Flat-equivalent distance: how far a trail race is on the flat
In flat kilometres, a hilly trail race is its distance plus about 0.4–0.8 km for every 100 m it climbs: on the 19 organisers' course files on Pendio's race pages, each 100 m of climbing added 438–820 m, and the courses came to 1.16–1.55 times their length. That is their flat-equivalent distance, the flat distance that would take as long at the same effort. UTMB's 2026 file, 173.8 km with ↑10,112 m by Pendio's count, comes to 247 flat km (153 mi). The popular rule of thumb, 1 km per 100 m of climbing, is higher than all 19. This post works the rate out grade by grade, shows why steepness moves it, and says what it leaves out.
TL;DR
- Flat-equivalent distance is the flat distance that would take as long at the same effort: 1.16–1.55 times the distance on the 19 organisers' course files on Pendio's race pages.
- There's no single exchange rate. In Pendio's grade-adjusted pace model, 100 m of climbing adds 445 m of flat distance at 5 % and 970 m at 30 %; 100 m of descent takes 235 m off at −5 % and adds 290 m at −30 %.
- On a hill run up and back down at one grade, each 100 m of height adds about 42 m per percentage point: 420 m at 10 %, 840 m at 20 %. ITRA's km-effort, 1 km per 100 m, matches a hill at about 24 % both ways.
- Two made-up 30 km courses with ↑1,000 m come to 34.2 and 38.4 flat km: at an effort worth 6:00 /km on the flat, the steeper one takes 25 minutes longer, though km-effort scores both 40.
- It converts the slope only: not the surface, the dark or the hours in your legs, and it isn't a finish-time prediction.
What is flat-equivalent distance?
Flat-equivalent distance is the flat distance that would take as long as a course at the same effort. A 2018 study built on the results of 616 Belgian races defines it the same way, “the distances that would be run in the same time if the ground was flat”, and concluded that equivalent distances “are more relevant than the actual distances as race time predictors” (de Smet et al., 2018).
Pendio counts each 10 m of the course as its length times a time factor for its grade (GPX course analyzer). The factors come from a fit to Strava’s heart-rate-based GAP model of 2017 (guide). Strava built that model from about 6 million runs by 240,000 athletes; its pace adjustment bottoms out at 0.88, at −9 %, and by −18 % is back to 1.0 (Robb, 2017). Pendio’s fit has the same shape: descents are quickest around −8 % and slower than the flat beyond about −16 %.
How much flat distance is 100 m of climbing worth?
In Pendio’s model, 100 m (330 ft) of climbing adds 445 m of flat distance at 5 % and 970 m at 30 %: the steeper the ground, the more each metre of height costs. Descents take distance off until they get steep, then add it. The table gives what each 100 m of height adds to its own length, from the model’s time factors (a kilometre at +10 % takes 1.55 times as long as on the flat, at −10 % 0.87 times; guide):
| Grade | 100 m of climbing | 100 m of descent | 100 m up, then down |
|---|---|---|---|
| 5 % | +445 m | −235 m | +210 m |
| 10 % | +550 m | −130 m | +420 m |
| 15 % | +655 m | −25 m | +630 m |
| 20 % | +760 m | +80 m | +840 m |
| 30 % | +970 m | +290 m | +1,260 m |
Read the last column for a race that ends where it starts: there, every metre climbed is also descended. A hill at 10 % both ways adds 420 m per 100 m of height, one at 20 % adds 840 m: on a hill climbed and descended at one grade, the cost grows in step with the grade, about 42 m for every percentage point.
Why do the rules of thumb disagree?
