How to Calculate Your Split Times in an Ultra-Trail? The Complete Guide

Most runners calculate their ultra-trail split times with a simple rule: distance divided by an average speed. The result? A plan that collapses at the first real climb. On an ultra, this approach is not just imprecise — it can cost you a finish outside the cut-off, a hypoglycaemia at mid-race or a missed crew handover at the wrong moment.

In this complete guide, you'll learn how to calculate your trail split times realistically: the 4 factors that really make the difference, a manual method you can apply without a tool, and how to automate all of it to get forecasts accurate to the minute — as at the Euskal Trail 130: target 20h59, actual 20h57, a 2-minute gap over 21 hours of racing.

Why classic methods don't work in ultra-trail

There are two classic approaches to estimating your trail split times. Both are insufficient as soon as you're talking about an ultra.

Naismith's rule: useful but incomplete

Naismith's rule estimates 1 hour for 5 km on the flat, plus 1 hour for every 600 m of D+. It's a good starting point for hiking — but it ignores technical descents, altitude, fatigue accumulated after 12 hours of effort, and the technicality of the terrain. In ultra-trail, it's not enough.

Average km/h: dangerous in the mountains

Even more widespread: "I run at 6 km/h, so my 100 km will take 16h40." The problem? 6 km/h on the flat at 400 m altitude has nothing to do with 6 km/h on a 15% climb at 2,200 m. Difficulty is not proportional to distance.

A worked example makes the point:

• Segment A: 10 km, flat, altitude 800 m → pace 6 min/km → 1h00
• Segment B: 8 km, 600 m D+, altitude 1,800 m → downhill coefficient −35%, altitude coefficient −8% → actual pace ~14 min/km → 1h52

Same distance (or nearly), but a ×2 factor on the time. That's why average km/h doesn't work for calculating an ultra-trail race plan.

The 4 factors that really affect your split times

To build a realistic trail race strategy, four parameters are essential. Ignore them and you're doomed to improvise during the race.

1. Positive and negative elevation

The impact of D+ is intuitive: climbing costs energy and slows you down. But D− is often underestimated. A steep 10 km descent after 15 hours of racing destroys the quadriceps. In the final hours of an ultra, descents can be as penalising as climbs — and must be budgeted into the plan.

2. Gradient

Two segments with the same D+ are not run in the same way. A 12% climb is run as a jog; a 25% climb is walked economically. An 8% descent is rolling; an 18% descent on technical terrain calls for caution. The mountain trail pace depends on the precise gradient, not just the raw elevation gain.

3. Altitude

Above 1,500 m, thinner air weighs on performance: roughly 3 to 5% less aerobic capacity for every extra 500 m. On an ultra with passes at 2,500 m, the cumulative effect is considerable — especially if you train on the flat. A segment at 2,400 m altitude with 800 m of D+ can take 18 to 22% longer than the same segment at 600 m.

4. The fatigue factor

This is the most underestimated parameter in ultra-trail preparation. Nobody runs at the same pace in hour 20 as in hour 2. Gradual slowing over time is inevitable — and predictable, if you build it into the plan from the start.

The fatigue factor: the key nobody calculates

In ultra-trail preparation, the trail fatigue factor is the great forgotten one. We talk about elevation, nutrition, training — rarely about the systematic slowing of pace over the kilometres. Yet this is often the parameter that determines whether you arrive within the cut-offs or collapse in the second half.

Definition and simplified formula

The fatigue factor measures the speed gap between the first and second half of a race:

Concrete examples

• Kilian Jornet, UTMB 2022: a fatigue factor of about +14% — he ran the second half 14% slower than the first. And that's him.
• Experienced amateur runner: a fatigue factor of +25 to 30% on average. Almost a third less speed in the second part.
• Starting too fast: a fatigue factor of +40 to +50% — a sign of poor management that often ends in a dropout or forced walking over the last 20 km.

This figure forces you to be honest with yourself: you won't hold your starting pace to the finish. Planning otherwise means planning to blow up. That's precisely why most bad finishes in an ultra are decided in the last 20 kilometres — not because the runner is under-trained, but because they mismanaged their first half.

