonen method. The five 10% zones are a common model, not a prescription.
Energy and calories
Walking, running and treadmill: the ACSM metabolic equations (walking VO2 = 0.1 S + 1.8 S G + 3.5; running VO2 = 0.2 S + 0.9 S G + 3.5; S in metres per minute, G the grade), with 4.9 kcal per litre of oxygen from ACSM's table. Leg cycling: VO2 = 1.8 x work rate / body mass + 7. Other activities: MET values from the 2024 Adult Compendium of Physical Activities, at 3.5 ml/kg/min per MET. Outdoor cycling: power from rolling resistance, slope and air drag, with drag area and rolling resistance shown as assumptions you can change, and 20% gross efficiency. Rowing: Concept2's watts = 2.80 / pace^3 and monitor calorie formula. Net calories subtract the resting 3.5 ml/kg/min (1 MET).
Slopes, track and age grading
Grade-adjusted pace and even-effort course splits use the energy cost of running on slopes from Minetti and colleagues (2002). Lane distances are calculated from World Athletics rule TR14 (lanes 1.22 m wide, measured 0.30 m from the kerb in lane 1 and 0.20 m from the line in other lanes). Age grading uses Alan Jones's 2025 road age-grading tables (CC0), approved by the USATF Masters Long Distance Running Council.
What we do not do
No medical, coaching or diet advice; no weight-loss promises; no data that changes every year such as qualifying standards or records; no accounts. Everything you type stays on your device. Each tool page lists its own sources at the bottom with the date they were checked. Full working notes on every source, including the ones we could not confirm and therefore did not use, are kept by the editor; corrections are welcome on the contact page.