How Many Calories Do I Burn Fasting For 16 Hours? | Science-Based Math

Guide / By

During a 16-hour fast, calories burned roughly match your baseline energy for that span, shaped by age, size, sex, and daily movement.

Calories Burned During A 16-Hour Fast: The Simple Math

Your body never idles. Breathing, circulation, and temperature control keep burning energy around the clock. That base demand is your resting burn. Health systems describe it as the minimum energy needed to keep you alive at total rest, and it varies with body size, age, and sex. A concise primer on this concept sits at Cleveland Clinic’s page on basal metabolic rate.

To get a sensible estimate for a 16-hour window, scale your daily resting burn to those hours. If your resting burn is 1,800 kcal per day, that’s 75 kcal per hour. Over 16 hours, you’d use about 1,200 kcal at rest. Light movement (walking, chores) adds on top.

Reference Table: From Daily Resting Burn To 16 Hours

The table below converts a range of common daily resting burns into hourly values and 16-hour totals. Pick the row that best matches you, or use it to sanity-check a calculator’s output.

Daily Resting Burn (kcal) Per Hour (kcal) 16-Hour Burn (kcal)
1,200 50.0 800
1,400 58.3 933
1,600 66.7 1,067
1,800 75.0 1,200
2,000 83.3 1,333
2,200 91.7 1,467

If you’d like a bigger picture of your daily energy use, scan our explainer on calories burned every day. It shows how rest and activity stack up across 24 hours.

How To Estimate Your Own Number

Here’s a simple, transparent method that lines up with what registered dietitians use in clinic. It leans on the Mifflin-St Jeor equations, which have been shown to track measured resting burn reasonably well across many body sizes.

Step 1: Estimate Your Resting Burn

Mifflin-St Jeor:

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

The American Journal of Clinical Nutrition published the original equations (Mifflin et al., 1990), and a later comparison found them reliable across many adults in practice. Those source details are in the original paper and the dietetics comparison in the Journal of the Academy of Nutrition and Dietetics.

Step 2: Scale To Sixteen Hours

Divide your daily result by 24 to get kcal per hour, then multiply by 16. That gives a clean base estimate for a 16-hour window.

Step 3: Add Real-World Movement

Resting numbers don’t include activity. To layer that in during your fasting window, use METs (metabolic equivalents). One MET is resting demand; sitting is usually around 1.3; easy walking often lands near 3. You can convert METs to kcal per hour with standard exercise physiology math shared by Texas A&M (METs to calories).

What Changes The Total During A 16-Hour Window

Body Size And Composition

More mass costs more energy to maintain. Muscle is metabolically active, so people with a higher lean-to-fat ratio tend to burn more at rest.

Age And Sex

Resting burn drifts lower with age. Sex differences show up through body size, hormones, and typical muscle mass.

Sleep And Temperature

Sleep trims energy use a bit compared with awake rest. Colder rooms nudge the number up as your body works to stay warm; warmer rooms do less of that.

Movement Inside The Window

The easy extras—standing, light chores, strolls—add more than people think. Those minutes stack up quickly over 16 hours.

What Fasting Does—And Doesn’t—Change

Time-restricted eating shifts fuel use toward stored energy between meals. A widely cited New England Journal of Medicine review describes the “metabolic switch”: as stored carbohydrate dwindles, the body increases fat use and produces ketone bodies. That switch can happen during longer fasting stretches and may start to emerge by the end of a 16-hour window, depending on prior meals and activity (NEJM review).

That fuel shift doesn’t mean a big bump in total energy burned. Short fasting windows usually leave resting burn near baseline. The main driver for 16 hours is still your resting number plus any movement.

Examples With Common Routines

These hourly burns assume a 70 kg person. To adjust, multiply the MET by 1.05 × your body weight in kg for a per-hour estimate. Then add up the hours that match your day. The MET method and values trace back to the standard Compendium used in research and coaching.

Activity Approx. MET kcal Per Hour (70 kg)
Sleeping 0.9 66
Sitting quietly 1.3 96
Desk work 1.5 110
Light housework 2.0 147
Leisure walk 3.0 221

If your 16-hour stretch includes 8 hours asleep, 6 hours sitting and desk time, plus 2 hours of easy walking, you’d total roughly: 8×66 + 6×(average of 96 and 110) + 2×221. That lands in the same range as the mid band from the card above. MET details and conversion steps are summarized by Texas A&M’s guide linked earlier, which points to the Compendium standard.

Accuracy Tips And Common Mistakes

Use Weight And Height In Metric For The Equation

Metric input avoids rounding errors. If you prefer calculators, pick one that shows the actual formula and units rather than just spitting out a number.

Don’t Double-Count Activity

Resting burn is always there. Activity is extra. If a calculator already applied an “activity factor” to reach a daily total, don’t add MET hours on top of that again.

Check The Hour Mix

Two people with the same resting number can end up far apart if one spends the window on their feet and the other spends it in a chair.

Safety Notes Before You Try Time-Restricted Eating

Some groups face higher risk: people managing diabetes, those on glucose-lowering medication, anyone with a history of disordered eating, and people who are pregnant or nursing. NHS sources recommend talking to your care team in those cases, and they acknowledge several patterns of fasting, including 16:8 and 5:2 (NHS guidance).

Proof And Sources Behind The Numbers

The concept of resting burn is covered by mainstream medical organizations, and the Mifflin-St Jeor equations remain a common choice in clinics. Cleveland Clinic offers a plain-language overview of resting burn and factors. The original Mifflin paper appeared in the American Journal of Clinical Nutrition; dietetics reviews have compared major equations and found Mifflin a strong pick across a range of adults.

Make The Math Work Day To Day

Pick a consistent window, keep fluids up, and plan protein across your eating hours so you hold on to lean tissue while you’re trimming calories. If you want a clearer daily plan, try our calorie deficit guide for a friendly walkthrough.