Free SAM Calculator — Your miles hit harder than you think

Your easy pace anchors everything

SAM is measured against your comfortable conversational pace (= 1.0× per mile). Faster than that and stress climbs super-linearly; slower and it drops off gently. Body weight is optional and nudges the multiplier slightly.

Your easy pace min/mi, e.g. 9:30

Body weight lbs, optional

Distance miles

Average pace min/mi

Elevation gain ft

Elevation loss ft

Surface

Enter a distance and pace to see what the run really cost.

Day	Miles	Pace	Gain ft	Loss ft	Surface	SAM

Rotate your phone (or use a wider screen) to add elevation gain/loss per run.

Fill in this week's runs — leave rest days blank — and see how hard the week really hit.

Track this automatically

RunIntensity reads every run from Apple Health or Google Health Connect — Apple Watch, Garmin, COROS, whatever you wear — and does this math for you, with real elevation and heart-rate data. Then it tells you whether today is a push day or a recovery day.

Every run scored in Stress-Adjusted Miles, automatically
Weekly SAM budget with mid-week pacing tips
Daily Recovery / Freshness readiness verdict from HRV, sleep and resting HR
Free. No account needed for the core app.

How SAM is calculated

No black box. These are the exact multipliers the app uses.

multiplier = (easy pace ÷ your pace) ^ k k = 1.5 slower · 2.6 faster
SAM = miles × multiplier × terrain × fatigue × surface

1. Pace intensity

Faster paces generate exponentially more tissue stress per step: higher ground-reaction forces, more muscle force, less contact time. A split power law anchored at your easy pace captures it — a ~10% rise in tissue stress roughly halves the loading cycles bone tolerates before failure.

2. Terrain

Downhill running increases eccentric muscle loading and peak impact forces; uphill costs more muscular work but lands softer.

Descent	+4% per 100 ft/mi
Ascent	+2% per 100 ft/mi

3. Cumulative fatigue

Glycogen depletion degrades mechanics on long runs — tibial acceleration rises 9–12% after prolonged running as muscles lose shock absorption.

0–6 miles	1.00×
6–13 miles	+1% per mile
13+ miles	+2% per mile (stacks)
Half marathon (13.1)
20-mile long run

4. Surface

Surface stiffness changes ground-reaction force; soft surfaces trade impact for metabolic cost and instability.

5. Body weight (optional)

Ground-reaction force scales with body mass, so heavier runners see more tissue stress per stride. The app uses a conservative exponent because bone cross-section partially compensates.

Weight factor	(lbs ÷ 150) ^ 0.3
Not entered	1.00×

Why not just miles?

Volume alone ignores the exponentially higher stress that faster paces place on bones, tendons and muscles. Heart-rate loads like TRIMP fix the physiology but not the biomechanics — high heart rates don't cause overuse injuries; high tissue loads do. SAM is a damage-equivalent mile.

Key references

Edwards WB (2018). Modeling overuse injuries in sport as a mechanical fatigue phenomenon. Exercise and Sport Sciences Reviews, 46(4), 224–231.
Warden SJ, Edwards WB, Willy RW (2021). Preventing bone stress injuries in runners with optimal workload. Current Osteoporosis Reports, 19, 298–307.
Edwards et al. (2009). Effects of stride length and running mileage on a probabilistic stress fracture model. Medicine & Science in Sports & Exercise, 41(12), 2177–2184.
Edwards et al. (2010). Effects of running speed on a probabilistic stress fracture model. Clinical Biomechanics, 25(4), 372–377.
Loundagin, Edwards, Schmidt (2018). Mechanical fatigue of bovine cortical bone using ground reaction force waveforms in running. J Biomechanical Engineering, 140(3).
Vernillo et al. (2017). Biomechanics and physiology of uphill and downhill running. Sports Medicine, 47, 615–629. · Eston et al. (1995). Muscle tenderness and peak torque changes after downhill running. J Sports Sciences, 13, 291–299.
Clansey et al. (2012). Effects of fatigue on running mechanics associated with tibial stress fracture risk. Medicine & Science in Sports & Exercise, 44(10), 1917–1923. · Derrick et al. (2002). Impacts and kinematic adjustments during an exhaustive run. MSSE, 34(6), 998–1002.
Dixon et al. (2000). Surface effects on ground reaction forces and lower extremity kinematics in running. MSSE, 32(11), 1919–1926. · Tessutti et al. (2012). Attenuation of foot pressure during running on four different surfaces. J Sports Sciences, 30(14), 1545–1550. · Pinnington & Dawson (2001). The energy cost of running on grass compared to soft dry beach sand. J Science and Medicine in Sport, 4(4), 416–430.
Banister (1975); Foster et al. (2001) — TRIMP and session-RPE training load. Gabbett (2016); Impellizzeri et al. (2020) — acute:chronic workload, and its limits.

Read more on the app's Recovery, Freshness & Readiness page. The calculator runs entirely in your browser; nothing you type is sent anywhere.

Your miles hit harder than you think.