Trail running calculators: what each one actually measures
Trail running calculators turn a hilly course into numbers you can plan with: grade-adjusted pace and flat-equivalent distance for how long the ground takes, energy cost for what it burns, VAM for how fast you climb, and carbs per hour for the fuel you plan to take in. Each answers a different question, and they disagree most on the way down: a 5 km descent at −20 % counts as 5.8 flat kilometres in time but about 2.5 in energy. This post explains each calculation from its source, shows where the models split, and points to the free GPX course analyzer.
TL;DR
- Time and energy are different currencies. Share out a target time with grade-adjusted pace or flat-equivalent distance; count kilocalories with the energy cost. On steep descents the two differ by more than a factor of two.
- In Pendio's time model a kilometre at +10 % takes 1.55× as long as on the flat at the same effort, at −10 % 0.87×, and descents steeper than about −16 % are slower than the flat.
- Minetti's measured energy cost of running is 3.40 J/kg/m on the level and 1.73 at −20 %: a steep descent is cheap in energy even when it's slow.
- VAM depends on the grade: in the time model, the same effort climbs 645 m/h on a 10 % slope and 794 m/h on a 20 % one. Compare VAM on the same climb.
- Carbs per hour is capped by absorption, not set by the course's kilocalories: up to about 60 g/h from one carbohydrate source, and about 90 g/h for ultras from multiple transportable carbohydrates (Jeukendrup, 2014).
Four questions, four calculations
Most calculator pages give you one number. Before you plan with it, ask which question it answers. A hilly course raises four, and each needs its own model:
| Question | Calculation | Unit | Built from |
|---|---|---|---|
| How long will this ground take at an even effort? | Grade-adjusted pace (GAP), flat-equivalent distance | min/km, flat km | Pace and heart rate from real runs |
| How much energy does it cost? | Energy cost of running on slopes | kcal | Treadmill measurements |
| How fast am I climbing? | VAM | m/h | Height gained and the clock |
| How many carbs do I plan per hour? | Carbs per hour | g/h | Absorption research |
The first two can both be expressed in flat kilometres. They don’t give the same answer.
Time: grade-adjusted pace and flat-equivalent distance
Grade-adjusted pace is the pace you’d run on the flat for the same effort. Strava’s version “adjusts your running pace for terrain grade”; its help page says the downhill adjustment peaks around −10 % and that GAP “does not account for the technical difficulty or condition of the terrain” (Strava Support).
Pendio’s GAP is a fit to Strava’s heart-rate-based GAP model of 2017 (guide). Compared with the flat at the same effort, a kilometre takes (guide):
- +20 %: 2.52× as long
- +10 %: 1.55×
- −10 %: 0.87×
- −20 %: 1.16×
- −30 %: 1.87×
Downhills save time down to about −16 %, the most around −8 %, and cost time beyond that (GPX course analyzer).
Flat-equivalent distance adds those factors up over a whole course, every 10 m counted as its length times the factor for its grade. Measured this way, a flat 10K counts as 10.0 km and a 10 km out-and-back with 1,000 m of climbing as 20.1 km (pendio.app).
A worked example
Three Passes 50 is the made-up demo course used on pendio.app, not a real race: 50.3 km (31.3 mi), ↑2,757 m ↓2,757 m. The GPX course analyzer reads it as 67.8 flat-equivalent kilometres.
Take the home page’s 7:30 target with 11 minutes of stops (pendio.app). That leaves 7:19 of moving time over 67.8 flat km: a grade-adjusted 6:28 /km (10:25 /mi), held the whole way. On a +10 % stretch the watch reads about 10:02 /km; on a −10 % one, about 5:38 /km. Race-plan splits come from the same sum: each leg gets the share of the moving time that its flat kilometres are of the total (guide).
That is what the time model is for. It doesn’t predict how fast you’ll be; it shares out a target you chose so the effort stays even.
Energy: the same hill in a different currency
Minetti and colleagues measured the energy cost of running on a treadmill in 10 runners, at slopes from −45 % to +45 % (Minetti et al., 2002). Running cost 3.40 J/kg/m on the level, 1.73 at −20 %, 3.92 at −45 % and 18.93 at +45 %.
Put that next to the time model and the descents come apart. At −20 % the energy cost is about half the flat’s (0.51×); the time model says each kilometre takes 1.16× as long. Take a 5 km descent at −20 %, 1,000 m (3,280 ft) of drop:
| Currency | Factor at −20 % | 5 km of descent counts as |
|---|---|---|
| Time (GAP model) | 1.16 | 5.8 flat km |
| Energy (Minetti, 2002) | 0.51 | about 2.5 flat km |
Both are a “flat-equivalent distance”, and they differ by a factor of 2.3. One describes how long the ground takes at the same effort; the other, how much energy each metre costs. A pace calculator that converts by energy gives a steep descent less than half the time the time model does; a fuel calculator that converts by time overstates what the descent burns.
On pendio.app the two stay apart: race-plan splits come from the time model, and the kilocalories on the Fueling card from Minetti’s energy cost of running on slopes, priced grade by grade at the weight you typed in the app (guide).
