Find your real number

The calculator gives you an estimate from a formula. This gives you a measurement from your own body. If you've tracked what you ate for a couple of weeks and weighed yourself along the way, you can stop guessing.

1

What you ate

Your average across the whole stretch, including the days it went badly. An honest average beats a flattering one. This only works if the input is real.

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How careful does the tracking need to be?

Careful. This is the number everything else depends on, and it's the one people get wrong. Under-reporting by a fifth or more is common and almost never deliberate. The oil in the pan, the handful from someone else's plate, the weekend, drinks.

If you weren't weighing food, treat the result as a rough direction rather than a figure. And if the answer comes out surprisingly low, the likeliest explanation isn't a slow metabolism. It's that the real intake was higher than the logged one.

2

What the scale did

Use weekly averages, not single mornings. Take every weigh-in from your first week and average them, then do the same for your last week. Single days swing by a kilo on salt and sleep alone.

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Why averages, and why two weeks minimum?

Body weight is mostly water on any given day. Salt, carbohydrate, a hard workout, poor sleep, hormones and what's still in your gut can move the scale by more than a week of real fat loss.

Averaging a week of weigh-ins cancels most of that noise. Comparing two averages a fortnight apart cancels most of the rest. One week is not enough, and comparing two single mornings is close to useless.

3

What the formula predicted

Optional. If you have the daily total from the calculator, put it here and you'll see how far off the estimate was.

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Leave blank if you don't have it.

Why this beats any calculator

Every formula on the internet, this site's included, guesses your energy needs from people who look roughly like you on paper. It can't see your genetics, your fidgeting, your thyroid, your sleep or fifteen years of dieting history.

The arithmetic here doesn't guess at any of that, because it doesn't need to. If you ate 1,750 calories a day for a fortnight and lost a kilogram, your body found that kilogram's worth of energy somewhere, and the only place it could come from is the gap between what you ate and what you spent. Run the sum backwards and the gap tells you what you spend.

The catch is that it inherits the quality of your inputs. Sloppy food logging in, meaningless number out. Two honest weeks are worth more than any calculator; two dishonest ones are worth less than nothing, because they'll convince you of something false.

Reading the result sensibly

  1. Expect it to disagree with the formula. A gap of 10% either way is completely normal and doesn't mean either number is broken.
  2. A surprisingly low result usually means under-reported food, not a broken metabolism. It's the single most common finding in the research on this, and it isn't a character flaw. It's how hard portions are to eyeball.
  3. A surprisingly high result usually means water. Starting a diet drops glycogen and the water bound to it, which can look like several thousand calories of fat that were never there. Longer stretches wash this out.
  4. Redo it after a big change. A new job, a new training block, a season, or 5–6 kg of weight change all move the real number.
  5. Then use it. Set your target from the measured figure, run another fortnight, and check again. Two or three rounds and you'll know your own number better than any calculator can tell you.

One caution before you start

Weighing food and weighing yourself daily suits some people and is corrosive for others. If tracking has ever tipped into something compulsive for you, this page is not worth the risk. The check-in method on the calculator page works fine with weekly weigh-ins and no food logging at all.

In the US, the National Alliance for Eating Disorders runs a helpline staffed by licensed clinicians on 1-866-662-1235, with free support groups and referrals at allianceforeatingdisorders.com. Elsewhere, your GP or family doctor is a good first call.