When will I reach my goal weight? Why the answer is a range

The arithmetic behind a single goal date

Suppose 20 lb remain and your trend shows a loss of 1.0 lb/wk. Twenty weeks, says the calculator. That figure assumes the rate is known exactly and will hold until the last pound.

Neither assumption is safe. A weekly rate measured from a month or so of noisy weigh-ins comes with its own range, and a reading of −1.0 lb/wk ±0.5 is consistent with anything from 0.5 to 1.5 lb a week. Run the division at both ends and the arrival lands somewhere between about 13 and 40 weeks out.

That span is lopsided. The fast end pulls the date in by seven weeks, and the slow end pushes it out by twenty. A single date quoted from the middle hides how much further the slow end reaches.

Why the pace of loss tends to slow

A straight line assumes each week looks like the last. Bodies don’t behave that way. A 2011 analysis in The Lancet, led by researchers at the NIH, modeled how body weight responds when energy intake changes and stays changed and found the response is slow, with a half-time of about a year. In that model, each permanent change of 100 kJ (about 24 kcal) a day eventually shifts weight by about 1 kg, with half of the change arriving in roughly a year and 95% in about three.

The familiar rule that a pound equals 3,500 kcal misses this. A 2013 commentary in the International Journal of Obesity found the rule grossly overestimates actual weight loss and described the real pattern as curvilinear. A companion letter from Kevin Hall and Carson Chow of the NIH named the flaw: the rule treats the energy gap as fixed, while that gap shrinks as weight falls, so the rule’s estimates keep falling and never reach a plateau.

For a goal date, the consequence is direct. A line fitted to the first two months of a campaign tends to point to an earlier finish than the data eventually delivers.

Some trajectories flatten above the goal

A slowing curve can still reach the goal, only later. It can also flatten at a level above the goal, and a straight line can never show that, because any falling line crosses every lower number eventually.

A forecast that fits a curve, with the leveling-off point left free to land wherever the data puts it, can say the curve is flattening short of the goal. It describes the path the recent readings are on, and that path can bend as new weigh-ins arrive.

Why a forecast sometimes shows no date

A forecast needs enough data to constrain the rate. After a long break or early in a campaign, the range of plausible dates can stretch over many months. A date drawn from the middle of that range would look precise while resting on almost nothing.

Leaving the date off in those cases is the accurate response. As weigh-ins accumulate, the range narrows, and a date range appears once the readings can support one.

Reading a forecast range

Treat the early end and the late end as equally real. Watch the width more than the midpoint: a range that narrows over several weeks means the data is converging, and one that widens usually reflects a gap in weigh-ins. A forecast that drifts later is telling you the trend has slowed, which is often the curve described above showing up in your own data. If a forecast is shaping decisions about eating or medication, talk with your clinician before acting on it.

The date you set for yourself is a separate thing. A plan says when you intended to arrive, and a forecast says when the measurements suggest you will. The two will usually disagree. A forecast that lands before the plan’s date means the trend is moving faster than the plan asked for.

Balance Point’s forecast model

Balance Point derives its forecast from measurements alone and presents it as a range, or shows no date while the recent record is too sparse to pin one down. The model it prefers leaves the curve’s resting level open, which lets it report that a trend “levels off above your goal.” That model holds off until the campaign spans 112 days and contains 20 weigh-ins, and the readings must show visible curvature. Otherwise the app falls back to a constant-rate model and brackets its answer with a range.

The chart names both lines. In the sample below, the orange dashed plan runs toward the dotted goal, and the faded violet forecast continues from the trend.

A campaign’s plan starts from a chosen date or a chosen pace, with the other derived, as the goals page describes, and the forecast page covers how the two are shown side by side.