How accurate is this calorie calculator?

The Mifflin-St Jeor equation we use predicts an individual to within roughly 10% either way, which is why we show a range of dates rather than a single one. The way to narrow it is to track what actually happens on the scale over two to three weeks and adjust from there.

Why is my timeline longer than other calculators say?

Most calculators divide your total weight loss by a fixed 7,700 calories per kilogram, which assumes your metabolism never changes. It does. As you get lighter you burn less, so the same intake becomes a smaller deficit. We simulate that week by week, which gives a later but far more realistic date.

How much protein do I need to lose weight?

Around 2 grams per kilogram of your goal bodyweight. Protein protects muscle while you are in a deficit, keeps you fuller than carbohydrate or fat, and takes more energy to digest.

Do I have to give my email to use it?

The calculator is completely free. We ask for your first name and email at the end so we can show your results and send you a copy of your plan to keep, along with the week-by-week schedule that goes with it.

Free tool

Your calories, your macros, and an honest timeline

A few quick questions and you will have your daily target, your macros, and a realistic date for hitting your goal. Takes about a minute.

Step 1 of 8

What would you like to do?

Your goal

The maths

How we work this out

No black box. Here is every step, so you can check our working — or argue with it.

What you burn at rest

We use the Mifflin-St Jeor equation, published in 1990. It is the current standard, and it holds up better across the general population than the older Harris-Benedict formula that a lot of calculators still run on.

Men    10×kg + 6.25×cm − 5×age + 5
Women  10×kg + 6.25×cm − 5×age − 161

Where kg is your weight, cm your height, and age your age in years. The answer is calories per day.

This is roughly what you would burn spending the entire day asleep. It is a floor, not a target — we never set your calories below it.

What you burn moving about

Most calculators ask one vague question here — sedentary, moderate, very active — and multiply your resting rate by a single number. Almost everybody picks one level too high, which makes this the biggest source of error in the whole calculation.

It is also just the wrong shape. Walking and training are different things: one is spread across your whole day and scales with how far you carry your bodyweight, the other is short, hard and scales with effort. Someone who sits at a desk but trains hard five times a week and someone who walks fifteen thousand steps and never trains are not the same person — yet a single multiplier calls them both “moderate”.

So we ask about them separately and add them up.

Walking

Carrying yourself a kilometre costs roughly half a calorie for every kilogram you weigh. We turn your step count into a distance using your own height, so the same ten thousand steps come out higher for someone tall and heavy than for someone short and light.

km   = steps × (height × 0.414) ÷ 1000
kcal = 0.5 × weight × km

Training

Sessions are measured in METs — multiples of what you burn sitting still. We subtract that one resting MET before counting, because your resting rate is already in the total and we are not going to charge you for it twice. The weekly figure is then spread across seven days.

kcal/min = (MET − 1) × 3.5 × weight ÷ 200

Easy3.5 MET
Mobility, yoga, a gentle swim — you could hold a conversation
Moderate6 MET
Weights with rest between sets, steady cardio, a fitness class
Hard9 MET
Circuits, intervals, running, Hyrox-style sessions

Intensity	Looks like	MET
Easy	Mobility, yoga, a gentle swim — you could hold a conversation	3.5
Moderate	Weights with rest between sets, steady cardio, a fitness class	6
Hard	Circuits, intervals, running, Hyrox-style sessions	9

Digesting your food

Eating costs energy too — roughly a tenth of what you take in. Since it scales with how much you eat rather than being a fixed amount, it goes in last, applied to the total.

daily burn = (resting + walking + training) ÷ 0.9

Add those together and you have what you burn in a day — built from things you can actually check on your phone, rather than an adjective you had to pick about yourself.

Your target, with limits

We set your deficit as a percentage of what you burn, not a flat 500 calories. A flat number is trivial for a 15 stone man and brutal for someone half his size.

Then we apply limits that we will not budge on, whatever you type in:

Floor: Never below your resting metabolic rate, and never below 1,200 calories for women or 1,500 for men.
Speed: Never faster than about 1% of your bodyweight a week — past that you start losing muscle along with the fat.
Honesty: If your goal needs a deficit we are not willing to prescribe, we stretch the timeline instead and tell you why.

We would rather give you a date you can actually reach than a number that looks impressive and makes you miserable.

How long it takes

This is where we part company with most calculators. The usual approach divides your total loss by a fixed 7,700 calories per kilo and hands you a date. It produces a straight line, and it is wrong.

It is wrong because you get lighter. A smaller body burns less, so holding your food steady means your deficit quietly shrinks every week until it reaches nothing. That is what a plateau actually is — not a failure of willpower.

So we simulate it a week at a time. Every week we recalculate what you burn at your new weight, apply your intake, and work out what actually changes. We also allow for adaptive thermogenesis — a real, measured effect where a body held in a deficit burns a little less than the equations predict.

The result is a curve that flattens, and a date that is usually a bit later than you were hoping. We would rather be honest about it now than have you think something has gone wrong in four months.

Why we show a range

These equations predict an individual to within about 10% either way. That sounds small, but stretched over months it is the difference between arriving in March and arriving in May.

So we show the range rather than pretending a single date is certain. The only way to narrow it is to watch what actually happens on the scale and adjust — which is exactly what the plan we email you is for, and exactly what we do with our coaching clients.

Your macros

Protein comes first and gets defended, because it protects muscle while you lose fat, keeps you fullest, and takes the most energy just to digest. Fat gets a floor for hormonal health. Carbohydrate takes whatever is left — it is what fuels your sessions, not the enemy.

Protein: 2.0 g per kg of your goal bodyweight
Fat: 25% of calories, and never under 0.6 g per kg
Carbs: Everything remaining
Fibre: 14 g per 1,000 calories

Before you start

These are estimates, and good ones — but they are still estimates. This is general guidance rather than medical advice, and it is written for healthy adults over 18. If you are pregnant or breastfeeding, managing a health condition, or have any history of disordered eating, please speak to your GP before changing how you eat.

Common questions

How accurate is this calorie calculator?: The Mifflin-St Jeor equation we use predicts an individual to within roughly 10% either way, which is why we show a range of dates rather than a single one. The way to narrow it is to track what actually happens on the scale over two to three weeks and adjust from there.
Why is my timeline longer than other calculators say?: Most calculators divide your total weight loss by a fixed 7,700 calories per kilogram, which assumes your metabolism never changes. It does. As you get lighter you burn less, so the same intake becomes a smaller deficit. We simulate that week by week, which gives a later but far more realistic date.
How much protein do I need to lose weight?: Around 2 grams per kilogram of your goal bodyweight. Protein protects muscle while you are in a deficit, keeps you fuller than carbohydrate or fat, and takes more energy to digest.
Do I have to give my email to use it?: The calculator is completely free. We ask for your first name and email at the end so we can show your results and send you a copy of your plan to keep, along with the week-by-week schedule that goes with it.

A number is a starting point. The rest is the hard bit.

Knowing what to eat and actually doing it, week after week, around a job and a family and everything else — that is where we come in. If you would like someone in your corner for it, have a look at how our coaching works.

See how coaching works