Weight Loss Calculator in Stone and Pounds

How to use this calculator

1. Choose the lower activity levelOne step up the scale is worth around 175 calories a day here — more than a third of a typical deficit. Overestimating activity is the single commonest reason these calculations disappoint.
2. Start with a 500 calorie deficitThat gives about a pound a week, which is sustainable and does not cost much muscle. Bigger is not better.
3. Check the target against the floorThe calculator flags a target below 1,200 for women or 1,500 for men. Below that, get proper support rather than pressing on.
4. Treat the date as optimisticThe arithmetic is linear and your body is not. As you get lighter, maintenance falls and the same intake produces a smaller deficit.

Where the numbers come from

There are three steps, and each carries a different amount of uncertainty.

**Resting metabolic rate** comes from the Mifflin-St Jeor equation, which uses weight, height, age and sex. It estimates what you would burn doing nothing at all — and for most people that is somewhere between 55 and 70 per cent of total daily energy use. Of the three steps this is the most reliable, though it still carries real individual variation.

**Maintenance** multiplies that by an activity factor, from 1.2 for a sedentary day to 1.9 for someone training twice daily or doing physical work. This is by far the weakest link in the whole calculation.

People consistently overestimate their activity. Three gym sessions a week is "lightly to moderately active", not "very active", and one step up the scale is worth roughly 175 calories a day at typical weights — more than a third of a standard deficit. **If you are between two levels, choose the lower one**, and let the scales correct you upward rather than downward.

**The deficit** is then converted to weight using roughly 3,500 calories per pound of fat. A 500 calorie daily deficit is 3,500 a week, which is a pound.

That rule is where most of the disappointment comes from, so it is worth being clear about it. **It is a linear approximation of a system that is not linear.**

As you lose weight, you get lighter, and a lighter body burns less — both at rest and in everything it does. So the same intake that produced a 500 calorie deficit at 15 stone produces a smaller one at 13 stone. The rate slows even if nothing about your eating changes.

There is also adaptive thermogenesis: energy expenditure falls somewhat more than body size alone predicts during a sustained deficit. And unconscious movement — fidgeting, walking pace, general restlessness — tends to fall too.

None of that means the arithmetic is useless. It means the early weeks run roughly to plan and the later ones do not, and the finish takes longer than the calculator says. **Treat the date as the optimistic end of a range.**

One more thing worth understanding before watching the scales. Week-to-week weight moves for reasons that have nothing to do with fat: water, salt, glycogen, hormones, bowel contents and the weather can swing it several pounds in a day. A single morning tells you almost nothing. A weekly average tells you a great deal.

Worked example: 15 st 4 lb down to 12 st

A 40-year-old woman, 5 ft 9 in, currently 15 st 4 lb, aiming for 12 st, lightly active, on a 500 calorie deficit.

**Weight to lose:** 15 st 4 lb is 214 lb and 12 st is 168 lb, so **46 lb — 3 st 4 lb**. That is 21.5% of body weight.

**Resting metabolic rate:** at 97.1 kg and 175 cm, the Mifflin-St Jeor equation gives **1,705 kcal a day**.

**Maintenance:** at the lightly active multiplier of 1.375, that is **2,344 kcal a day**.

**Target intake:** 2,344 − 500 = **1,844 kcal a day**. Comfortably above the 1,200 floor, so no flag.

**Weekly rate:** 500 × 7 ÷ 3,500 = **1.00 lb a week**, which is 0.47% of body weight — well inside the usual guidance of 1 to 2 lb.

**Time:** 46 lb at a pound a week is **46 weeks**, or about 10.6 months.

Forty-six weeks is a long time, and seeing it written down is the point of the exercise. Losing three stone is a year of work, not a project for the spring.

BMI falls from 31.6 to 24.8 over that journey.

Now change the deficit and watch what it buys. At 250 calories: 0.50 lb a week, 92 weeks, eating 2,094. At 750: 1.50 lb a week, 30.7 weeks, eating 1,594. At 1,000: 2.00 lb a week, 23 weeks, eating 1,344.

