Subtract starting weight from ending weight, divide by starting weight, and multiply by 100. To find the average weekly rate, divide that percentage by the number of days between measurements and multiply by seven.
Keep the sign. A negative number means the ending weight is lower. A positive number means it is higher. If you only want the size of the difference, use the absolute value after calculating the signed result.
This is an observational formula. It answers what happened between two measurements. It does not answer what should happen next.
The percentage weight change formula
((Ending weight - starting weight) ÷ starting weight) × 100
The starting weight is the denominator because the question is how large the change was relative to where the period began. Pounds and kilograms produce the same percentage as long as both measurements use the same unit.
For weight loss, the answer is negative. Some articles display weight loss as a positive magnitude instead by calculating starting weight minus ending weight. Both conventions can work, but the label must make the convention clear. Mori's weight change percentage calculator keeps the signed result and adds plain-language direction.
Worked examples for loss and gain
Suppose a starting average is 180.14 lb and the ending average is 179.44 lb:
- 179.44 - 180.14 = -0.70 lb.
- -0.70 ÷ 180.14 = -0.003886.
- -0.003886 × 100 = -0.39%.
The ending average is 0.39% lower than the starting average.
For a gain example, suppose a starting average is 70 kg and the ending average is 70.7 kg. The change is 0.7 kg. Divide 0.7 by 70 and multiply by 100 to get +1.0%.
Percentage gains and losses are not perfectly symmetrical because the denominator changes. Moving from 100 to 90 is a 10% decrease. Moving from 90 back to 100 is an 11.11% increase. That is ordinary percentage math, not a calculator error.
Convert the observed period to an average week
First calculate the total signed change. Then scale it to seven days:
- Average weekly weight change = total weight change ÷ days × 7
- Average weekly percentage change = total percentage change ÷ days × 7
If the two averages are 28 days apart and total change is -2.0%, the linear weekly average is -0.5% per week. The same method works for 14, 21, or another known number of days.
Use the number of calendar days between the centers of the two averaging periods. Two consecutive Monday-to-Sunday averages are seven days apart because their midpoints are seven days apart. Do not count the seven measurements inside each average as seven extra days.
This weekly result is a summary, not a prediction. It assumes the total change is spread evenly for the purpose of describing the period. Real weight rarely moves in a straight line, and next week's change can be different.
Use comparable averages at both ends
A single weigh-in includes more than fat and muscle. Water, glycogen, sodium, food in the digestive tract, training-related inflammation, menstrual-cycle changes, illness, alcohol, travel, and measurement timing can all move the number.
Use the same scale on the same hard surface and measure under a routine you can repeat. Many people choose roughly the same time after waking and using the bathroom, before eating or drinking. No routine removes all variation, but comparable conditions reduce avoidable noise.
The weekly average weight calculator can turn daily measurements into two arithmetic means. Use the same number of days when practical. If a day is missing, leave it out of both the sum and the divisor instead of entering zero.
Keep a genuine surprising measurement. Correct an obvious entry mistake, wrong unit, or scale error, but do not delete a real high or low reading simply because it changes the result. A short context note is often more useful than edited history.
The result cannot identify what changed
A scale measures total body mass. A lower average could include changes in water, glycogen, digestive contents, fat, lean tissue, or several of these. A higher average has the same ambiguity. Percentage change does not solve that measurement problem.
Read the rate beside the length and circumstances of the period. A sharp change after travel, illness, a large carbohydrate change, or an unusually hard training week may not continue. A slower direction repeated across several comparable averages is easier to interpret than one seven-day jump.
Current community discussions show why context matters. In an August 2026 r/MacroFactor post, a user reconsidered an app-selected rate after hunger began interrupting sleep: “Tanking my sleep is not part of my optimal scenario.” The quoted experience does not establish a medical threshold. It does show why a calculated percentage should never overrule symptoms, recovery, or individual guidance.
If you also have complete calorie logs over a stable period, Mori's TDEE from weight change calculator can make a rough retrospective maintenance estimate. That tool adds more assumptions. It should come after reviewing the weight averages and the food log, not before.
Why this guide does not publish one universal target
Studies and position statements discuss rates for particular populations, goals, and settings. Applying one number to everybody would erase those differences.
A randomized study compared two intended weight-loss rates in 24 elite athletes who also resistance trained. The slower group targeted 0.7% per week and the faster group 1.4% per week. Body-composition and strength results favored the slower intervention on several measures. The sample was small, young, athletic, and supervised. It does not prove that 0.7% is the correct target for every adult.
A study of resistance-trained people compared smaller and larger energy surpluses. Faster body-mass gains were more clearly related to increased skinfold thickness than to greater improvement in most muscle-size and strength outcomes. The authors discussed conservative surplus and gain rates, but that evidence belongs to resistance training, not recovery from illness, pregnancy, childhood growth, or unintentional weight change.
An off-season bodybuilding narrative review recommends assessing average weekly changes from daily or several-times-weekly weigh-ins because daily fluctuation can obscure direction. Its specific gain suggestions are for bodybuilders. The measurement lesson travels more safely than the target number: average comparable observations before making a decision.
The National Athletic Trainers' Association position statement emphasizes gradual changes and warns against dehydration and unsafe restriction in sport. Combat-sport weight cutting is a separate, high-risk context and should not be reverse-engineered from this calculator.
Stop calculating when the next step is clinical
Unexpected or rapid weight change, persistent hunger, faintness, sleep disruption, missed periods, dehydration, declining performance, or other concerning symptoms deserve more than spreadsheet interpretation. Speak with a qualified clinician or registered dietitian instead of selecting a new calorie target from the percentage alone.
This page cannot set a safe rate for children, adolescents, pregnancy, breastfeeding, eating-disorder treatment, chronic disease, medication-related change, older adults at risk of muscle loss, or competitive weight-class sport. It also cannot assess whether a person's starting or ending weight is appropriate.
If weighing or calorie tracking causes guilt, compulsive checking, food restriction, binge and restrict cycles, or body-checking distress, put the scale and calculator aside. A useful trend is never more important than psychological safety.
This guide uses the elite-athlete weight-loss-rate trial, the resistance-training surplus study, the off-season bodybuilding narrative review, and the National Athletic Trainers' Association position statement. Public community discussions informed the practical questions, not the clinical claims.
Common questions
Should weight loss percentage be positive or negative?
A signed change is negative when ending weight is lower. Some reports show the magnitude of loss as a positive number. Either is understandable when clearly labeled. Do not compare two figures until you know which convention each one uses.
Can I use pounds for one weight and kilograms for the other?
No. Convert one measurement first. The percentage is unit-independent only when both inputs use the same unit.
Is average weekly percentage compounded?
Not in this calculator. It scales the observed total change linearly to seven days. It does not calculate a compound growth rate or forecast future weight.
How many weeks should I compare?
There is no universal window. One week is easy to calculate but vulnerable to fluid and routine changes. Several comparable weeks usually provide more context, provided the routine and goal did not change halfway through.