Calorie Deficit Calculator
Find your daily calorie target and see how your weight is likely to change over a year.
Calorie deficit calculator
Built on the National Academies' 2023 energy equations and the NIH weight-change model, with the classic 3,500-calorie rule shown for comparison.
How many calories should I eat to lose weight?
To lose weight, start about 500 calories below your maintenance calories — roughly 1 lb a week at first. Don't drop below 1,200 calories a day (women) or 1,500 (men) without medical supervision. Expect your weekly loss to shrink over the months, because your body burns less as you get lighter.
How this calculator works
Your maintenance calories come from the 2023 National Academies energy equations, which were built on doubly labeled water measurements — the benchmark method for measuring how much energy people actually burn as they go about normal life.
We deliberately don't use the common 1.2 to 1.9 activity multipliers other calculators apply to a resting-metabolism formula. That multiplier table is a textbook convention we couldn't trace to a primary source, so we use equations that build activity in directly.
The year-long projection comes from the NIH dynamic weight-change model (Hall et al., The Lancet 2011) — the same model behind the NIH Body Weight Planner. It accounts for the fact that a lighter body burns fewer calories, so the same intake yields ever-smaller losses. We draw the old 3,500-calorie rule next to it, for comparison only.
Worked example
Take a 40-year-old woman, 5 ft 5 in (165 cm), 176 lb (79.8 kg), low active, eating 500 below maintenance. Her maintenance is about 2,354 calories a day (range 2,108 to 2,600), so her target is about 1,854.
Here is her weight by week — the NIH model versus the 3,500-calorie rule (kg in parentheses):
| Week | NIH model | 3,500-rule |
|---|---|---|
| 4 | 171.8 lb (78.0 kg) | 172.0 lb (78.0 kg) |
| 12 | 166.4 lb (75.5 kg) | 164.0 lb (74.4 kg) |
| 26 | 158.2 lb (71.8 kg) | 150.0 lb (68.0 kg) |
| 52 | 146.9 lb (66.6 kg) | 124.0 lb (56.2 kg) |
At that intake the model has her levelling off near 126.7 lb (57.5 kg) over the following years — losses keep shrinking as she gets lighter.
| Week | NIH model | 3,500-rule |
|---|---|---|
| 0 | 176.0 lb | 176.0 lb |
| 4 | 171.8 lb | 172.0 lb |
| 8 | 169.0 lb | 168.0 lb |
| 12 | 166.4 lb | 164.0 lb |
| 16 | 163.9 lb | 160.0 lb |
| 20 | 161.5 lb | 156.0 lb |
| 24 | 159.3 lb | 152.0 lb |
| 28 | 157.2 lb | 148.0 lb |
| 32 | 155.2 lb | 144.0 lb |
| 36 | 153.3 lb | 140.0 lb |
| 40 | 151.6 lb | 136.0 lb |
| 44 | 149.9 lb | 132.0 lb |
| 48 | 148.4 lb | 128.0 lb |
| 52 | 146.9 lb | 124.0 lb |
Typical starting numbers
Maintenance, target and weight after 52 weeks for common starting points (age 40, low active, 500-calorie deficit; 52 weeks = day 364):
| Person | Maintenance | Target | 52 wk — NIH model | 52 wk — 3,500 rule |
|---|---|---|---|---|
| Woman 5 ft 4 in, 150 lb | 2,194 | 1,694 | 121.1 lb (54.9 kg) | 98.0 lb (44.5 kg) |
| Woman 5 ft 4 in, 180 lb | 2,359 | 1,859 | 151.0 lb (68.5 kg) | 128.0 lb (58.1 kg) |
| Woman 5 ft 4 in, 210 lb | 2,525 | 2,025 | 180.9 lb (82.1 kg) | 158.0 lb (71.7 kg) |
| Man 5 ft 10 in, 180 lb | 2,844 | 2,344 | 151.4 lb (68.7 kg) | 128.0 lb (58.1 kg) |
| Man 5 ft 10 in, 220 lb | 3,115 | 2,615 | 191.6 lb (86.9 kg) | 168.0 lb (76.2 kg) |
| Man 5 ft 10 in, 260 lb | 3,386 | 2,886 | 231.7 lb (105.1 kg) | 208.0 lb (94.3 kg) |
Calculated with the 2023 equations and the NIH model; your numbers depend on age, height and activity.
