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2026-05-24 · 5 min read


VDOT vs FTP vs CSS — which to use and why they don't translate across sports


If you train more than one sport (run + bike, or full triathlon), you've probably bumped into all three: VDOT in running, FTP in cycling, CSS in swimming. The three measure something similar — your sustainable capacity near threshold — but they are not interchangeable. Here's when to use each and why.


The common idea


All three measure the same thing physiologically: the pace or intensity you can hold for 45-60 minutes before lactate drags you down. That's the functional threshold — a critical anchor for periodizing training.


What differs:

  • Discipline (run / bike / swim).
  • Underlying model (VDOT bakes in running economy; FTP is raw watts; CSS is velocity through water).
  • How it's measured (race vs 20-min test vs 400+200m test).

    • VDOT (running)


    What it is: Jack Daniels' model combining functional VO2max with running economy. It comes out of a near-maximal effort over 5K, 10K, half or full marathon.


    How it's calculated: feed a recent race time into the formula. Faster time → higher VDOT. Each VDOT defines paces for 5 zones (E, M, T, I, R).


    Strength: it includes running economy (the efficiency of burning oxygen per km). Two athletes with the same VO2max but different economy will have different VDOTs.


    Limitation: it loses precision on trail terrain. Use GAP (Grade Adjusted Pace) for it to make sense in the hills.


    When to use it: any serious running — road, track, trail with GAP.


    FTP (cycling)


    What it is: Functional Threshold Power. The power you can sustain for 60 minutes flat out. Measured in watts.


    How it's calculated: 20-minute all-out test, then take 95% of that average power (classic Coggan). There are alternatives (ramp test, 8-min). Seven zones (P1-P7) are built on top of FTP.


    Strength: power is objective — independent of terrain or wind. It's the one metric that measures what YOU put in (not what the bike does for you).


    Limitation: you need a power meter (USD 300-1000) or a smart trainer. Without measurement, FTP is a rough estimate.


    When to use it: cycling in any flavor — road, MTB, indoor, gravel. If you have a power meter, FTP always wins over HR or speed.


    CSS (swimming)


    What it is: Critical Swim Speed. The fastest velocity you can theoretically hold "indefinitely" aerobically — in practice, around 30 minutes flat out.


    How it's calculated: dead-simple test. Swim 400m all out, rest 10 min, swim 200m all out. CSS = (400m - 200m) / (T400 - T200), expressed in seconds per 100m.


    Strength: very precise prescription for swimming. Clean zones (S1-S5) and specific sets.


    Limitation: pool-specific. Doesn't translate to open water (where you have current, chop, sighting).


    When to use it: structured swimming. For an amateur triathlete, retesting every 8-12 weeks is enough.


    Why they DON'T cross over


    Three different athletes might have:

  • VDOT 50 (competitive amateur running, 10K in 39:00).
  • FTP 220 W (amateur cyclist, 65 kg, 3.4 W/kg, middling).
  • CSS 1:35/100m (average amateur swim, ~10 min for 600m).

    • These don't equate to each other. Each one depends on how much you've trained THAT specific sport. An elite runner picking up cycling has VDOT 60 but absolute-beginner FTP. And vice versa.


    So how do you handle this if you do triathlon?


    This is the critical question. Three options:


    ### 1. Per discipline (recommended)

    Every sport trains on ITS metric:

  • Run on VDOT (Daniels pace zones).
  • Bike on FTP (Coggan zones).
  • Swim on CSS (swim zones).

    • Your brain has to juggle 3 systems, but each one is precise.


    ### 2. HR only

    Everything by heart rate, one zone per intensity. Simpler but less accurate (HR in the pool is misleading because of the horizontal posture, HR on the bike drifts with heat, etc).


    ### 3. RPE

    Perceived effort (1-10). Works if you have lots of experience. For beginners it's too subjective.


    What elite triathletes actually do: option 1 (each discipline on its own metric), with HR as a secondary check.


    How Vetta handles it


    Vetta uses all three depending on the discipline and what you've got available:

  • Run: VDOT from your history (median of the top-3 of the last 28 quality activities). If you enter a PR on /onboarding, it anchors to your real VDOT.
  • Bike: FTP if you have a power meter. Otherwise HR with an estimated LTHR.
  • Swim: CSS if you enter 400m + 200m times in your profile. Otherwise HR, carefully.

    • When you pick your philosophy in /training Engine Settings, the system knows which metric to use for each discipline.


    What you (or your coach) should re-check


    Every 8-12 weeks:

    1. Is my VDOT still valid? (run a distance to confirm).

    2. Is FTP still valid? (20-min indoor ramp to retest).

    3. Is CSS still valid? (400+200m pool retest).


    All three metrics degrade with detraining and improve with a proper block. If you don't retest, you'll train on stale numbers and your paces will be off (too easy or too hard).


    Bottom line


  • Just a runner: VDOT.
  • Just a cyclist: FTP.
  • Just a swimmer: CSS.
  • Triathlete: all three, separately.
  • No equipment to measure them: HR + RPE as a backup, but with less precision.

    • [Calculate your VDOT here](https://vettatrainer.com/herramientas/vdot) if you're just starting out. For FTP and CSS, the tests happen in the sport-specific apps (Wahoo/Zwift for the bike, pool app or manual stopwatch for swimming).