VAM: climbing speed, and why the grade matters
VAM is the height you climb per hour, in metres per hour (Strava Support). Polar’s watches show it in trail running and hiking profiles, among others (Polar Support). Divide the height gained by the time it took: as an example, 400 m in 30 minutes is 800 m/h.
It’s simple, and it hides the grade. Run the time model backwards with an example effort worth 6:00 /km on the flat:
- On a +10 % slope that effort is 9:18 /km, which climbs 645 m/h.
- On +20 % it’s 15:07 /km, which climbs 794 m/h.
Same effort, about 23 % more VAM on the steeper slope, because every metre of trail lifts you twice as high. A VAM figure compares well on the same climb, or between climbs of a similar grade; across grades it partly measures the slope. It also inherits the model’s limits: like Strava’s GAP, it knows nothing of rocks, roots or mud.
The Pendio app recognises the same climb every time you run it and lines its efforts up, ranked by score, VAM and GAP (guide): the like-for-like comparison VAM needs.
Carbs per hour: capped by the gut, not by the course
The energy cost says what the ground burns. It doesn’t say what to eat, because the carbohydrate your body can use from what you eat is limited by absorption. Jeukendrup’s 2014 review puts it plainly: “exogenous carbohydrate oxidation is limited by the intestinal absorption of carbohydrates” (Jeukendrup, 2014). The same review says:
- a single carbohydrate source can be oxidised at up to about 60 g/h;
- for ultra-endurance events the recommendation is about 90 g/h, and at such rates the carbohydrate must be multiple transportable carbohydrates;
- athletes should practise the strategy in training.
So a carbs-per-hour figure is a rate times a duration, with a ceiling from that research, not a conversion of the course’s kilocalories.
The Fueling card’s Auto rate picks 60, 90 or 100 g/h from how hard the target is for you (that needs the threshold pace you typed in the app; without it, 90 g/h), or you choose 60–120 g/h. The plan holds the full rate for three hours, then about 4 % less each hour, never below 70 %, and from 90 g/h it reminds you to practise that rate in training (guide). That taper is Pendio’s planning choice, not a figure from the review. On Three Passes 50 at a 7:30 target and 90 g/h, it comes to 630 g of carbs, 84 g/h on average (pendio.app).
Where to run the numbers
- The free GPX course analyzer: flat-equivalent distance, D+ and D−, the hardest climb and descent, D+ per 10 km, the 13 grade bands and the biggest climbs and descents. The GPX is read in your browser and never uploaded.
- pendio.app/app: race plans split by flat-equivalent distance, the Fueling card’s energy and carbs, and a one-page pace card PDF. The guide shows the method step by step.
- The iPhone app, in private beta: GAP for every run, and VAM on its climbs.
Sources
- Minetti AE, Moia C, Roi GS, Susta D, Ferretti G. “Energy cost of walking and running at extreme uphill and downhill slopes.” Journal of Applied Physiology 93(3): 1039–1046, 2002. doi.org/10.1152/japplphysiol.01177.2001
- Jeukendrup A. “A step towards personalized sports nutrition: carbohydrate intake during exercise.” Sports Medicine 44 (Suppl 1): S25–S33, 2014. PMC4008807
- Strava Support. “What is Grade-Adjusted Pace (GAP) on Strava?” Checked 16 October 2026. support.strava.com
- Strava Support. “What is Vertical Ascent in Meters (VAM) on Strava?” Checked 16 October 2026. support.strava.com
- Polar Support. “Vertical speed & VAM.” Checked 16 October 2026. support.polar.com
- Pendio. “How Pendio works”: split times, fueling and the training numbers. pendio.app/guide
- Pendio. GPX course analyzer, with the demo course Three Passes 50. pendio.app/tools/gpx-course-analyzer
- Pendio. Home page: the flat 10K and the 1,000 m grind, and the Three Passes 50 race plan and fuel. pendio.app
Questions
Which trail running calculator should I use to pace a race?
One built on a time model, grade-adjusted pace or flat-equivalent distance, so a target time is shared out at one even effort and the climbs simply take longer. A calculator that converts the course to flat kilometres by energy gives a −20 % descent less than half the time the time model does.
Why do two GAP calculators give different numbers for the same run?
Some are built on energy measured on a treadmill, others on pace and heart rate from real runs, and they treat elevation differently. The GPX course analyzer resamples the course every 10 m and smooths the elevation over 70 m before it adds anything up; a tool that counts every GPS bump reports more climbing.
What is a good VAM for trail running?
There is no single number, because VAM depends on the grade as well as on you. Compare it on the same climb over time, or between climbs of a similar steepness.
How many carbs per hour do I need in an ultra?
Jeukendrup's 2014 review gives about 90 g/h for ultra-endurance events, from multiple transportable carbohydrates, and up to about 60 g/h from a single source, and recommends practising the strategy in training. What your gut handles is individual; if fuelling gives you gut problems, talk to a doctor or a sports dietitian.
The bottom line
Before you trust a trail running calculator, check which currency it counts in. Use the time model (grade-adjusted pace, flat-equivalent distance) to share out a target time, the energy model for kilocalories, VAM only on like-for-like climbs, and carbs per hour as a rate the research caps rather than a conversion of the energy. To see your race in flat kilometres and where its climbing sits, drop its GPX in the free GPX course analyzer.