The formula
How to calculate weight loss
This works out a daily calorie target from your BMR, activity level and a chosen weekly weight loss rate, then projects a rough date you reach your goal weight at that rate.
BMR is calculated with the Mifflin-St Jeor equation, then scaled by an activity multiplier to get TDEE — total daily energy expenditure. The daily target subtracts a deficit sized to the weekly loss rate you picked, using the standard 7,700 kcal per kg of fat.
Fill in the following:
- Gender
- Age (years)
- Height (cm)
- Current weight (kg)
- Goal weight (kg)
- Activity level
- Desired weekly loss
- Start date — pick today, or the date you plan to start
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
Some of the fields above will accept figures that seem unusual for your own situation, and that is deliberate: the formula behind weight loss works the same way regardless of scale, so the calculator does not stop you testing a hypothetical scenario a long way from your actual numbers — often the fastest way to see which input the result is most sensitive to.
Why weight loss matters
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind weight loss is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
Worked example
A concrete run-through, using the values already in the fields:
- Gender: male
- Age: 30 years
- Height: 170 cm
- Current weight: 80 kg
- Goal weight: 70 kg
- Activity level: moderately_active
- Desired weekly loss: 0.5
- Start date: today
That gives:
- BMR: 0 kcal/day
- TDEE: 0 kcal/day
- Daily calorie target: 0 kcal/day
- Daily deficit: 0 kcal/day
- Total weight to lose: 0 kg
- Estimated weekly loss: 0 kg/week
- Estimated date at goal: —
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
This is a linear estimate: it assumes the same daily deficit holds steady all the way to your goal. In reality, TDEE falls as you lose weight — a lighter body needs less energy — so the real date you reach your goal is usually somewhat later than this simple projection, and the deficit needs recalculating every few kilos.
Where this goes wrong. A deficit larger than about 25% of TDEE risks losing muscle along with fat and is hard to sustain — picking a faster weekly loss rate than 1 kg/week usually falls into that range for most people. This is general information, not medical advice; check with a healthcare professional before making significant changes to your diet.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
It doesn't automatically — this calculator gives a single straight-line projection from today's numbers. Because TDEE actually drops as you lose weight, recalculating partway through with your new current weight will usually push the estimated date a bit later than the original projection.
Not necessarily. 1 kg/week is a faster rate that implies a larger deficit, which is more likely to exceed the 25% of TDEE guideline for smaller or less active people. Slower rates like 0.25-0.5 kg/week are generally easier to sustain and less likely to cost muscle mass.
It returns bMR. With 30 years age, 170 cm height and 80 kg current weight, that comes to 0 kcal/day. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.