Why your TDEE calculation is wrong if you train Hyrox (and how to fix it)

If you've calculated your TDEE (Total Daily Energy Expenditure) using a standard formula like Mifflin-St Jeor and you're using it to plan Hyrox nutrition, you're almost certainly under-eating. The formulas were designed for the general population — they don't model the specific energy cost of strength-endurance compromised work like Hyrox.

Here's why, and how to actually feed yourself across a 15-week Hyrox build.

What standard TDEE formulas assume

Mifflin-St Jeor (the most widely used formula) gives you a Basal Metabolic Rate (BMR) and multiplies it by an activity factor:

• Sedentary (1.2) — desk job, no exercise
• Lightly active (1.375) — exercise 1-3 days/week
• Moderately active (1.55) — exercise 3-5 days/week
• Very active (1.725) — daily intense exercise
• Extra active (1.9) — twice-daily training, physical job

If you're training Hyrox 4-5 sessions per week, you'd naturally pick "Very active" (1.725). That gives you a daily calorie target based on this multiplier × BMR.

Why this underestimates Hyrox training

The activity multipliers were calibrated against a mixed population: walkers, joggers, gym-goers, weekend athletes. They assume "exercise" = roughly 45-60 minutes of moderate aerobic activity. Hyrox training is not that.

A typical Hyrox Build-phase session involves:

• 10-15 minutes of warm-up
• 40-60 minutes of compromised running + station efforts at near-max heart rate
• Repeated transitions that prevent any aerobic "rest" between efforts
• Heavy strength components (sleds, lunges, wall balls) that recruit large muscle groups under load

The metabolic cost per minute of this kind of work is 30-50% higher than steady-state moderate cardio. A 60-minute Hyrox compromised-running session burns 600-900 kcal for a 75-80 kg male — compared to 400-550 kcal for the same duration of "moderate" running that the formula assumed.

Net effect: standard TDEE underestimates daily expenditure by 200-400 kcal during the Build and Peak phases of a Hyrox plan.

How to correct it

Three options, ranked by accuracy:

Option 1: Move up one activity level

If you'd otherwise pick "Very active" (1.725), pick "Extra active" (1.9). This is the lazy correction and works reasonably well for most Hyrox athletes. Adds roughly 250-350 kcal/day to the target.

Option 2: Phase-based adjustment

Use different multipliers per phase:

• Base phase (weeks 1-5): standard "Moderately active" (1.55) — volume is mostly aerobic, lower intensity
• Build phase (weeks 6-10): "Very active" (1.725) — compromised running enters, strength stays high
• Peak phase (weeks 11-13): "Extra active" (1.9) — full simulations are extremely costly
• Taper (weeks 14-15): drop to "Lightly active" (1.375) — you're cutting volume by 60%, eating maintenance is the goal

This matches your nutrition to your actual training load week by week. Better than a single setting for 15 weeks.

Option 3: Heart-rate-based calculation

If you wear a heart-rate monitor or chest strap, your watch's daily calorie estimate (Garmin, Apple Watch, Whoop) tends to be more accurate than any formula. They use real-time HR + movement data to estimate per-session burn, then add it to BMR.

The catch: most watches still UNDERESTIMATE strength training calorie burn by 20-30% because HR doesn't spike the same way during heavy lifts as during running. So add 100-150 kcal to whatever your watch says on station-heavy days.

What to actually eat

For an 80 kg male, intermediate fitness, 4-5 Hyrox sessions/week:

Phase	Daily Calories	Protein	Carbs	Fat
Base	2700-2900	160 g	320 g	90 g
Build	2900-3200	170 g	370 g	95 g
Peak	3100-3400	180 g	410 g	95 g
Taper	2500-2700	160 g	280 g	85 g

Adjust proportionally for your body weight: ±10 kg ≈ ±200-300 kcal/day.

The carb-loading trap

Many Hyrox athletes carb-load aggressively the week before the race ("race week" carbs). For an 80-minute race, this is usually overkill — you have enough glycogen for the full event from your normal Peak-phase carb intake.

What actually works:

• Race week (Mon-Thu): normal Peak-phase macros
• Race day -1 (Fri/Sat): bump carbs by 50-80 g (one extra rice/pasta serving). Cut fibre to avoid GI issues.
• Race day morning: 80-100 g of carbs 2-3 hours before, easy-to-digest (oatmeal, banana, white toast + honey). Avoid heavy fats and high-fibre foods.
• 30 min before race: 30-40 g fast carbs (sports drink, gel)

Going beyond this is more likely to cause GI distress than to improve performance.

The tool that handles this automatically

The TDEE & Macro Planner uses Mifflin-St Jeor but lets you pick your training intensity correctly, and surfaces a per-meal breakdown (3, 4, or 5 meals) so each meal has its own calorie + macro target. Combined with the Hyrox Race Planner which knows your current training phase, the cross-product context shares your goal calories and macros with the Hyrox AI Race Coach when it recommends nutrition tweaks per phase.

Side benefit: when you're in Taper, the Hyrox Coach will tell you to drop your TDEE one tier so you don't accidentally gain 1-2 kg in race week.

FAQ

I'm cutting weight for a Pro division — different rules?

Yes. If you're cutting (e.g. going from Open to Pro requires dropping a weight class), you need a calorie deficit during the Base and early Build phases, then return to maintenance for Peak and Taper. Cutting during Peak destroys race performance. Use the TDEE & Macro Planner's "Cut" mode for the first 8-10 weeks, then switch to "Maintain" for the last 5.

What about caffeine?

Solid pre-race ergogenic aid — 3-6 mg/kg body weight 45-60 min before race. Don't try this for the first time on race day; test in training. Many athletes get GI distress at higher doses.

Carb sources during the race?

Most Hyrox athletes don't fuel mid-race because of the duration. If you're over 90 min: one gel (25 g carbs) at run 5 or run 6. Don't take in real food — too slow to absorb under high HR.