How the prediction works
A trail finish time is really two problems stacked on top of each other. You are projecting your fitness across a new distance, and you are paying for the climbing. Here is the logic this calculator runs on, so the numbers up above make sense.
Step one: project across distance with Riegel
It all starts with Riegel’s endurance formula, T2 = T1 * (D2 / D1)^k, which takes a finish time you already ran and predicts your finish time at a new distance. The exponent k is just how much you slow down as the distance grows. The classic 1.06 fits road races from 5K to the marathon well. For ultra distances we raise it, up toward 1.10 above 100K, because the longer you are on your feet, the more the result comes down to fatigue resistance and not raw speed.
We also work out your Daniels VDOT, a single fitness score pulled from your recent race, and show you the equivalent flat target pace it implies. That gives you a second read on the projection from a different angle, so you are not putting all your trust in one formula.
Step two: pay the vert tax
Distance-only math assumes every mile costs the same, and that is exactly where flat calculators fall apart for trail runners. So we fold in your course climbing with a grade-adjusted-pace model: every 100 ft of climb per mile adds roughly 18 seconds per mile to your pace versus running flat. A 100K with 8,000 ft of gain averages about 128 ft per mile, and that little penalty piles up across every one of those miles into a real chunk of your finish time.
And you can see the cost for yourself. Flip the vert off and you get the flat projection, flip it on and you get the vert-aware one, and the flat-versus-with-vert comparison shows you exactly how many minutes the climbing is adding. No black box here. It is just the same coefficient the Summit Line app runs on your activities.
Why climb-only, and what it leaves out
This model pays for climbing but not for descending. Fast runnable downhills can claw some of that time back, and a true grade-adjusted model would handle it, but the downhill benefit swings a lot more from runner to runner and trail to trail than the climbing cost does, so a generic tool that tried to guess at it would steer you wrong more than it would help. Leaning toward the climb-only penalty keeps the number honest and a touch conservative, and conservative is the right way to lean when you are planning a race.
It also cannot see technical footing, heat, altitude, night running, or where the steep pitches actually fall on the course. Those are real, and sometimes they are huge. But they are also the exact things that hang on your own training and your specific course, which is what the full Summit Line app is built to model.
Garbage in, garbage out: pick a good input race
The projection is only as good as the race you hand it. Use a recent, complete, hard effort at a distance reasonably close to your target. A strong half marathon or marathon projects cleanly up to 50K and 100K. The bigger the leap, say a 5K stretched out to 100 miles, the more the number is resting on endurance you have not actually shown yet, and the more you should treat it as a loose ceiling instead of a plan.
When you are not sure, run the projection off a couple of different recent races and see where they land. If a 50K and a marathon both point to a similar 100K time, you can trust that more than either one on its own.
This calculator gives you a general projection built on standard endurance-running models (Riegel cross-distance prediction and a grade-adjusted-pace penalty). It is not a guarantee of how your race goes. Real finish times come down to training, terrain, weather, pacing, and the day. Always check your assumptions against your own training data.