How Many Kilojoules per Day Do You Need?

How many kilojoules per day you need comes down to your body size and how much you move. A 35-year-old woman of 165 cm and 68 kg needs about 8,700 to 10,950 kJ, and a man of 178 cm and 80 kg about 11,130 to 14,270 kJ. Those are estimates from the NASEM 2023 equations, and a typical miss is around 1,000 to 1,400 kJ.

How many kilojoules per day does an adult need?

It depends on four things: sex, age, height and weight, plus how much you move. The 2023 equations from the US National Academies turn those inputs into a daily energy estimate, and they were built from doubly labeled water studies, which measure the energy people actually use. I ran them for two adults and converted at 4.184 kJ per kcal.

Person and activity	Equation	kcal a day	kJ a day
Woman, 35, 165 cm, 68 kg, inactive	584.90 − 7.01×35 + 5.72×165 + 11.71×68	2,080	8,700
Woman, low active	575.77 − 7.01×35 + 6.60×165 + 12.14×68	2,240	9,390
Woman, active	710.25 − 7.01×35 + 6.54×165 + 12.34×68	2,380	9,970
Woman, very active	511.83 − 7.01×35 + 9.07×165 + 12.56×68	2,620	10,950
Man, 35, 178 cm, 80 kg, inactive	753.07 − 10.83×35 + 6.50×178 + 14.10×80	2,660	11,130
Man, low active	581.47 − 10.83×35 + 8.30×178 + 14.94×80	2,880	12,030
Man, active	1,004.82 − 10.83×35 + 6.52×178 + 15.91×80	3,060	12,800
Man, very active	−517.88 − 10.83×35 + 15.61×178 + 19.11×80	3,410	14,270

Take the first row. The equation gives 2,079.6 kcal, and 2,079.6 × 4.184 is 8,701 kJ, which I rounded to 8,700. Each step up in activity adds from 578 to 1,470 kJ. The biggest jump is the man’s, from active to very active: 1,470 kJ.

Australia’s reference tables point the same way. Eat For Health says a reference man of 76 kg aged 19–30 needs 10.8 to 13.8 MJ from sedentary to moderate activity, and a reference woman of 61 kg needs 8.1 to 10.5 MJ. In kilojoules that is 10,800 to 13,800 and 8,100 to 10,500.

These equations come with a published error. The root mean square error was 246 kcal a day for women and 339 kcal for men. That is 1,029 and 1,418 kJ. Plan with a band of about a thousand kilojoules either side. A single number is false precision.

To switch any of these between units, use the kJ to Calories converter. The factor behind it, and how Australian labels use it, is in calories to joules. Today, find the row that looks most like you and read the range. Don’t copy the figure to the nearest kilojoule.

What do inactive, low active and very active mean?

They are bands of a number called the physical activity level, or PAL: your total daily energy, or TDEE, divided by your resting energy. For adults, NASEM sets inactive at a PAL below 1.53, low active from 1.53 to under 1.68, active from 1.68 to under 1.85, and very active from 1.85 to under 2.50.

Australia’s own reference page describes lifestyles with a similar scale. I lined its lifestyle examples up against the NASEM bands.

Lifestyle (Eat For Health)	PAL	NASEM band
Seated work, little strenuous leisure	1.4–1.5	Inactive
Seated work with some walking and standing	1.6–1.7	Low active, or active at the top
Mostly standing or walking work	1.8–1.9	Active, or very active at the top
Heavy work or highly active leisure	2.0–2.4	Very active

The same page says that 30 to 60 minutes of sport or strenuous leisure, 4 to 5 times a week, adds about 0.3 PAL units.

If you are between two bands, start with the lower one. Your weight trend will correct it. On Eat For Health’s scale a desk job is 1.4 to 1.5, and a few gym sessions a week add 0.3, so that person lands at 1.7 to 1.8: low active or active. A job spent on your feet starts at 1.8. “Very active” means heavy physical work or serious training, 2.0 and up.

What changes my kilojoule number the most?

Activity moves it most. Going from inactive to very active adds 2,250 kJ for the woman in my table and 3,140 kJ for the man. Next comes weight, then age and height. The equations give a fixed change per unit, so you can read each one straight off the coefficient. For the low active group, I multiplied each coefficient by 4.184 to get kilojoules.

Change	Women (low active)	Men (low active)
Each extra kg of weight	+51 kJ	+63 kJ
Each extra cm of height	+28 kJ	+35 kJ
Each extra year of age	−29 kJ	−45 kJ
10 kg heavier	+508 kJ	+625 kJ
10 years older	−293 kJ	−453 kJ

Take the woman’s weight coefficient: 12.14 × 4.184 is 50.8 kJ per kilogram. Ten kilos is 508 kJ. Ten years take off only 293 kJ, so your weight moves the answer more than your age does.

Equations know nothing about your muscle mass or how much you fidget. That is part of the 1,000 kJ band.

Is 8,700 kJ the amount I should eat?

No. The 8,700 kJ on Australian labels is a reference for working out percentage daily intake. Standard 1.2.8 of the Food Standards Code requires a footnote saying the percentages are based on an average adult diet of 8,700 kJ. It is 2,079 kcal, and it’s the same on every pack.

It sits close to one particular person: the inactive woman in my table, at 8,701 kJ. A taller, heavier or more active adult needs more. A smaller or older one needs less. So the label number is a yardstick for comparing foods, and it can’t tell you what to eat.

Here is the yardstick at work. A 40 g serve of dry rolled oats is 634 kJ, which a label would show as 7% of the reference. Set against the inactive man’s 11,130 kJ it is 5.7%, and against the very active man’s 14,270 kJ it is 4.4%. Same bowl, three different shares of a day.

How good are predictions for one person? Eat For Health says predictive estimates are much less accurate for individuals than for groups, and that energy needs can vary a lot even between people who look alike. To see how much two official sources disagree, I compared its reference table with the NASEM equations for an adult aged 40 at a BMI of 22. The pairing is mine: PAL 1.4 against inactive, and 1.6 against low active, which are the closest bands.

Reference adult	Eat For Health, PAL 1.4	NASEM inactive	Eat For Health, PAL 1.6	NASEM low active
Woman, 160 cm, 56.3 kg	7,600 kJ	7,860 kJ	8,700 kJ	8,510 kJ
Man, 180 cm, 71.3 kg	9,900 kJ	10,440 kJ	11,300 kJ	11,330 kJ

The two sources land within about 5.5% of each other in every cell. That’s much tighter than the 1,000 kJ miss for one person.

How do I find my own kilojoules per day?

Start from the equation, then let your weight correct it. The calorie calculator takes your age, height, weight and activity, uses these NASEM equations and can show kilojoules. Log your food for three weeks in a calorie counting app that handles kilojoules, and weigh yourself a few mornings a week. If you’d rather do the arithmetic yourself, how to calculate my TDEE walks through it by hand.

Flat weight trend? Then the average of what you ate is your real requirement, whatever the equation said. Add up the kilojoules from all 21 days and divide by 21. If the trend drifts instead, the gap between your log and the trend shows which way to move. Eat For Health calls doubly labeled water the best method of assessing energy needs. A flat weight trend is the nearest thing you can do at home.

This article is for adults. The NASEM adult equations start at 19. Pregnancy, breastfeeding, illness and some medicines change energy needs, and then your doctor or midwife sets the number, not a calculator.

Calsa sets your goal from a few questions in kcal or kJ, refuses to save one below a safe floor, and shows your weight as a smoothed trend you can hold up against it.