Free tool
Are you really in a deficit?
You've been logging for weeks and the scale hasn't moved the way the maths says it should. This works out what's actually happening from your own numbers, and tells you how far off your logging is.
Runs in your browser. Your numbers never leave this page unless you ask for the result by email.
Your data
Paste from a spreadsheet, MyFitnessPal, Cronometer or CalorieBot. One row a day: date, weight, and calories if you've them. Calories are optional, but they're what unlocks the comparison.
About you
Only used to compare what you burn against what a standard formula would guess. Leave it blank and you still get the deficit from your weight.
The last 28 days by default. The first fortnight of any diet is mostly water and makes the number look better than it is.
Paste your numbers and press the button.
How this works, and what it can't know
Daily weight moves by one to two kilos on water, salt, glycogen and what's still in you. So this doesn't subtract last week's weight from today's. It fits a robust trend line through every weigh in, using the median of the slopes between each pair of days, which means a travel week or a bad Saturday can't drag the answer around. Days that sit miles off the local trend are dropped and the page tells you how many.
From that slope it works backwards. If you're losing weight, you're in a deficit, and the size of it's the slope multiplied by the energy in the tissue you're losing. Add your maintenance to your intake and you get what you actually burn, measured rather than guessed.
The prediction is simulated day by day, not divided. As you get lighter you burn less, roughly 14 calories a day less per kilo lost, so a deficit that starts at 500 doesn't stay at 500. Any calculator that divides pounds by 3,500 is telling you a date that won't happen.
What it can't know. If your logs and your weight disagree, it can't tell you which one is wrong. It can tell you how big the gap is and what usually causes it. A standard maintenance formula is routinely out by ten per cent for any individual, so a gap smaller than that's agreement, and this says so rather than inventing a problem.
It's not medical advice, and it won't give you a target if you tell it something that needs a person rather than a page.
Knowing the number is the easy half
The reason the logs and the scale disagree is almost never knowledge. It's that logging a meal properly costs more effort than you've on a Wednesday. CalorieBot takes a few seconds, typed or photographed, which is cheap enough to keep doing on the days you can't be bothered.