How to Calculate a Calorie Deficit

To calculate a calorie deficit, estimate your maintenance calories and subtract the gap that matches the pace you want. A pace of 0.5 kg (1.1 lb) a week costs about 550 kcal a day. For a 90 kg man in the inactive category, the NASEM equation gives 2,737 kcal, so that pace means eating about 2,187 kcal.

How to calculate calorie deficit: where do I start?

The equations come from the 2023 National Academies (NASEM) report, which built them from measured energy use. The inputs are age in years, height in centimeters, weight in kilograms and an activity category. The result is a daily total. For an inactive man aged 19 or over, the equation is 753.07 − (10.83 × age) + (6.50 × height) + (14.10 × weight). Each sex and activity category has its own numbers.

I ran all four categories for three adults, so you can see how much the choice matters.

Adult	Inactive	Low active	Active	Very active
Man, 42 years, 180 cm, 90 kg	2,737 kcal	2,965 kcal	3,155 kcal	3,557 kcal
Woman, 55 years, 160 cm, 82 kg	2,075 kcal	2,242 kcal	2,383 kcal	2,607 kcal
Woman, 28 years, 170 cm, 62 kg	2,087 kcal	2,254 kcal	2,391 kcal	2,636 kcal

Here are two worked by hand. The 42-year-old man, inactive: 753.07 − (10.83 × 42) + (6.50 × 180) + (14.10 × 90) = 753.07 − 454.86 + 1,170 + 1,269 = 2,737 kcal. The 28-year-old woman, inactive: 584.90 − (7.01 × 28) + (5.72 × 170) + (11.71 × 62) = 2,087 kcal. Table 5-5 of the NASEM report has them all. The equations can’t see your muscle, your body fat or how efficiently your body runs. Five inputs, nothing else.

To skip the arithmetic, enter the same inputs in the calorie calculator. It runs these equations and never returns less than 1,200 kcal for women or 1,500 kcal for men.

The NIDDK Body Weight Planner is a second opinion, built on the same kind of inputs.

Which activity category should I pick?

Pick the lowest category that honestly fits your week. In NASEM’s Table 7-1, inactive is daily living only, and low active adds 60 to 80 minutes of walking at 3 to 4 mph. Active adds 30 to 50 minutes of walking plus 45 minutes of moderate cycling and 40 minutes of doubles tennis.

A desk job plus a few workouts a week usually lands in low active, not active. Being one category too high costs real calories. For the three adults above, the step from inactive to low active, or from low active to active, adds 137 to 228 kcal a day to maintenance. That is a quarter to nearly half of a 550 kcal gap. If you are torn between two, take the lower one.

How do I turn maintenance into a calorie deficit?

Choose a pace, then convert it to calories with the old rule of about 7,700 kcal per kg, which comes from the 3,500 kcal per pound rule that Hall and colleagues describe. It makes 1 kg a week cost 1,100 kcal a day. Half of that, 0.5 kg, costs 550 kcal. In pounds it’s simpler. One pound a week is 3,500 ÷ 7 = 500 kcal a day. The rule simplifies how a calorie deficit works in a real body, so I use it for planning and nothing finer.

A walk already inside your category shouldn’t be subtracted again from your food target, because that counts it twice. Add a category only when your week really changes.

The NHS suggests 1 to 2 lb (0.5 to 1 kg) a week. Two more limits keep the result sane. The first is a floor. The prescribed ranges in the AHA, ACC and TOS guideline start at 1,200 kcal a day for women and 1,500 kcal for men. The second is 1% of your body weight a week, a ceiling taken from a review by Helms and colleagues that is written for lifters. I apply both and let the smaller gap win. For UK readers, the NHS calorie deficit advice frames the same range as a daily cut.

