A loose link between rate of perceived exertion and respiratory exchange ratio, rough way to gauge fat vs. carb burn at a given intensity. Useful for fueling sanity checks; not a substitute for measured VO2 testing. Type RPE or RER, the other one fills in.
Set total kcal for the session, then either RPE 1–10 or RER 0.70–1.00. The other field auto-fills via a linear mapping (with the standard caveats, see below).
And what it doesn’t.
RER ≈ 0.70 = predominantly fat. RER ≈ 1.00 = predominantly carb. 0.85 sits roughly 50/50. The math is the volume of CO2 you exhale divided by O2 you consume, fat oxidation produces less CO2 per O2, carb produces equal.
Protein contributes a small, variable amount RER can’t isolate. Ketone oxidation has an effective RQ near fat, can’t separate them. Phosphocreatine powers the first ~3–10 s of all-out efforts and uses no oxygen, so it’s invisible to RER.
At very high intensity, lactate is buffered and exhales as extra CO2. RER can climb past 1.0 even with no actual carb-only oxidation, substrate inference becomes unreliable. Reading is most useful at RPE 4–8 / RER 0.85–0.95 when you’re in steady-state aerobic work.
The map is rough, but the order is right.
Predominantly oxidative. Fat > carb. Aerobic base work, recovery sessions, all-day Zone 1. Long-term metabolic adaptations live here.
Oxidative. Mix of fat + carb. The endurance sweet spot. Where most race-pace effort happens for events of 90 min+. Fat ox peak typically lives in here.
Oxidative + glycolytic. Carb > fat. Sustainable for 30–60 min. Where lactate threshold sits; fueling decisions matter most because carb burn rate is high.
PCr → glycolytic dominant. RER becomes meaningless due to lactate buffering. Don’t infer substrate split here, fueling at this intensity is a different conversation.
This calculator’s linear RPE→RER mapping is a teaching tool. In reality, two athletes at the same RPE can have very different RER values:
If you actually need your crossover and peak-fat-oxidation rate, for race-day fueling planning, weight goals, or zone setup, metabolic profiling at the lab measures it directly. The number you walk out with is the difference between guessing your fuel needs and knowing them.
Why fueling matters even when RER says you’re “mostly burning fat.”
Long, under-fueled efforts pull amino acids from muscle. The liver (and kidneys) convert them, plus lactate (Cori cycle) and glycerol, into glucose via gluconeogenesis. Muscle itself doesn’t export glucose; it supplies substrates (e.g. alanine via the glucose-alanine cycle).
Chronic under-fueling raises reliance on amino acids. Net: lean mass and power both decline. Athletes feel this as “legs heavy for no reason” weeks before the scale moves.
Appropriate intra-workout carbohydrate + adequate daily protein protect performance and muscle. RER says “mostly fat” at low intensity, but that doesn’t mean fasted long sessions are free.
Indirect calorimetry at the lab measures your oxygen and CO2 across a graded intensity protocol, gives you measured RER, fat oxidation rate (g/min), and the exact wattage / pace where carb takes over. The fueling plan you build on top of that is in a different league.
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