Nutrition
Energy Availability Calculator — RED-S Risk Screening
Work out your energy availability in kcal per kg of fat-free mass and see whether you are fuelling enough to avoid Relative Energy Deficiency in Sport.
% — an estimate is fine; it only sets fat-free mass.
kcal per day, averaged over the week.
minutes
Exercise cost is estimated from the Compendium of Physical Activities (MET × body weight × hours) and is gross, not net of resting metabolism. Laboratory protocols subtract resting expenditure for the training period, so this figure runs slightly conservative. This is a screening tool, not a diagnosis — persistent low readings warrant a sports dietitian or physician.
What energy availability actually measures
Energy availability (EA) is the energy left for everything your body does *other* than exercise. It is not the same as calorie balance. The equation used here is the one from the sports-science literature:
EA = (Energy Intake − Exercise Energy Expenditure) ÷ Fat-Free Mass
The result is expressed in kilocalories per kilogram of fat-free mass per day. Fat-free mass is used rather than total body weight because fat tissue is not metabolically expensive to maintain.
The thresholds
- 45 kcal/kg FFM/day and above — the figure associated with normal
- 30 to 45 — reduced availability. Some athletes hold here through a
- Below 30 — low energy availability, the condition underlying Relative
Why endurance athletes drift low without meaning to
Low energy availability is usually not deliberate restriction. It is more often appetite suppression after hard sessions, a training load that rose faster than intake did, or fuelling built around body composition goals during a heavy block. The arithmetic is unforgiving: a 60 kg runner at 18% body fat has roughly 49 kg of fat-free mass, so a single 700 kcal session eats 14 kcal/kg FFM of headroom before anything else is accounted for.
What this tool does not do
This is a screening calculation, not a diagnosis. Exercise cost is estimated from MET values rather than measured, and the equation cannot see menstrual status, bone density, bloodwork or training history. Persistent low readings are a reason to talk to a sports dietitian or physician, not a reason to self-treat.
Frequently Asked Questions
What is a good energy availability number?
45 kcal per kg of fat-free mass per day and above is the level associated with normal physiological function in athletes. Between 30 and 45 is reduced availability, which some athletes sustain through a training block but is not a long-term state. Below 30 is classified as low energy availability and is the condition underlying RED-S.
Is energy availability the same as being in a calorie deficit?
No. A calorie deficit compares intake against total daily expenditure including exercise. Energy availability subtracts the cost of training first, then asks how much is left for basic physiological function relative to fat-free mass. An athlete can be in calorie balance on paper and still have low energy availability if their training load is high.
Does low energy availability only affect female athletes?
No. RED-S was defined to replace the older female-athlete-triad framing precisely because the syndrome occurs in men as well. The research on male-specific thresholds is less settled than for women, and some reviews suggest the threshold may sit lower in men, but the mechanism — insufficient energy left after training — is the same.
How accurate is the exercise cost estimate?
It is an estimate. This tool uses MET values from the Compendium of Physical Activities multiplied by body weight and duration, which gives gross exercise energy expenditure. Laboratory protocols subtract resting metabolism over the training period, so the figure here runs slightly conservative and your true energy availability may be marginally higher than shown.
Should I use this every day?
Averaging across a week is more informative than a single day. Energy availability varies with the training schedule, and one hard session does not indicate a problem. A pattern of readings below 30 across a normal training week is the signal worth acting on.
Data sources & references
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