How long until I reach my goal weight

Most answers to "how long until I reach my goal weight" use the same sum: work out how many kilos you want to lose, work out how much you lose a week, and divide. It's simple, and it is wrong in a way that matters. It assumes you'll keep losing at the same speed until the very last kilo.

You won't, and nobody does. That isn't a problem with you. It is a problem with the sum, and it's why so many people decide their diet has stopped working when it is doing exactly what it should.

Why the simple sum is too optimistic

The usual rule says a kilo of fat is about 7,700 calories (a pound is about 3,500). Eat 600 calories a day below maintenance, and you'll lose about half a kilo a week, every week, until you stop.

The rule misses one thing: as you get lighter, you burn less. A smaller body needs less energy to run and to move around. So the same daily intake that was a 600-calorie deficit at the start is a smaller deficit a few months later, and a smaller one again after that. Weight loss on a fixed intake doesn't go in a straight line. It curves, getting slower over time until it levels off.

Researchers at the US National Institutes of Health built a model of this from measured data, and it's what the NIH's own Body Weight Planner uses. Their rule of thumb: every 10 calories a day of permanent change eventually moves your weight by about one pound. Half of that change happens in roughly the first year, and nearly all of it within three.

What the difference looks like

Take someone at 90 kg aiming for 80 kg, eating about 600 calories a day below their starting maintenance.

• The simple sum says 19 weeks.
• A model that allows for the slowdown says about 25 weeks, somewhere between 21 and 29.

That's six extra weeks, and they come at the end, when motivation is lowest. Someone who was promised 19 weeks reaches week 20 with three kilos still to go and concludes they've failed.

The gap grows with the size of the goal. From 100 kg to 80 kg at a similar pace, the simple sum says 34 weeks. Allowing for the slowdown, it's about 58, more than a year.

There's one more thing the simple sum gets wrong in the other direction. The first month usually goes faster than any model predicts, because some of the early loss is water and stored carbohydrate, not fat. So month one looks better than the forecast, months two and three come back to it, and if you were judging by month one, it feels like a stall. It isn't. It's the forecast catching up.

Work out your own timeline

The calculator below allows for the slowdown and gives you a range of weeks rather than a single date. People differ, and a date to the day would be pretending to know more than anyone does.

The goal weight calculator is also on a page of its own if you'd like to come back to it as your weight changes.

When a goal is out of reach at your current pace

Sometimes the calculator tells you that, at the pace you chose, your weight levels off before it reaches your goal. That isn't an error. It's the most useful thing it can tell you.

Because loss slows as you get lighter, any fixed deficit has a limit: a weight where you level off, however long you keep going. At 90 kg, a gentle pace of about 250 calories a day can take you down roughly 11 kg in total. A moderate pace of about 600 calories a day reaches roughly 27 kg. A goal beyond that limit can't be reached by holding the same intake for longer. It needs the plan to change along the way.

That's normal, and it's what happens on any long diet that works. The calorie target that was right at 95 kg is wrong at 80 kg, so it has to be lowered as you go. The problem is that most plans set the number once, at the start, and never change it. So when the loss slows down, as it was always going to, nothing tells you why, and the only advice left is to try harder.

How to use a timeline without it running your life

A timeline is useful for one thing: deciding whether your plan is reasonable before you start. Once you've started, it becomes a source of stress if you check yourself against it every morning.

• Judge progress by the weekly average, not by individual mornings. Your weight moves by one to two kilos day to day for reasons that have nothing to do with fat. Why the scale lies explains those swings.
• Re-check every month or so. If your four-week trend matches the forecast, carry on. If it's clearly slower for a month or more, something has changed, and why am I not losing weight in a calorie deficit walks through the likely reasons in order.
• Choose a pace you can keep. A faster pace shortens the timeline on paper. It only shortens it in practice if you can hold it. How many calories to lose fat covers how big a deficit makes sense.

The honest answer

How long it takes depends on your starting point, your goal and your pace, and it's almost always longer than the simple sum says. Expect the loss to slow as you go, plan to adjust your target along the way, and judge the trend over weeks rather than days. How long it really takes to see results covers the rest of the wait, beyond the scale: strength, how clothes fit and the milestones that come before the mirror changes.

Let Sophia re-plan as you go

A forecast made on day one is a guess. What makes it reliable is checking it against what actually happens and adjusting.

That's Sophia's job. She compares what you log with what your weight actually does, reads the weekly trend rather than any one morning, and lowers your target when your body says it's time, rather than leaving you to find out at week 20. When you're on track, she tells you so. When the loss slows, she tells you why, and what changes.