How this calculator works
BMI is your weight in kilograms divided by your height in metres squared. It is a crude measure that takes no account of muscle, but it is the figure surgical criteria are written around.
Ideal weight uses the Devine formula, adjusted for frame size. Measure your wrist to find your frame: under 16 cm is small, 16 to 18 cm medium, over 18 cm large for most adults.
Excess weight is your current weight minus your ideal weight. This is the number bariatric results are quoted against, which is why surgical figures look so much larger than balloon figures.
Percentages shown are excess weight loss at 12 to 18 months. Losing 65% of excess weight is not the same as losing 65% of your body weight — the difference is enormous and often misunderstood.
BMI alone does not decide eligibility. A BMI of 35 with type 2 diabetes is treated very differently from a BMI of 35 with no other condition.
To check your eligibility against the full surgical criteria, use the weight-loss surgery calculator.
Frequently asked questions
Excess weight is how much you are above your ideal weight. If you weigh 130 kg and your ideal is 70 kg, your excess weight is 60 kg. A procedure quoted at 65% excess weight loss would be expected to take off around 39 kg — not 65% of 130 kg. This distinction is the single most common source of disappointment.
For most people it is a reasonable screening tool, but it is crude. It does not distinguish muscle from fat, so very muscular people can register as obese, and it takes no account of where fat is carried. Surgical criteria still use it because it is simple and comparable between clinics.
Wrap a tape measure around your wrist at the narrowest point, below the wrist bone. For most adults, under 16 cm indicates a small frame, 16 to 18 cm medium and over 18 cm large. Frame size shifts ideal weight by roughly 8% either way.
The Devine formula, used clinically since 1974 and still standard for drug dosing and bariatric assessment. We adjust it for frame size, which the original formula does not do.
The standard criteria are a BMI of 40 or above, or 35 or above with a weight-related condition such as type 2 diabetes, sleep apnoea or high blood pressure. Some patients between 32 and 35 with significant comorbidities are considered individually.
Because non-surgical options genuinely achieve less, and because they are often advertised as a percentage of total body weight rather than excess weight, which makes them sound closer to surgery than they are. Everything here is shown as excess weight loss so the comparison is honest.
Most patients do not, and that is a normal outcome rather than a failure. Losing 65 to 80% of excess weight typically leaves someone in the overweight range rather than the healthy range, with their weight-related health conditions substantially improved or resolved.
The formulas do not adjust for age, but outcomes do vary. Younger patients tend to lose slightly more and faster. Older patients often see the greatest benefit in joint pain, mobility and medication reduction rather than in raw numbers.
