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Free Health and Nutrition Calculators
Four calculators that answer the questions every weight-loss, weight-gain, or maintenance plan starts with — body mass index, daily calorie burn, macro targets, and fluid intake. Free, no signup, and everything runs in your browser.
Each one answers a different question, and together they answer the whole first day of a plan: where you are now, how much you burn, what to eat, and how much to drink.
Nothing here is stored. Work out the numbers, use them, close the tab. When you want them to follow you week to week — recalculated as your body changes, and counted against every meal you eat — that is what the app is for.
3-day free trial · Cancel anytime · No card charged during trial
BMI Calculator
Enter your height and weight. Nothing is uploaded and nothing is stored — the maths runs in your browser.
Your BMI
25.5
Overweight · 25.0 – 29.9What this means
Body weight is above the healthy-weight band. A modest, steady calorie deficit is the usual starting point — not a crash diet.
Work out your maintenance calories first, then take a moderate deficit off it. A 0.5 kg per week pace is the one most people actually hold.
This number changes every week. SlimAI keeps it current.
Sign up once and SlimAI stores your height, weight, and BMI history, recalculates your healthy range as you go, and adds an AI body-fat estimate BMI cannot give you.
3-day free trial · Cancel anytime · No card charged during trial
BMI is a screening ratio, not a body-composition measurement. It cannot tell muscle from fat, and it reads high for muscular people and low for people carrying little lean mass. It is not medical advice.
The Four Numbers, in the Order They Matter
BMI Calculator
Where am I now?
See your BMI category and the exact weight range that puts you inside the healthy band.
Open calculator →ThenTDEE Calculator
How many calories do I burn and how many should I eat?
Get maintenance calories plus a goal-adjusted daily target, from BMR to deficit.
Open calculator →ThenMacro Calculator
What should those calories be made of?
Convert calories into daily protein, carb, and fat grams, with a protein sanity check.
Open calculator →AlongsideWater Intake Calculator
How much should I drink while doing all of it?
Get a daily fluid target from weight, age, training, and climate — in glasses or bottles.
Open calculator →Four numbers on a page, or one profile that keeps them current.
SlimAI stores your targets, recalculates them as your body changes, and counts every meal against them.
3-day free trial · Cancel anytime · No card charged during trial
Short Answers to the Four Questions
How do you calculate BMI?
BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). For adults, the WHO classifies a BMI under 18.5 as underweight, 18.5–24.9 as a healthy weight, 25.0–29.9 as overweight, and 30.0 or above as obesity. BMI is a screening ratio, not a body-composition measurement: it cannot separate fat from muscle.
- Formula: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
- Healthy adult range: 18.5 to 24.9
- BMI does not measure body fat, and reads high for muscular people
What is TDEE and how is it calculated?
TDEE (total daily energy expenditure) is the number of calories you burn in a day, including everything you do. It is calculated by working out your BMR — the calories you burn at complete rest — and multiplying it by an activity factor from 1.2 for sedentary to 1.9 for extra active. Eating at your TDEE holds your weight; eating below it loses weight; eating above it gains weight.
- BMR via Mifflin-St Jeor, or Katch-McArdle if you know your body-fat percentage
- TDEE = BMR × activity multiplier (1.2 to 1.9)
- A deficit of about 550 kcal a day is roughly 0.5 kg of fat a week
- Estimates, not measurements — adjust against real results after 2–3 weeks
How do you calculate macros?
Start from your daily calorie target, choose a percentage split across protein, carbohydrate, and fat, then convert each share into grams — protein and carbohydrate provide 4 calories per gram, fat provides 9. A common balanced split for fat loss is 30% protein, 40% carbohydrate, 30% fat, and most guidance puts protein between 1.6 and 2.2 g per kilogram of body weight.
- Protein: 4 kcal per gram · Carbohydrate: 4 kcal per gram · Fat: 9 kcal per gram
- Grams = (calories × share%) ÷ calories-per-gram
- Protein target: roughly 1.6–2.4 g per kg while losing fat
- Calories decide whether weight moves; macros decide what that weight is made of
How much water should you drink a day?
A common working guideline for healthy adults is 30–40 ml of total water per kilogram of body weight per day, which is about 2.3–3.1 litres for a 78 kg adult. Roughly 20% of that arrives through food, so the amount to actually drink is closer to 1.9–2.5 litres. Add about 350 ml for every 30 minutes of exercise, and more again in hot or humid conditions.
- Baseline: about 35 ml per kg of body weight for adults aged 31–55
- Around 20% of total water intake comes from food, not drinks
- Add roughly 350 ml per 30 minutes of sweating exercise
- Urine colour is the most practical day-to-day check: pale straw is the target
A Calculator Gives You a Number. A Plan Needs the Number to Keep Up.
Every calculator on this page has the same limitation: it knows you for exactly as long as you have the tab open. It cannot know what you weighed last month, whether you hit your protein yesterday, or that your maintenance calories have dropped six kilograms into a diet.
That is not a flaw in the arithmetic — the formulas are the same ones any app uses. It is a flaw in the format. A one-off answer to a question that changes weekly goes stale almost immediately.
SlimAI Calorie Tracker takes the same inputs during a short setup — sex, age, height, weight, activity level, goal and pace, target weight, body type, preferred diet, favourite cuisine — and keeps them. Your calorie goal, gram-level macro targets, healthy weight range, water goal, BMI, and body-fat trend all recalculate as you log, and every meal you record counts against them automatically.
You can also override any calculated figure and set your own daily calorie goal and gram-level protein, carbohydrate, and fat targets, which is what athletes, coaches, and anyone following a prescribed plan usually needs.
FAQ
Still Wondering About SlimAI
Yes. All four are free, require no account, and run entirely in your browser — your height, weight, and other inputs are never sent to a server or stored between visits. SlimAI Calorie Tracker is also free to download, with a Premium tier for unlimited AI scans and AI Body Assessment photo analysis.
Start with BMI to see where you are, then TDEE to find out how many calories you burn and what your daily target should be, then the macro calculator to split that target into protein, carbohydrate, and fat. The water intake calculator runs alongside all three.
They use the same standard formulas found in clinical and sports nutrition practice — WHO BMI thresholds, Mifflin-St Jeor and Katch-McArdle for energy expenditure, Atwater factors for macros, and body-weight scaling for fluid needs. All of them are population estimates rather than measurements of you specifically, and predictive energy equations typically land within about 10% of measured values. Verify against real results over two to three weeks and adjust.
Yes. Every calculator has a metric and imperial toggle, and switching converts your existing entries rather than clearing them. Water results are shown in litres or fluid ounces to match.
No. The calculations run entirely in your browser and nothing is uploaded or saved. If you want your numbers kept and updated over time, that requires an account in the SlimAI app, where your profile, history, and targets are stored and recalculated as your body changes.
No. SlimAI is not a medical app, a disease-management app, or a diagnostic tool, and these calculators are general fitness and nutrition estimates. If you have a medical condition, are pregnant or breastfeeding, take medication, or have a history of disordered eating, consult a qualified health professional before changing your diet, training, or fluid intake.
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