Why the 3,500-calorie rule overshoots
The 3,500-calorie rule traces to Wishnofsky (1958) and assumes every pound lost costs exactly 3,500 calories (Thomas et al. 2014). Real bodies don't work that way.
As you lose weight you burn less, so the same intake produces ever-smaller losses. Hall et al. (2011) put numbers on it: every 10 calories a day of lasting change leads to about 1 lb of eventual weight change — half of it in about a year and 95% in about three years. And the energy value of the weight you lose depends on how much body fat you start with (Hall 2008).
For the worked example, the 3,500 rule predicts 52 lb lost in a year; the NIH model predicts about 29 lb.
Calculated on this page with the Hall et al. 2011 model.
How to choose your activity level
Inactive means everyday living only — little or no activity at work and no regular exercise (not bed rest). Low active adds some regular activity; active adds a good amount; very active means a physically demanding job, plenty of exercise, or both.
These levels come from the 2023 National Academies report, which split measured adults into quarters from least to most active. The report calls choosing your level the hardest step in the whole estimate. Pick your honest best guess, then adjust from your real weigh-ins (2023 report, chapter 7).
Safety limits
Harvard Health Publishing advises not eating below 1,200 calories a day for women or 1,500 for men without a health professional's supervision, so this calculator caps every plan at that floor — if your target lands lower, we run the plan at the floor instead.
It isn't for anyone under 19, pregnant or breastfeeding. If you have a history of disordered eating, talk to a professional before counting calories. If a goal weight would put you below the healthy range for your height, we don't project it (CDC adult BMI categories).
My story
I'm Marko. I built this calculator while losing weight myself. I went from 220 to 183 lb (37 lb) over about 34 weeks, aiming for roughly 500 calories a day below maintenance and judging progress by weekly averages, not any single day. More about me and how I check the math is on the about page.
Ways to use this calculator
Set your first target. Enter your details, pick a 500-calorie deficit, and start with the number the tool gives you. Confirm it against a couple of weeks of weigh-ins, then adjust. New to this? See how to calculate a calorie deficit.
Check a plan from an app or coach. If a program handed you a calorie number, compare it with your maintenance and target here. If it's far below the floor, that's a flag — read minimum calories per day.
Figure out why the scale slowed. Losses shrink as you get lighter, so a stall can just be the curve bending. Rerun with your current weight and compare with the solid line. More causes: in a deficit but not losing weight.
Test whether a goal date is realistic. Add a goal weight and see when each method reaches it — the model is the honest one. For aggressive plans, see is a 1,000-calorie deficit safe.
Frequently asked questions
How do I figure out my calorie deficit?
Subtract the deficit you choose from your maintenance calories. For the worked-example woman, that's 2,354 minus 500 = about 1,854 calories a day.
How long does it take to lose 20 pounds with a 1,000-calorie daily deficit?
For the worked-example woman, eating about 1,354 a day, it takes about 12.4 weeks by the NIH model, versus 10 weeks by the 3,500 rule. Note that eating only 1,000 calories a day in total would be below the 1,200 floor.
How big a deficit do I need to lose 2 pounds a week?
The 3,500 rule says 1,000 calories a day. The model shows that pace fading: for the worked-example woman it's about 2.1 lb a week in the first four weeks, easing to about 1.2 lb a week by weeks 21 to 24. For smaller or less active people, a 1,000 deficit can push intake below the floor.
Is 1,500 calories a day enough to lose weight?
For the worked-example woman (maintenance about 2,354), yes — that's about 854 below maintenance. For a 40-year-old, 5 ft 10 in, 180-lb low-active man (maintenance about 2,844), 1,500 is his floor and about 1,344 below maintenance — more than the 500 to 1,000 a day Harvard Health describes for a 1 to 2 lb pace.
Is 1,200 calories a day enough?
It's the lowest intake Harvard Health advises for women without supervision. For men it's below the 1,500-calorie floor, so it isn't recommended without a health professional.
Sources
- National Academies (2023), Dietary Reference Intakes for Energy, Table S-1
- National Academies (2023), chapter 7, activity levels
- Hall KD et al., The Lancet 2011 (NIH author manuscript)
- NIDDK, model appendix for Hall et al. 2011
- Hall KD, International Journal of Obesity 2008
- Thomas DM et al., Journal of the Academy of Nutrition and Dietetics 2014
- Harvard Health Publishing, Calorie counting made easy
- CDC, Adult BMI categories