16:8 Fasting Macro Calculator

The 16:8 protocol, in one paragraph

Sixteen hours of fasting, eight hours of eating. Most people skip breakfast, eat lunch around noon, and finish dinner by 8 p.m. That's it. The popularized version comes from Martin Berkhan's Leangains protocol, which paired the window with calorie cycling around training days. The form on this page implements the practical version: pick a deficit, pick a window length, and tell us whether you're training that day.

Where the math comes from

We compute your TDEE with the Mifflin-St Jeor equation (the same one used in the TDEE calculator), apply the deficit you choose, and produce a daily calorie target. On a moderate cut (-23%), a 2,200 TDEE delivers a daily target near 1,700 — the typical "mild fat loss" window for people using 16:8 for body recomposition.

We then split the daily target into two meals. The protein floor sits at ~0.9 g per lb of bodyweight, split evenly across the window because muscle protein synthesis caps per-meal (more protein in one sitting doesn't buy more muscle). Fat is the larger of 0.3 g/lb or 20% of total calories — enough for hormonal function, not so much that it crowds out protein. Carbs fill the rest of the budget.

Training-day carb shifting

On a training day, we move 70% of your carbs to the post-workout meal (typically dinner if you lift in the late afternoon, which most 16:8 practitioners do). The rationale: your muscles are most insulin-sensitive in the hours after training, so putting carbs there is marginally better for glycogen replenishment and recovery. The effect size is small — total daily intake still matters much more than timing — but if you're lifting hard, it's a free optimization.

The honest take on 16:8

16:8 is one of the easier fasting protocols to actually live on. Skipping breakfast suits most people's natural appetite curve. The 8-hour window is wide enough to be social. You can train inside or outside the window with minor adjustments.

But none of that means 16:8 is metabolically superior to a straight calorie deficit. The research on equal-calorie comparisons consistently shows similar fat loss between 16:8 and ad-lib timing. So the question to ask yourself is: does compressing my eating window help me eat less overall? For some people, the answer is yes — eating less when you can't snack in the morning becomes automatic. For others, the answer is no — they just shovel the same calories into a smaller window, plus an extra dessert because they "earned it." Track for two weeks and judge by the scale.

Pairing 16:8 with the PE Diet

The PE Diet (protein to energy ratio) is a complementary framework. Where 16:8 controls when you eat, PE controls what. They stack well: a 16:8 day with high-PE foods (lean protein anchors, non-starchy vegetables, modest starch portions) is the path of least resistance to a deficit with high satiety.

If you want a deeper look at how protein interacts with fasting windows, see PE Diet with intermittent fasting.

Frequently asked questions

Related tools

• OMAD Calorie Calculator — narrower window, single meal.
• 5:2 Diet Calorie Calculator — weekly average instead of a daily window.
• Macro Calculator — set macros independent of a fasting frame.
• PE Diet Calculator — check your meals against the PE ratio.