How Many Calories Do You Burn Fasting For 72 Hours? | Real-World Math

Calorie Burn During A 72-Hour Fast: What To Expect

Calories burned over three days come from your resting energy needs plus any movement you still do. Even without meals, your heart, brain, lungs, and basic cell housekeeping keep running. That baseline is your resting energy expenditure. Daily steps add a bump.

Short fasts don’t shut metabolism off. Research shows the body shifts fuels—glycogen first, then more fat—while resting needs stay in the same ballpark early on. That fuel shift, sometimes called “metabolic switching,” is described in a clinical review from the New England Journal of Medicine, which outlines how ketone production ramps up after glycogen falls.

How This Article Estimates Your Number

To keep things practical, the ranges below assume a quiet routine: desk work or time at home, short walks, no hard training. The math starts with a common predictive method (Mifflin-St Jeor) for resting needs and adds a small activity factor for steps and chores. Individual numbers vary with size, age, height, sex, muscle mass, temperature, and medication effects.

Quick Table: Typical Burn Over Three Days

The table uses a modest activity day (light steps). If you’re taller, more muscular, or you rack up lots of steps, you’ll trend to the high side; if you’re smaller or very still, you’ll sit near the low side.

Body Weight	Per-Day Estimate (kcal)	72-Hour Total (kcal)
50 kg (110 lb)	1,250–1,500	3,750–4,500
60 kg (132 lb)	1,400–1,700	4,200–5,100
70 kg (154 lb)	1,600–1,900	4,800–5,700
80 kg (176 lb)	1,800–2,150	5,400–6,450
90 kg (198 lb)	2,000–2,350	6,000–7,050
100 kg (220 lb)	2,150–2,550	6,450–7,650

These ranges line up with what you’d expect from resting needs plus light movement. Once you know your baseline, you can think about timing and fluids. Snacks fit better once you set your calorie deficit math.

How To Estimate Your Own Burn

You can ballpark your personal burn with two steps. First, calculate resting needs with Mifflin-St Jeor. Second, add a small activity bump if you’ll keep up your steps.

Step 1 — Resting Needs (Mifflin-St Jeor)

Formulas widely used by dietitians estimate resting energy from weight, height, and age. That’s your 24-hour baseline without purposeful movement. (Clinicians use these equations daily; the original research appears in the American Journal of Clinical Nutrition.)

• Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (y) + 5
• Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (y) − 161

Step 2 — Add A Small Activity Bump

On a three-day fast, most people keep movement light. A conservative bump is 10–20% above resting needs for casual steps and chores. Multiply your resting number by 1.1–1.2 for a safer range. Multiply by three for the full 72 hours.

Example Walkthroughs

Example A: 60-kg woman, 165 cm, 30 y. Resting ~1,360 kcal. With light steps (×1.15) ~1,565 kcal/day. Over three days ~4,695 kcal.

Example B: 80-kg man, 178 cm, 40 y. Resting ~1,760 kcal. With light steps (×1.15) ~2,025 kcal/day. Over three days ~6,075 kcal.

Example C: 100-kg man, 183 cm, 35 y. Resting ~2,020 kcal. With light steps (×1.2) ~2,425 kcal/day. Over three days ~7,275 kcal.

What Fuels You Use Across Three Days

Your body cycles through stored fuels. Early time windows rely more on glycogen; later windows lean on fat and ketones. A major medical review explains that after glycogen falls, the liver ramps up ketone output, which tissues can use in place of glucose. That switch helps spare muscle while energy needs continue.

Hours Into Fast	Main Fuel Source	What It Means
0–12	Meal glucose → liver glycogen	Post-meal energy; insulin tapers
12–36	Liver glycogen → rising fat use	Metabolic switch begins; ketones start to rise
36–72	Fatty acids + ketones	Greater fat reliance; stable energy for easy activity

What Changes Your Number

Body Size And Muscle

Bigger bodies and more lean mass burn more across the same 72 hours. Muscle tissue carries a higher resting demand than fat tissue, so lifters tend to sit higher in the ranges.

Movement

Steps count even when you’re not eating. Long walks or full-day errands can push totals up by several hundred calories per day. Hard training is risky on a water fast; fatigue and dizziness creep in fast.