The manual method: calculating your split times segment by segment

If you want to calculate your trail split times without a digital tool, here is the method in 5 steps. Useful to understand, even if, let's be honest, doing it by hand for a 100 km takes several hours — and remains error-prone.

1. Split the course

Identify natural split points: official aid stations, notable passes, towns you pass through. Aim for 6 to 12 segments for a 100 km — fine enough to be useful, not so many that it becomes unmanageable.

2. Identify the D+ and D− of each segment

Use the course elevation profile (available on the organiser's website) to note the positive elevation, negative elevation and median altitude of each section.

3. Estimate a base speed

Depending on your level: 4.5 km/h for an ultra beginner, 6–7 km/h for an intermediate level, 8–9 km/h for a good level. This speed applies on the flat, early in the race, at low altitude.

4. Apply the elevation and altitude coefficients

For each segment, adjust your base speed according to the D+ (roughly −5% per 100 m of D+ for an intermediate level) and altitude (−4% per 500 m above 1,500 m). You get a corrected pace for each segment.

5. Build in the fatigue factor on later segments

On the second-half segments, multiply the times obtained by your fatigue factor (e.g. ×1.15 for a 15% factor). The further you go into the race, the more the coefficient applies.

To be transparent: this method is tedious, prone to calculation errors, and does not account for the precise gradient angle, cumulative altitude effects or the technicality of the terrain. For a truly reliable plan, a dedicated trail GPX analysis tool is essential.

How Segly automates all of this

Segly is an application created by two engineers passionate about ultra-trail. It automates the calculation of trail split times by integrating all the parameters the manual method doesn't handle well.

GPX import → automatic analysis

You import your GPX file (exported from Garmin, Suunto or Strava, or downloaded from the race website). Segly analyses the course segment by segment and calculates for each: distance, D+, D−, average gradient, median altitude. No account required, no data shared without your consent.

The 3 criteria built into the calculation

• Positive and negative elevation: real impact on your pace, differentiated uphill/downhill
• Gradient: distinction between steep climb / gentle descent / technical terrain
• Altitude: coefficient automatically applied from 1,500 m

An adjustable fatigue factor

You set your trail fatigue factor according to your level and goal. 10% for a short trail, 15–20% for a 100 km, 25% for a long ultra. Segly applies this coefficient progressively on the later segments — a realistic degradation, not a simple binary multiplier.

Manual segmentation and export

You can re-split the automatic segments to align segment ends with your actual aid stations. You get precise aid-station split times, exportable as a PDF, shareable with your crew via a dedicated read-only link.

The proof? On the Euskal Trail 130 (128 km, 7,500 m D+), the Segly plan forecast 20h59. Actual result: 20h57 — a 2-minute gap over 21 hours of effort. Read the full feedback →

Worked example: preparing your split times on a 100 km

Here is a practical case, fictional but realistic: an intermediate runner, goal 18 hours on a 100 km / 5,500 m D+ course. Fatigue factor set at 15%. Course split into 6 segments aligned with the official aid stations:

This table lets your crew know exactly when to expect you at each aid station. If you reach Aid station 2 25 minutes late, the signal is clear. If you're ahead on the early segments, it's better to manage than to accelerate.

Common mistakes to avoid in your race plan

Even with a good method, these pitfalls come up again and again in ultra-trail preparation.

• Going out too fast: the number-one mistake. Start enthusiasm masks fatigue. If you reach mid-race 1 hour ahead, you'll lose it again in the second part — often with interest.
• Underestimating night fatigue: at night, everything slows — reflexes, alertness, morale. A plan that ignores the night section is an incomplete plan.
• Ignoring the impact of altitude: on a course with passes above 2,000 m, not correcting your paces for altitude means planning in a vacuum.
• Not allowing a margin on technical segments: a scree slope or an exposed ridge can double your time over 2 km. These sections deserve a specific coefficient you can adjust.
• Not sharing your plan with your crew: a plan kept to yourself only serves you. Your team needs your split times to be in the right place — and to know when to worry.