So doubling the deficit from 500 to 1,000 halves the time — but it also means eating a thousand calories below maintenance every day for five months, and losing at the very top of the advised rate. **Faster costs more muscle and is far harder to sustain**, and a plan abandoned at week eight beats no plan only marginally.

Now the activity multiplier, which is where the real sensitivity lives. Keep everything else the same and change only the activity level:

Sedentary at 1.2 gives maintenance of 2,046 and a target of 1,546. Moderately active at 1.55 gives 2,643 and a target of 2,143.

That is a **597 calorie spread in the target** from one dropdown. If you pick "moderately active" when you are actually lightly active, you will eat 300 calories a day more than you meant to and the pound a week will simply not appear.

Which is why the honest instruction is to pick the lower level and adjust after a fortnight of actual scale data rather than trusting the estimate.

One last figure. A stone is 14 lb, or about 49,000 calories. At a 500 deficit that is 14 weeks per stone — a useful number to carry around, and a sobering one.

What the deficit buys

The last three rows make the point about the activity multiplier. The deficit is 500 in all of them, so the predicted rate and the time are identical — but what you actually eat swings from 1,546 to 2,441 calories, a difference of nearly 900. Pick the wrong level and either you are eating far more than you think, or you are eating far less than you need to.

What the arithmetic leaves out

A calorie model is a useful frame and a poor description of a person. Some things it does not capture are worth knowing before you rely on the date.

**Losing weight and losing fat are not the same thing.** In a deficit the body draws on fat and on lean tissue, and how much of each depends largely on protein intake and whether you are doing any resistance training. Losing three stone of which a large share is muscle leaves you lighter, weaker, and with a lower maintenance than you started with — which makes keeping the weight off harder.

Adequate protein and some form of strength work are what protect against that. They do not change the arithmetic on this page; they change what the weight loss is made of.

**The scales lag reality.** Water retention from a hard session, a salty meal, or the days before a period can mask a fortnight of genuine progress. People abandon plans that are working because of a number that had nothing to do with fat.

Weigh at the same time, in the same state, and judge by the weekly average.

**Maintenance is the real problem.** Most people can run a deficit for a few months. Far fewer keep the weight off, and the reason is that the end of a diet is usually the end of the plan. Deciding in advance what you will eat at the goal weight matters more than the route there.

**Adherence beats optimisation.** A 400 calorie deficit you can actually sustain for a year will take you further than an 800 you abandon in March. Every study of this points the same way, and it is worth choosing the deficit you can live with rather than the one that produces the best number on this page.

A few practical points. Track for a fortnight before adjusting anything, because two weeks of data beats any equation. If the scale has not moved in three weeks and you are confident about the tracking, drop 100 to 150 calories rather than making a dramatic cut. Do not eat back exercise calories from a watch — those estimates run high.

**And the boundaries.** This is a general estimate, not a diet plan and not medical advice.

Do not use it if you are under 18, pregnant or breastfeeding, or have a history of an eating disorder. If you are worried about your relationship with food or with your body, that is a conversation with a GP rather than a calculator, and it is worth having early.

Speak to a GP first if you have a medical condition, take medication that affects weight or appetite, or want to lose a large amount of weight — there is real support available and it works better than doing it alone.

And weight that comes off without trying is not a success. **Unexplained weight loss is a reason to see a doctor**, not a result to be pleased about.

Assumptions and limitations

• Predictive equations describe populations — individuals vary by hundreds of calories
• The activity multiplier is the weakest input and is routinely overestimated
• The 3,500 calories per pound rule is linear and real loss is not
• Maintenance falls as you get lighter, so the rate slows over time
• Does not distinguish fat lost from lean tissue lost
• Does not account for medical conditions or medication affecting weight
• Not for under 18s, pregnancy, breastfeeding, or a history of an eating disorder

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