Sleep And Stress

Short sleep pushes appetite hormones around on refeed day and can make the fast feel tougher. Good sleep supports steady energy and safer activity choices.

Temperature

Cold environments raise needs through shivering and thermogenesis. Hot weather can change fluid and electrolyte needs first, which matters more than tiny shifts in burn.

Caffeine And Non-Calorie Drinks

Plain coffee and tea don’t add calories. They may raise activity a bit by making you feel more alert, which nets out through more steps.

Safety First: Who Should Skip A Three-Day Fast

Extended fasting isn’t for everyone. People with diabetes or those who take glucose-lowering or blood pressure medication need individualized guidance to avoid low sugars and lightheaded episodes. National guidance for adults also sets clear weekly activity targets; pairing that baseline movement with a mild calorie deficit is a steadier route for many.

For physiology background, a respected medical review details the switch from glycogen to fat and ketones. For movement targets, see the CDC adult activity guidance that anchors weekly minutes and strength days. Those pages help set expectations and timing for safer experiments between eating windows.

Planning A 72-Hour Fast Without Guesswork

Hydration And Electrolytes

Drink to thirst with a plan: water plus zero-calorie electrolytes spaced through the day. Sodium and potassium help steady blood pressure and head off headaches. Many people feel better with a pinch of salt in one or two glasses.

Light Movement Only

Casual walks, easy mobility, light chores. Avoid sprints, hot yoga, max lifts, and long hikes. Save those for refeed day or later in the week.

Track Steps Instead Of Time In The Gym

Pick a step band—3–6k for a restful approach or 8–12k if you feel steady. Steps translate into a small, predictable calorie bump and help sleep that night.

Plan The Refeed

Start with a modest plate: lean protein, a fist of carbs (like rice or potatoes), and a thumb of fats. Chew well. A second small plate after an hour often feels better than one huge meal. That keeps the next day’s appetite calmer.

Sample Day-By-Day Timeline

Day 1 (Hours 0–24)

Fluids and electrolytes only. Short walks. Expect the first hunger wave at usual meal times; it passes. Mental focus often improves late afternoon.

Day 2 (Hours 24–48)

Glycogen is low; fat use climbs. Breath may smell fruity as ketones rise. Keep steps easy. Sleep 7–8 hours if you can.

Day 3 (Hours 48–72)

Energy feels different—calmer for many people. Stay gentle with movement. Prep a balanced refeed for hour 72.

Realistic Outcomes Over Three Days

Weight On The Scale

Glycogen binds water, so fasts show a quick drop from water loss early on. Fat loss is much slower than the scale suggests. Expect a partial rebound when you reintroduce carbs and salt with your first meals.

Fat Loss

The calorie gap you create is the driver. If your three-day burn is ~6,000 kcal and you take in near zero, you’ve created a noticeable deficit. A portion of that shows up as fat loss over the next several days as fluids settle.

When A Three-Day Fast Isn’t The Best Tool

If your goal is steady fat loss, a mild ongoing deficit with planned meals can be easier to stick with and easier to recover from. The science describing the fuel switch during fasting is clear, yet weekly progress still comes down to the balance between intake and expenditure across the whole week.

For a deeper read on the physiology behind that switch—ketones rising as glycogen wanes—see the NEJM review on metabolic switching.

Who Should Get Advice Before Trying This

People with type 1 or type 2 diabetes, anyone on insulin or sulfonylureas, those with kidney or heart conditions, pregnant or breastfeeding people, and anyone with a history of eating disorders should not attempt extended fasting without individualized medical input. Clinicians regularly adjust medications around fasting windows to avoid low sugars or fluid swings.

Putting It All Together

Over three days, your burn mostly mirrors your resting needs with a small lift from easy movement. Smaller bodies usually land near 4,500–5,500 kcal; average builds land near 5,500–6,500; larger builds or high-step days can push 6,500–7,500. Match fluids and electrolytes to comfort, keep steps easy, and plan a measured refeed. If you want a primer on fasting beyond the three-day window, you may enjoy our take on prolonged fasting basics.