Adult (activity used)	Maintenance	Target at 0.5 kg a week	Biggest gap by the floor	Biggest gap by 1% of weight
Man, 90 kg (inactive)	2,737 kcal	2,187 kcal	1,237 kcal	990 kcal
Woman, 82 kg (low active)	2,242 kcal	1,692 kcal	1,042 kcal	902 kcal
Woman, 62 kg (inactive)	2,087 kcal	1,537 kcal	887 kcal	682 kcal

For the 62 kg woman the 1% limit is 0.62 kg a week, or 682 kcal a day. A 0.75 kg pace, which needs 825 kcal, is over it. A pace of 0.5 kg fits. In every row the weight limit is the tighter one, so it decides for all three.

If your labels are in kilojoules, the 82 kg woman’s target of 1,692 kcal is 7,079 kJ, because 1 kcal = 4.184 kJ according to the FAO.

What is my calorie deficit for today or this week?

Subtract what you ate from your maintenance, then look at the week and not the day. That difference is the calorie deficit itself. Take the 82 kg woman again, with a maintenance of 2,242 kcal and a target of 1,692 kcal. The week below is from her diary, and the conversion uses the 7,700 kcal per kg rule.

Day	Mon	Tue	Wed	Thu	Fri	Sat	Sun	Week
Eaten (kcal)	1,950	2,100	1,800	2,300	1,900	2,600	2,000	14,650

Her maintenance for the week is 2,242 × 7 = 15,694 kcal. The deficit is 15,694 − 14,650 = 1,044 kcal, or 149 kcal a day. By the 7,700 kcal per kg rule that is about 0.14 kg, not the 0.5 kg she planned. She ate above her 1,692 kcal target every day, and Saturday alone ran 358 kcal over maintenance.

Plan the week, not the day. If you eat 300 kcal over on a Saturday, spread the cost across the other six days (50 kcal each) and the weekly total holds.

How do I check my number against my own weight?

Log everything you eat for two to four weeks, and weigh on at least three mornings each week. Then work backward. Your maintenance is your average intake plus the energy behind the weight you lost. That energy is the drop in kilograms, divided by the days, times 7,700.

Say the 82 kg woman averages 2,150 kcal a day. Her weekly average weight falls from 82.6 kg in week 1 to 81.8 kg in week 4, which is 0.8 kg over 21 days. That is 0.038 kg a day, or about 293 kcal a day. Her maintenance is then about 2,150 + 293 = 2,443 kcal, and the equation said 2,242. The gap is 201 kcal. The NASEM equations have a typical error of 246 kcal a day for women and 339 kcal for men, so both numbers are plausible.

I built this check from the 3,500 kcal rule. No paper publishes it. It works only if the log is complete, because a missing oil or a skipped weigh-in turns it into noise. With a complete log, use your own number and set the target from it, but with a patchy log stay with the equation.

What if the numbers leave no room for a deficit?

In that case the gap shrinks or disappears. The guideline starts its prescribed range for women at 1,200 kcal. Say a woman’s maintenance comes out at 1,650 kcal. That leaves 450 kcal of room, and a pace of 0.5 kg needs 550, so she can’t run it without going under the floor. A gap of 275 kcal, or 0.25 kg a week, fits.

Short, older or light adults get small maintenance numbers, because small bodies burn less. The guideline also reserves diets under 800 kcal a day for medical care, and no calculator should hand you a target near that. These equations cover adults only. If you are under 18, pregnant, breastfeeding or on a medication or treatment that touches weight, your doctor or midwife sets the target.

Write your maintenance number and its category on paper. Log for two weeks, compare the averages, then change the target in small steps. How long to stay in a calorie deficit is the next question once the numbers settle.

If counting calories has started to feel hard to stop, or eating feels out of control, talk to someone: ANAD in the US (1-888-375-7767), Beat in England (0808 801 0677), NEDIC in Canada (1-866-633-4220) or the Butterfly Foundation in Australia (1800 33 4673). If you’re in danger right now, call your local emergency number. Other UK numbers and crisis lines are on our crisis support page.