Methodology
What each tool computes, the model it uses, and where that model stops being reliable.
Race Split Planner
GPX route + goal time
Pace model
Splits are built by converting each mile into a "flat-equivalent" distance using a grade-dependent cost factor, then pacing evenly against that adjusted total rather than the raw mileage. That is an even-effort strategy, not an even-pace one. The cost factor is Strava's empirical Grade Adjusted Pace model (Robb, 2017, An Improved GAP Model, Strava Engineering), fit to heart-rate-equivalent pace across ~6 million real runs rather than to treadmill oxygen cost. Strava's fitted curve is proprietary, so both branches are reconstructed from its published figures: downhill, a parabola through the published minimum (0.88 at about -9% grade) and back to flat cost by -18%, held there for steeper descents; uphill, the published worked example (5:00/km at +5% grade → GAP 4:27/km) gives +2.5% cost per 1% of grade, used as a linear slope. Strava also reports its uphill penalty is pace-dependent, which a single slope does not capture. Grades past +/-45% are clamped, since real GPX grades beyond that are almost always sensor noise on a short segment rather than terrain.
Terrain technicality
Trail and hybrid race types apply an extra downhill cost multiplier (1.15x and 1.075x), since technical footing (roots, rocks, loose surface) keeps runners from converting a descent's gravity assist into speed the way pavement does. This multiplier is a heuristic, not a fitted or cited figure. Treat trail/hybrid splits as an approximation rather than precisely calibrated.
Grade computation
Grade is rise over horizontal (2D) distance, not the diagonal (3D) distance actually run. That is the standard %grade convention; using slope distance instead systematically understates grade, worse as terrain steepens. Because the cost model describes sustained effort (heart rate lags terrain by tens of seconds), raw adjacent-trackpoint deltas are aggregated into ~100 m chunks before computing grade, damping point-to-point GPS/elevation sensor noise that would otherwise be read as real terrain change.
Pace Calculator
Two of pace, time, distance
Units
Everything is converted to meters and seconds, computed there, and converted back, so the result does not depend on which units were typed in. One mile is 1609.344 m and one yard is 0.9144 m, both exact by definition. A marathon is 42.195 km and a half marathon 21.0975 km, the distances World Athletics specifies.
Arithmetic
There is no model here, only time = distance / speed. Given any two of pace, time
and distance the third is exact; the only approximation is rounding for display. Times and paces
are rounded to the nearest second, so reading a rounded pace back into the tool can move a long
finish time by a second or two.
Finish time projection
The projection assumes you hold your current average pace for the rest of the distance. It is arithmetic, not a prediction: it does not model fatigue, terrain or a finishing kick. For a plan that accounts for gradient, use the Race Split Planner.