Guide
How the weight loss calculator by date works
Every number in your plan comes from a published formula you can check by hand. This guide shows how each one is made.
1. The calories you burn at rest
Your basal metabolic rate (BMR) is the energy your body uses at rest to breathe, pump blood and repair tissue. The calculator estimates it with the Mifflin-St Jeor equation (Mifflin et al., 1990). It needs only your sex, age, height and weight.
WomenBMR = 10 × kg + 6.25 × cm − 5 × age − 161
MenBMR = 10 × kg + 6.25 × cm − 5 × age + 5
Example10 × 78.0 + 6.25 × 165.1 − 5 × 38 − 161 = 1,461 kcal
The example is the woman from the calculator: 172 lb (78.0 kg), 5 ft 5 in (165.1 cm), 38 years old.
Add your body fat % under Edit and the calculator switches to Katch-McArdle: BMR = 370 + 21.6 × lean mass in kg. It is more accurate when your body fat number is reliable, because it works from lean mass, the part of you that burns the most energy. Two people with the same height and weight can carry very different amounts of muscle, and this formula sees the difference. Not sure of yours? The body fat calculator gives you a number in a minute.
2. How active you are
BMR covers rest only. To find your maintenance calories, the calories that hold your weight steady, the calculator multiplies your BMR by an activity factor. If you track steps, use the daily steps column to pick your level.
| Activity level | What it looks like | Daily steps | Factor |
|---|---|---|---|
| Sedentary | Desk job, little planned exercise | Under 5,000 | 1.2 |
| Lightly activeExample | Light exercise 1 to 3 days a week | 5,000 to 7,499 | 1.375 |
| Moderately active | Exercise 3 to 5 days a week | 7,500 to 9,999 | 1.55 |
| Very active | Hard exercise 6 to 7 days a week | 10,000 to 12,499 | 1.725 |
| Extra active | Hard daily training or a physical job | 12,500 or more | 1.9 |
In the example: 1,461 × 1.375 = 2,009, shown as 2,010 kcal a day. Step bands follow Tudor-Locke and Bassett, Sports Medicine, 2004. Walking more lifts this number, and the walking weight loss calculator shows by how much.
3. Solving for your date
Your body burns less as it gets smaller, so the calculator plays your plan forward one day at a time. Each day it compares what you eat with your maintenance calories at that day's weight, and turns the gap into weight at 7,700 kcal per kg (about 3,500 kcal per lb).
Then it finds the one daily target that lands on your goal weight on your date. You eat the same amount every day, and your deficit shrinks a little each week. That is why your chart is steeper at the start and eases toward your goal.
The example plan, day by day
- Maintenance
- Your target
Recalculate after every 5% you lose. Every result here is an estimate, and real bodies vary. Track your weight for two to three weeks and compare it with the plan. Then recalculate each time you lose 5% of your body weight, because a smaller body needs fewer calories.
Safe weight loss limits behind every plan
A plan you can keep beats a plan that looks fast on paper. Every result is checked against five limits before you see it. If a plan breaks one of the four hard limits, the calculator does not show it as a plan. It tells you why in one sentence and offers safer options. The fifth, your resting burn, is a softer line: a plan under it still shows, marked doable but tough.
- Weekly pace2 lb a week at most
- Or 1% of your body weight, if that is lower. The calculator checks your first week, because loss is fastest then. Health agencies call 1 to 2 lb (0.5 to 1 kg) a week a healthy pace. Faster loss costs more muscle and raises the risk of gallstones.Source: CDC healthy weight guidance
- Calorie floor1,200 women, 1,500 men
- The fewest calories a day the calculator plans. Below these levels it is hard to get enough protein, vitamins and minerals from food. Lower intakes belong under a doctor's care.Source: AHA/ACC/TOS obesity guideline, 2013
- Resting burnA soft check
- Eating under the energy your body uses at rest is hard to keep up, because hunger pushes back. The calculator still shows the plan, marked doable but tough, with both numbers so you can judge.Source: Olive safety rule
- Biggest daily deficit1,000 kcal at most
- A deficit of 500 to 1,000 kcal a day matches a pace of 1 to 2 lb a week. Bigger gaps are hard to keep and cost more muscle.Source: NHLBI clinical guidelines, 1998
- Healthy goal weightBMI 18.5 or higher
- The calculator never plans a goal below a BMI of 18.5, the World Health Organization's cutoff for underweight. If you enter one, it offers the lowest healthy weight for your height instead. At 5 ft 5 in, that is 112 lb.Source: WHO BMI categories
When your goal date is too soon
Some goals need more time than the calendar allows. Take the same woman with a new goal: 150 lb in 10 weeks. That needs 2.2 lb a week on about 840 calories a day, which breaks three of the four hard limits. The calculator tells her so in one sentence, with the calories as the reason, and offers three ways forward.
Move the date. Keep your goal weight and take the earliest safe date instead. For her, that is 150 lb in about 15 weeks, 5 weeks later than she planned, at about 1,210 calories a day. That is under her resting burn of 1,460, so the plan is doable but tough.
Keep the date. Aim for the lowest weight you can reach safely by then. For her, that is 157 lb in 10 weeks, down 15 lb, at about 1,220 calories a day. The chart keeps her first goal in view: a dashed line goes on to 150 lb at a safe pace, about 5 weeks later, and the plan says what to eat from that date.
Talk to a clinician. If you have a lot to lose, ask about medical weight loss. At the average pace seen in clinical trials (STEP 1 and SURMOUNT-1), losing 22 lb (12.8% of body weight) on GLP-1 treatment takes about 16 to 35 weeks. Results vary, and a licensed clinician decides if treatment is right for you.
When a deficit isn't enough
A calorie target is simple on paper. Hunger makes it hard. When you eat less, your body pushes back with a bigger appetite and louder cravings, and the plan starts to slip.
GLP-1 medicines work on those signals. They slow how fast your stomach empties and act on appetite centers in the brain, so you feel full sooner and think about food less. A steady deficit can feel possible again.
- 14.9%
- Average weight loss at 68 weeks on semaglutide 2.4 mg. STEP 1, Wilding et al., NEJM 2021.
- 20.9%
- Average weight loss at 72 weeks on tirzepatide 15 mg. SURMOUNT-1, Jastreboff et al., NEJM 2022.
Both trials paired the medicine with diet and activity changes. Results vary.
Habits first. A clinical push when your body needs it. A licensed clinician decides if treatment is right for you.
Compounded medications are tailored to a patient's needs and are not reviewed or approved by the FDA for safety or effectiveness. Prescription required. Eligibility, prescribing, and dosing are determined individually by a licensed clinician based on your medical history. Treatment is not appropriate for everyone. Results vary.