Weight Loss Simulator

How the weight loss simulator works

Most calculators assume you lose the same amount every week forever. Real weight loss doesn't work like that: as you get lighter, your body burns fewer calories, so the same diet creates a smaller gap each week. Researchers at the NIH have shown that this is why the old 3,500-calorie rule overestimates long-term weight loss.

This simulator steps through your plan one day at a time:

1. It works out your maintenance calories today with the Mifflin-St Jeor equation and your activity level, the same way as our TDEE calculator.
2. It sets your daily intake: maintenance minus your chosen deficit. You eat that amount every day, the way most people follow a plan.
3. Each day, it recalculates maintenance at your new weight, adds your walk, and changes your weight by the gap, at about 7,700 calories per kilogram (3,500 per pound).

The result is a curve that starts steep and flattens out, plus a goal date you can plan around.

Choosing a deficit

The CDC suggests 1 to 2 pounds a week for weight loss that lasts. The simulator flags plans that go faster and never sets intake below 1,200 calories for women or 1,500 for men, levels where it gets hard to meet nutrient needs without medical supervision. Our 1300 calorie diet shows a full week just above that floor. For what supervised plans below that look like, see the 800 calorie diet guide.

Adding walking to lose weight

Walking burns calories on top of your diet, so it speeds the plan up without eating less. A brisk 3 mph walk burns about 3.8 calories per kilogram of body weight per hour. That's about 155 calories for 30 minutes at 181 pounds. In the example, that one daily walk cuts 8 weeks off the plan. Use the slider as a free walk calculator to lose weight, or see the full numbers in our walking calorie calculator.

Zig-zag diet calculator

Zig-zag dieting, also called calorie cycling, keeps the same weekly total but moves calories between days: a couple of higher days and five lower ones. Weight loss is the same as eating evenly, because the week adds up to the same number. Some people find it easier, for example eating more on weekends or training days. Pick "Zig-zag days" and the zig-zag calorie calculator shows your week.

Reverse dieting calculator

Reaching your goal isn't the end of the plan. Your maintenance at the new weight is lower than it was at the start: about 1,920 calories for the example woman at 154 pounds, against 2,080 at 181. Reverse dieting means stepping your calories back up gradually, about 100 a week, until you're at the new maintenance. It gives you time to watch the scale and adjust. The simulator works out how many weeks that takes.

When do you start seeing weight loss?

The scale often drops in the first week or two, partly from water and stored carbohydrate. Steady fat loss of about 1 pound a week shows on the scale within 2 to 4 weeks. Most people start noticing it in clothes and photos after 6 to 8 weeks. So how long does it take to see weight loss others notice? Often 2 to 3 months.

Weigh yourself at the same time each day and look at the weekly average; day-to-day swings of 1 to 3 pounds are normal.

Turning the numbers into a weight loss plan

A goal date is a starting point for an online weight loss plan, not a promise. To make it work:

• Set your calories with the simulator, then split them with the macro calculator to keep protein high.
• Fill up on fiber: our fiber calculator helps you hit 25 to 38 grams.
• Drink water instead of soda or juice; water has no calories, and a 20 oz soda has about 258.
• Log a typical week in the calorie counter to check you're really eating the planned amount.
• If skipping breakfast makes the gap easier, map out an eating window with the fasting calculator.
• Need meals to match? Our 7-day meal plan lands at about 1,500 calories a day and scales to 1,200, 1,800 or 2,000.
• Re-run the simulation every month with your new weight; real progress is rarely a perfect curve.

Weight loss simulator FAQ