The rules of thumb disagree because each uses one rate for every metre of climbing, taken from different people on different ground:
| Source | Flat distance per 100 m of climbing |
|---|---|
| ITRA’s km-effort | 1,000 m |
| Scarf, from fell-running records | 800 m for men, 1,000 m for women |
| Naismith’s rule, hill walking | 792 m |
| Datasport’s trail pacing rule | 600–700 m |
| Scarf, a mountain road relay | 440 m |
| Scarf, treadmill experiments | 330 m |
| Pendio’s model, climbing at 5–30 % | 445–970 m |
ITRA counts 1 km of distance as 1 km-effort and “+100m vertical meters” as another, from “the distance and the elevation gain” (ITRA), and calls the result “a flat equivalent distance (km-effort)” (ITRA). Naismith’s rule “implies that 1 m of ascent is equivalent to 7.92 m of horizontal travel”; Scarf’s analysis of fell-running records supports it and recommends that “male runners and walkers use a 1:8 equivalence ratio and females a 1:10 ratio”, against 1:3.3 from treadmill experiments and 1:4.4 from a mountain road relay (Scarf, 2007). Datasport adds “600 to 700 additional “flat” meters” for every 100 m of gain (Datasport, 2025).
Km-effort and Datasport’s rule count only the climbing. Set against the model, each is right at one grade: km-effort’s 1,000 m is a hill at about 24 % both ways, Datasport’s 600–700 m climbing at about 12–17 %.
Can two courses with the same distance and climbing differ?
Two courses with the same distance and climbing can differ: the steeper one is longer on the flat. Take two made-up 30 km courses, each ↑1,000 m ↓1,000 m:
| Course A | Course B | |
|---|---|---|
| Climb | 10 km at +10 % | 5 km at +20 % |
| Descent | 10 km at −10 % | 5 km at −20 % |
| Flat | 10 km | 20 km |
| Flat-equivalent distance | 34.2 km | 38.4 km |
| Km-effort | 40 | 40 |
| Moving time at an effort worth 6:00 /km on the flat | 3:25 | 3:50 |
Course B’s climb costs 760 m per 100 m against A’s 550 m, and its descent adds 80 m per 100 m where A’s takes 130 m off. Km-effort can’t tell them apart.
What do real race courses come to?
On the 19 organisers’ course files on Pendio’s race pages, each 100 m of climbing added 438–820 m of flat distance, always less than km-effort’s 1,000 m; km-effort, counted on the same file, put every course 8–18 % higher than the model. Seven of them, lowest rate first, as Pendio measured each organiser’s file in October 2026:
| Course (year of the organiser’s file) | Distance | D+ | Flat-equivalent | × the distance | Flat distance per 100 m of D+ |
|---|---|---|---|---|---|
| Chianti Marathon Trail (2027) | 46.3 km | 1,650 m | 53.5 km | 1.16 | 438 m |
| Ultra Trail Chianti Castles (2027) | 122.0 km | 5,006 m | 146.1 km | 1.20 | 483 m |
| Lavaredo 120K (2026) | 119.5 km | 5,471 m | 151.3 km | 1.27 | 582 m |
| OCC (2026) | 60.1 km | 3,363 m | 83.2 km | 1.38 | 684 m |
| UTMB (2026) | 173.8 km | 10,112 m | 247.0 km | 1.42 | 724 m |
| TDS (2026) | 148.2 km | 9,421 m | 222.4 km | 1.50 | 788 m |
| Val d’Aran EXP (2026) | 32.2 km | 2,148 m | 49.8 km | 1.55 | 820 m |
The rate follows the steepness. On the Chianti Marathon Trail’s file, 350 m of the 46.3 km is steeper than ±20 %; on the EXP’s, 9.1 km of the 32.2 km. Each course page lists its grade bands and links the organiser’s race page and file. The pages are unofficial: Pendio isn’t affiliated with these races.
Is a flat-equivalent kilometre a road kilometre?
A flat-equivalent kilometre matches a flat one only for the slope: it turns the gradient of every 10 m into time at one effort, and leaves the rest of the ground and the day out:
- The surface. Strava says its GAP “does not account for the technical difficulty or condition of the terrain” (Strava Support). A rocky descent at −20 % counts the same as a smooth one.
