Subtract two measurements, interpret the sign, and calculate percentage change using the starting value. Animated scales and number lines make the difference between direction and magnitude visible.
Open the weight difference calculator 2 · More Health and Fitness videos
The calculation
Signed change = final − initial; percent change = (final − initial) ÷ initial × 100
Worked example
80 kg to 78 kg gives −2 kg and −2 ÷ 80 × 100 = −2.5%. Returning from 78 kg to 80 kg gives +2 kg but about +2.56%, because the starting value changes. Across four weeks, −2 kg is an average of −0.5 kg per week.
Use consistent units and comparable measurements. The calculation describes the recorded change; it does not establish the cause or guarantee a future trend.
Video chapters
- 0:00 — What changed between two measurements?
- 0:16 — Final minus initial
- 0:32 — Direction and size are different
- 0:47 — Use the starting weight as the base
- 1:03 — Reversing the journey changes the percentage
- 1:21 — An average rate needs elapsed time
- 1:38 — Check units and the meaning of the result
- 1:56 — Subtract → divide by the start → ×100
Read the full transcript
A first measurement is eighty kilograms. A later measurement is seventy-eight kilograms. How much did the weight change, and what percentage is that? Two answers tell different parts of the story: the signed difference gives the direction, while the percentage compares that difference with where you started.
Keep the measurements in the same unit and label their order. The rule is final minus initial. Seventy-eight minus eighty equals negative two kilograms. On a number line, moving from eighty down to seventy-eight is a two-kilogram move in the negative direction.
The signed change is negative two kilograms. Its magnitude, or size, is two kilograms. If the second reading were eighty-two, the change would be positive two. If it were still eighty, the change would be zero. Preserve the sign when the direction matters.
For percentage change, divide the signed difference by the starting weight, then multiply by one hundred. Negative two divided by eighty is negative zero point zero two five. Multiplying by one hundred gives negative two point five percent. The final reading is two point five percent below the first.
Now reverse the journey: seventy-eight kilograms back to eighty. The difference is positive two kilograms, but the percentage base is now seventy-eight. Two divided by seventy-eight times one hundred is about two point five six percent. Equal kilogram changes can have different percentages because their starting values differ.
If the original two-kilogram decrease happened over four weeks, divide negative two by four. The average rate is negative zero point five kilograms per week. This is an average across the interval. It does not mean the reading fell by exactly the same amount every week.
Never subtract a kilogram reading from a pound reading without converting one first. Percentage change also needs a nonzero starting value; division by zero is undefined. And a difference between two readings cannot explain its cause. For body weight, the arithmetic alone cannot tell you what changed inside the body.
Label the initial and final measurements, match their units, and subtract initial from final. Divide by the initial value for a percentage, or by elapsed time for an average rate. In our example, eighty to seventy-eight kilograms is negative two kilograms, or negative two point five percent.