How this instrument works
Percent excess weight loss compares how much weight has come off to how much weight needed to come off. "Ideal weight" here follows the common convention of a BMI of 25, the upper edge of the normal range, applied to a person's actual height: ideal weight in kilograms equals 25 times height in metres, squared. Everything above that figure at the pre-operative weigh-in counts as excess; %EWL is the share of that excess that current weight has actually shed.
The reason %EWL became the standard, rather than kilograms lost outright, is comparability. A patient who starts at 130 kg and one who starts at 190 kg can lose an identical amount and land in very different places relative to their own targets — %EWL accounts for that by measuring progress against each patient's own excess rather than against a shared absolute number. The convention traces to Deitel and Greenstein's 2003 recommendations in Obesity Surgery, written specifically to standardize how bariatric outcomes get reported across studies and surgeons.
%EWL is a different figure from %TBWL, total body weight loss, which simply divides weight lost by starting weight with no reference to a target at all. A patient can post an unremarkable %TBWL while posting a strong %EWL, or the reverse, depending on how far above the BMI-25 line they started. Both numbers circulate in bariatric literature, and it is worth checking which one a given report means before comparing it against another source.
"Ideal weight" itself is not a settled concept — BMI 25 is only one convention among several, alongside older Metropolitan Life insurance tables and other reference standards, and different studies have leaned on different ones. A 2019 systematic review of the bariatric literature found ongoing inconsistencies in how weight loss gets reported, tracing a meaningful share of it back to exactly this: no single agreed definition of the target weight surgery is measured against. This calculator uses the BMI-25 convention because it is the one Deitel and Greenstein's recommendations popularized, not because it is the only defensible choice.
- Enter Pre-Op Weight — the weight recorded before surgery.
- Enter Current Weight — the most recent weigh-in.
- Enter Height in metres, centimetres, or feet and inches — the unit menu converts before computing.
- Read Ideal Weight and %EWL; the working block shows the excess-weight arithmetic behind both.
Worked example — 130 kg pre-op, 95 kg current, 1.65 m
Ideal weight at 1.65 m is 25 × 1.65² = 25 × 2.7225 = 68.06 kg. That puts the excess at the pre-operative weigh-in at 130 − 68.06 = 61.94 kg — the total this patient's frame carried above the BMI-25 line. Weight actually lost so far is 130 − 95 = 35 kg. Divide the two: 35 ÷ 61.94 × 100 = about 56.5% — a %EWL of roughly 56.5%, meaning a little more than half of the original excess is gone.
A taller, heavier starting point changes the arithmetic but not the method: 150 kg pre-op, 100 kg current, at 1.70 m gives an ideal weight of 25 × 1.70² = 72.25 kg, an excess of 150 − 72.25 = 77.75 kg, and 50 kg actually lost. 50 ÷ 77.75 × 100 = about 64.3% — a higher %EWL than the first patient despite losing more absolute weight against a larger excess, which is exactly the comparison %EWL exists to make fair.
Questions
Why use %EWL instead of just reporting kilograms lost?
Because raw kilograms don't account for how much weight a person was carrying above a reasonable target to begin with. Two patients losing the same amount can represent very different outcomes if one started far above a BMI-25 target and the other started only modestly above it. %EWL puts both results on the same scale — progress against each patient's own excess — which is what let Deitel and Greenstein's 2003 recommendations establish it as the standard bariatric outcome measure.
What's the difference between %EWL and %TBWL?
%TBWL, total body weight loss, is simply weight lost divided by starting weight — no target involved. %EWL divides weight lost by the excess above a target weight, here the BMI-25 figure, so it only counts progress toward that reference point. A patient starting only modestly overweight can post a striking %EWL off a small excess while showing an unremarkable %TBWL, so the two numbers answer different questions and are not interchangeable.
Why does this calculator define "ideal weight" as BMI 25?
Because it is the convention Deitel and Greenstein's widely cited 2003 recommendations used, and it remains the most common in bariatric reporting — but it is not the only one. A 2019 systematic review of bariatric outcome literature documented ongoing inconsistencies in weight loss reporting, part of which traces back to studies using different definitions of ideal weight, from BMI 25 to older Metropolitan Life insurance tables. Comparing %EWL figures across two studies is only valid if both used the same target-weight convention.
Can %EWL go above 100%?
Yes. If current weight drops below the BMI-25 ideal-weight figure, the patient has lost more than their original excess, and the arithmetic produces a number over 100%. It is not an error — it just means the starting excess has been fully cleared and then some, and the formula keeps counting past that point.
Is %EWL specific to gastric sleeve surgery?
No — the metric is used across bariatric procedures generally, including gastric bypass and adjustable banding, not just sleeve gastrectomy. Deitel and Greenstein's recommendations were written to standardize reporting across the field, so the same formula applies regardless of which procedure produced the weight loss being measured.
When after surgery should %EWL be checked?
At intervals, not just once at the end. Bariatric outcomes are commonly tracked at checkpoints such as 6, 12, 18, and 24 months post-surgery, since the rate of loss slows over that window and a single late measurement misses the shape of the curve. Running this calculation at each checkpoint, using the same pre-operative weight every time, tracks progress against the same excess-weight target throughout.
References
- Deitel M, Greenstein RJ 2003, Obes Surg — Recommendations for Reporting Weight Loss (PubMed)
- Mocanu V et al. 2019, Obes Surg — Ongoing Inconsistencies in Weight Loss Reporting Following Bariatric Surgery (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.