- The hours. The model holds one effort from start to finish. Pendio’s race plans have two optional factors for slowing late or in the dark, fatigue drift and a night factor, off by default (guide).
- Energy. In kilocalories a steep descent is cheap rather than slow; fuel needs another model (trail running calculators).
So 247 flat km doesn’t mean UTMB runs like 247 km of road. It means that at one even effort, its ground takes as long as 247 km of flat would.
How do you find your race’s flat-equivalent distance?
Drop the organiser’s GPX into the free GPX course analyzer: it shows the flat-equivalent distance next to the distance and D+, counted every 10 m, and reads the file in your browser without uploading it. Finding the right file: race elevation profile from a GPX.
- A race course page gives the course’s flat-equivalent kilometres and each leg’s share of the effort (how Pendio measures courses).
- A race plan at pendio.app/app: Show flat km adds each leg’s flat-equivalent distance, the figure the split times share the target by (guide). Building the chart: ultra pace chart with aid station splits.
- By hand, multiply each climb’s and descent’s height by the table’s rate for its average grade, and expect a low result. A 1 km climb at 10 % on average, half at 5 % and half at 15 %, counts as 1.60 flat km, not 1.55: the steep half costs more than the gentle half saves.
Sources
- de Smet D, Verleysen M, Francaux M, Baijot L. “Long-distance running routes’ flat equivalent distances from race results and elevation profiles.” Proceedings of the 6th International Congress on Sport Sciences Research and Technology Support (icSPORTS): 56–62, 2018. doi.org/10.5220/0006937000560062
- Scarf P. “Route choice in mountain navigation, Naismith’s rule, and the equivalence of distance and climb.” Journal of Sports Sciences 25(6): 719–726, 2007. doi.org/10.1080/02640410600874906
- Robb D. “An Improved GAP Model.” Strava Engineering, 3 October 2017. medium.com
- Strava Support. “What is Grade-Adjusted Pace (GAP) on Strava?” Checked 2 November 2026. support.strava.com
- ITRA. “Discover trail running.” Checked 2 November 2026. itra.run
- ITRA. “ITRA Race Score.” Checked 2 November 2026. itra.run
- Datasport. “Pacing on Trails.” 18 May 2025, checked 2 November 2026. datasport.com
- Pendio. Race course pages: each organiser’s file measured with Pendio’s engine, October 2026. pendio.app/blog/races
- Pendio. “How Pendio measures race courses.” pendio.app/blog/races/how-we-measure
- Pendio. GPX course analyzer. pendio.app/tools/gpx-course-analyzer
- Pendio. “How Pendio works.” pendio.app/guide
Questions
Is 100 m of climbing really worth 1 km on the flat?
Only as a rough rule. ITRA's km-effort counts every 100 m of climbing as 1 km at any grade and leaves descents out. In Pendio's grade-adjusted pace model, 100 m of climbing adds 445–970 m between 5 % and 30 %, and on 19 organisers' course files km-effort came out 8–18 % higher than the model.
Do descents make a trail race shorter in flat kilometres?
Gentle ones do, steep ones don't. In Pendio's model, 100 m of descent takes 130 m off at −10 % and adds 290 m at −30 %: a descent counts for less than its length only down to about −16 %.
Can flat-equivalent distance predict my finish time?
Not on its own. It says how long the ground takes compared with the flat at one effort; your finish also depends on fitness, stops, the surface and how much you slow down. Pendio doesn't guess how fast you'll be: a race plan takes a target time you choose and shares it out by flat-equivalent distance (guide).
The bottom line
Count a hilly race on the flat grade by grade, not with one exchange rate: steep climbing costs more per metre than gentle climbing, and gentle descents give time back where steep ones take it. Drop the organiser's GPX for your edition into the free GPX course analyzer to read its flat-equivalent distance, and use it to compare courses and share out a target you chose, not to predict one.