How to predict your weight loss from your deficit

You can guess your weight loss from your daily deficit, but the simple maths runs too fast. Here is how to shave the number down honestly so your hopes match your real curve.

1 Aug 2026 · Burnie Academy
deficit weight-loss-maths prediction burnie

You cut your calories. Now you want to know how much weight you will lose, and when. That is a fair question. You want to plan, not just hope. This short read shows you a simple way to predict your own weight loss from your deficit. Not a fantasy. A real forecast you can trust.

Your starting guess: the simple maths

Start with the easy maths. Your body stores about 7700 kcal in each kilo of fat. So a 500 kcal daily deficit cuts 3500 kcal in a week. That is about 0.45 kg, or one pound. By this maths, ten weeks should drop about 4.5 kg. Burnie works out your daily deficit as food calories minus your BMR and active calories. Hold this number. It is your starting guess, not your promise.

Sources: Hall KD, 2008 - What is the Required Energy Deficit per unit Weight Loss? (Int J Obes, PMC)

Your real weight loss will match the simple maths.

mostly false

In a real test on over 100 people, the simple rule guessed 27.6 lbs of loss. People actually lost 20.1 lbs. That is about 7.4 lbs less than the rule said. It is roughly a quarter less. So take your starting guess and shave off about one quarter. That smaller number is your honest forecast. Real loss also curves, then slows. It does not drop in a straight line.

Sources: Thomas et al., 2013 - Can a Weight Loss of One Pound a Week be Achieved With a 3,500 kcal Deficit? (Int J Obes, PMC)

The three brakes that slow your loss

Water and sugar first

Week one drops fast, but most of it is water and stored sugar called glycogen, not fat. Week two feels slower. This is normal.

Your body burns less

After about two weeks of eating less, your body slows down. It burns about 120 kcal per day less. This is called adaptive thermogenesis.

Logging drift

People often eat more than they write down. In one study they under-reported their food by about 47 percent. Small misses hide hundreds of calories.

Sources: Heymsfield et al., 2011 - Voluntary weight loss: systematic review of early phase body composition changes (Obes Rev, PubMed) · Muller MJ, Bosy-Westphal A, 2013 - Adaptive thermogenesis with weight loss in humans (Obesity, PubMed) · Lichtman et al., 1992 - Discrepancy between self-reported and actual caloric intake and exercise in obese subjects (N Engl J Med, PubMed)

Build your honest forecast in four steps

The bottom line

A 500 kcal daily deficit is a fine aim. The simple maths says about 0.45 kg a week. Your honest forecast is smaller, closer to 0.3 kg a week over the months. Burnie is FREE, so run the maths as often as you like. Trust your trend over weeks, not the scale on one day. The honest number is smaller, and it still adds up.

Sources: Thomas et al., 2013 - Can a Weight Loss of One Pound a Week be Achieved With a 3,500 kcal Deficit? (Int J Obes, PMC)

How to predict your weight loss from your deficit

Your starting guess: the simple maths

The three brakes that slow your loss

Build your honest forecast in four steps

Track your food the honest way