If you were born in January, or in the first few days of February, there is a good chance every Kua number you have ever been given is wrong — including on sites that look careful.
The reason is a single step that is easy to leave out and impossible to notice once it is missing.
The year the calculation uses is not the calendar year
The Kua number is worked out from your year of birth. But the year it means is the Chinese solar year, which does not begin on 1 January.
It begins at Li Chun — the start of spring in the Chinese solar calendar — which falls around 4 February, moving by a day either side depending on the year.
So somebody born on 20 January 1990 was born in the solar year 1989, not 1990. Feed 1990 into the formula and you get a different number, different directions, and no warning at all.
This is not an obscure edge case. It is everybody born in January plus the first few days of February — roughly one person in ten.
Why it is so easy to get wrong
Because the formula still returns an answer. Give it the wrong year and it produces a perfectly plausible number between 1 and 9, with four confident directions attached. There is no error, nothing looks broken, and the result is indistinguishable from a correct one unless you already know your answer.
A calculator that skips this step is not visibly worse than one that does not. That is precisely why it stays broken.
The lunar new year makes the confusion worse, because it is the date people have heard of. It moves between late January and late February and it is not the boundary this calculation uses. The solar term is, and it barely moves at all.
Check it in ten seconds
If you were born between 1 January and roughly 4 February, the year for this calculation is the year before your birth year.
You do not need to trust that. Try your own date on a calculator both ways — as your birth year and as the year before — and see whether it gives you two different numbers. If it gives the same answer for 20 January 1990 and 20 January 1989, it is not applying the boundary.
Ours applies it and says so on the page, including the exact date it used and why, so you can see the substitution happen rather than take it on trust.
The other step that gets skipped
While you are checking: there is no Kua 5.
The formula produces 5 for some people, and 5 has no seat in the eight-direction scheme — the centre is not a direction. The tradition substitutes: 2 for men and 8 for women. A calculator that reports Kua 5 and then lists four directions for it has invented them, because there is no such set to look up.
There is a third thing, quieter still. The arithmetic is cyclic in nine, so a calculation that lands on 0 has completed a cycle and means 9. Handled wrongly it produces a Kua of 0 — which, again, has no directions. That bug is real: this site had it, for men born in 2009, 2018 and 2027, and the page displayed “0” above the previous person’s directions until it was found and fixed.
What the number is and is not
The Kua number is arithmetic on a date. It names four directions and it does so by a rule anyone can follow and check.
What a direction means for a room, a bed or a desk is a practitioner’s judgement, and it is not something a formula can hand you. Getting the arithmetic right is the part a website can be held to — and it is the part most of them are getting wrong for one person in ten.
What this page is. It explains how the Kua number is calculated and which step most calculators miss. It does not tell you what will happen in a room. Nothing here is medical, legal or financial advice.