Fasted Workout Calculator

Where in the fast to train depends on the goal. Fasted cardio for fat loss goes between hours 12 and 16. Lifting for fat loss goes in the two hours after you break the fast. Muscle work goes within an hour either side of it. Performance work goes one to three hours after eating. The rules are all printed below.

Fast starts at

When you stop eating

Fast ends at

When you start eating

Workout Type

Your Goal

Workout Duration: 45 min

15 min 60 min 120 min

How the window is chosen

Clock arithmetic plus a set of rules, both of which are printed here. Your fasting hours are the difference between the start and end times you give; your eating hours are the rest of the 24.

The window it recommends depends on your goal and the kind of session. For fat loss with cardio or interval work it places the session between hours 12 and 16 of the fast, the stretch where the body is furthest into fat oxidation and still has enough left for moderate intensity. For fat loss with weights or yoga it places the session in the two hours after you break the fast, because lifting hungry costs more muscle than it saves fat. For muscle it places the session in the hour either side of breaking the fast, so that protein is never far away. For performance it places it one to three hours after eating, because high intensity runs on glycogen.

Three checks then run, and any of them adds a warning above the plan rather than quietly adjusting it: weights on a fast longer than 16 hours, a session over 60 minutes on a fast longer than 16 hours, and any session over 90 minutes. None of these rules is a measurement of you. They are the site's own reading of where the trade-offs sit, and the one that matters most is the last: if a session is long, being fed beats being fasted whatever your goal.

Where this comes from

1. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis — Areta et al., The Journal of Physiology, 2013 (PubMed)
2. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting — Anton et al., Obesity, 2018 (PubMed Central)

Nothing on this site is medically reviewed — it cites its sources so you can check them. Read our editorial policy. Method last checked 21 August 2026.

The timing of your workout during intermittent fasting can dramatically affect your results. Exercise during the wrong phase of your fast can leave you drained and compromise recovery, while training at the right time amplifies fat burning and preserves muscle mass.

This calculator matches your specific workout type and goal to your fasting schedule, giving you a clear window for optimal training. Whether you want maximum fat loss from fasted cardio or need to preserve muscle during strength training, the timing recommendation adapts to your needs.

Frequently Asked Questions

Should you work out while fasting?

It depends on your goal and workout type. Fasted cardio and light exercise are generally safe and can enhance fat burning. However, heavy strength training while deep into a fast (18+ hours) may compromise performance and muscle preservation. The key is matching your workout timing to your fasting phase.

Is fasted cardio better for fat loss?

Research is mixed but promising. Fasted cardio may increase fat oxidation during the session, particularly at moderate intensity. However, total daily calorie balance matters more for long-term fat loss. Fasted cardio works best when done 12-16 hours into a fast, when your body is actively using fat for fuel.

Can you build muscle while intermittent fasting?

Yes, but timing matters. Schedule your strength training close to your eating window so you can consume protein within a reasonable timeframe after training. Aim for at least 25-40g of protein in your post-workout meal. Many athletes fast, train near the end of their fast, and break their fast immediately after.

What should you eat before and after a fasted workout?

Before: nothing if truly fasted, though black coffee is acceptable and may enhance performance. After: prioritise protein (25-40g) and carbohydrates to replenish glycogen. If your goal is fat loss, you can delay your post-workout meal slightly. For muscle gain, eat within 30-60 minutes of finishing.

Is it dangerous to exercise while fasting?

For most healthy adults, moderate exercise during a fast is safe. However, avoid high-intensity training beyond 18 hours fasted, stay hydrated, and listen to your body. Stop immediately if you feel dizzy, nauseous, or lightheaded. People with medical conditions should consult their doctor first.

Go deeper

lifestyle · 11 min read

Working Out While Fasting: The Complete Guide

Can you work out while fasting? Learn how to combine intermittent fasting and exercise for fat loss, muscle retention, and peak performance.

weight loss · 13 min read

Does Fasting Cause Muscle Loss? What the Science Says

Worried about losing muscle while fasting? What the studies say about intermittent fasting and lean mass, how to tell fat loss from weight loss, and how to keep the muscle you have.

nutrition · 11 min read

Protein and Intermittent Fasting: How Much Do You Need?

How much protein during intermittent fasting, when to eat it, and what to eat it from. Daily targets, per-meal distribution, training-day timing, and the best sources.

Fasted tracks your fasting phases so you always know your optimal workout window

See exactly which metabolic phase you are in at any moment. The Fasted app shows you when to train for maximum fat burning, muscle preservation, or performance.

Download on the App Store Get it on Google Play

Free to use — no account required