Short answer
There is no single daily kilojoule number that fits everyone. The calculator estimates resting energy from your measurements, applies an activity factor, and reports the result in kJ. The 8,700 kJ you see on packaging is a labelling reference, not your requirement.
The reason this page exists is unit mismatch. If your labels are in kilojoules, converting every calculator output from Calories adds a step and an error each time you do it. Working in the unit your food is already labelled in removes both.
Where 8,700 kJ comes from
8,700 kJ is prescribed in the Australia New Zealand Food Standards Code as the average adult daily energy intake. It exists so that percentage figures on a label have a common denominator, which lets you compare two products without doing arithmetic against your own requirement.
It is explicitly an average. Actual daily energy needs vary considerably across the population with age, sex, body size, metabolism, and how much a person moves. Treating 8,700 as a personal target is the same mistake as treating a shoe-size average as your shoe size, and the calculator above will usually return something other than 8,700 for exactly that reason.
In Calories, 8,700 kJ is about 2,079. If you want to convert a specific label value rather than a daily total, the kJ to Calories chart covers everything from 10 kJ to 20,000 kJ.
What %DI on the panel is telling you
Percentage daily intake on an Australian or New Zealand nutrition information panel expresses what one serving contributes against that average adult diet. Including it is voluntary, but where a panel does show it, the panel must also carry the statement that percentage daily intakes are based on an average adult diet of 8,700 kJ.
That footnote is the important part and it is easy to skim past. A serving listed at 15% of daily intake means 15% of 8,700 kJ, not 15% of your own estimate. If the calculator above gave you 9,800 kJ, every %DI figure on every label you read is quietly understating your share; if it gave you 7,400 kJ, they are overstating it.
The %DI column is still useful for comparing two products against each other, since both are measured against the same denominator. It is comparison information, not personal guidance.
Keeping the log in one unit
Pick kilojoules or Calories and stay there. The most common way logs go wrong across unit systems is not the conversion arithmetic; it is a log holding a mix of both, where a few entries were converted and a few were not, and nothing on screen says which is which.
If your packaged food is labelled in kJ and your recipes or restaurant estimates come back in Calories, convert at one point in the process rather than several. Converting once as you enter a food, and storing only the converted figure, is easier to audit later than converting at review time.
The kilojoules to Calories guide covers the naming convention and the label rounding that makes a pack's two printed energy figures fail to divide out to each other exactly.
Testing the estimate against your own data
An equation plus an activity category is a starting assumption. Your food log and weight trend are the review, and after a few weeks they are better evidence than any formula.
The TDEE from weight change calculator works out the maintenance your own intake and weight trend imply over 14 to 28 days, and the maintenance calories guide explains how much weight to give a formula estimate in the meantime. Both work in Calories, so convert the kJ figure above once when you compare them.
If you want to see the resting-energy step on its own, the resting energy calculator runs two published equations side by side so the disagreement between them is visible.
Limits
The activity categories are broad descriptions of a changing week compressed into one multiplier, and moving one step changes the result by hundreds of kilojoules. If you already count exercise separately, do not also pick a high activity category; that counts the same movement twice.
This tool is general information for adults. It is not for children, pregnancy, breastfeeding, eating-disorder treatment, chronic disease, or setting a medically necessary intake. Speak with a qualified clinician or an Accredited Practising Dietitian for personal guidance, and stop tracking if it causes distress, guilt, or compulsive checking.
The 8,700 kJ reference and the requirement to state it alongside percentage daily intake come from the Australia New Zealand Food Standards Code, summarised by FSANZ and the NSW Food Authority. Resting energy uses the original Mifflin-St Jeor equation. The 4.184 conversion factor follows NIST's definition of the calorie.