The character 兆 appears on both sides of the Taiwan Strait, in Japanese law, and in the specification of every megawatt of Chinese wind capacity. It does not mean the same number in all of them.
In the People's Republic it means a million. In Taiwan it means a million million. The gap is six orders of magnitude. Both values sit in instruments that are current, and each side assigns the other's word to the other's number — a swap rather than a difference of dialect or drift.
The two tables, and what they actually say
The mainland's assignment comes from the annex to the State Council order of 27 February 1984, which set out the legal units of measurement. Its table of prefixes gives 10⁶ as 兆, with the symbol M, and 10¹² as 太［拉］, with the symbol T.
Taiwan's Bureau of Standards, Metrology and Inspection publishes the opposite. Its guide to the statutory units lists T as 兆 and M as 百萬 — literally "hundred ten-thousands". The underlying instrument dates from 1974 and was amended in 1992.
The symmetry is exact. Mega is 兆 in Beijing and 百萬 in Taipei; tera is 太 in Beijing and 兆 in Taipei. Each side has a word for each magnitude. They have simply exchanged them.
The arithmetic is confirmed by each side's own money. Taiwan's central government budget for 2026 records expenditure of 3兆0,350億元 — the composition of 兆 with 億 proves 兆 is a million million, because there is no other reading in which the figure parses. Mainland documents do the reverse, keeping 兆 for megawatts and reaching for 万亿 when they need 10¹².
The mainland's own instrument says to avoid the collision
The ninth note attached to that 1984 annex goes further. It states that 10⁴ is called 万, 10⁸ is called 亿, and 10¹² is called 万亿 — and that the use of these numerals is unaffected by the prefix names, but they must not be confused with them.
So writing 万亿 for a million million is not a habit that grew up around the standard. It is prescribed by the same document that assigns 兆 to mega, and the reason is stated: the numerals and the prefixes are different systems that happen to share characters.
That distinction is why a mainland document can use 兆瓦 for megawatt and 万亿 for 10¹² on the same page without contradiction. 兆 is a prefix there and nothing else.
Japan has no kanji prefixes at all
Japan is often listed alongside Taiwan as a 10¹² jurisdiction. The number is right; the mechanism described is not.
The Japanese measurement order lists every SI prefix in katakana — テラ for tera, ギガ for giga, メガ for mega. There is no kanji prefix table to disagree with. But the same order uses 兆 as a numeral, and does so in its own definitions: the candela is defined in terms of 五百四十兆ヘルツ, 540 terahertz, and parts-per-trillion is written 質量一兆分率 against parts-per-billion as 質量十億分率.
Japanese law therefore fixes 兆 at 10¹² inside the very document that names tera in katakana. The finance ministry's budget does the same in prose: a line printed as 382,778億円 in the table appears as 38.3兆円 in the text, and the two only reconcile at 兆 = 10¹².
Only Taiwan actually assigns 兆 to tera as a prefix. Japan's 兆 is purely a numeral.
Taiwan does not agree with itself
The sharpest disagreement sits inside a single government.
The standards bureau's guide, in its worked examples of correctly written units, gives 兆赫 as the Chinese for terahertz. The Ministry of Education's revised dictionary defines 兆赫 as megahertz, in the form "1兆赫(MHz)＝1000千赫". The same dictionary defines the bare character 兆 as 億的萬倍 — ten thousand times a hundred million, which is 10¹².
So the dictionary contradicts the standards bureau, and it also contradicts itself: 兆 alone is a million million, but 兆赫 is a million hertz.
The telecommunications regulator appears to have noticed. Its regulation on radio frequency use does not contain 兆赫 at all — the string does not appear. Where it needs megahertz it writes 百萬赫, and for gigahertz it writes 吉赫. Faced with a word its own government cannot agree on, the regulator stopped using it.
Where 10¹² actually comes from, which is not where people say
The usual account is that 兆 = 10¹² is the classical value and the mainland's million is a modern reassignment. The classical texts do not support it.
The old sources describe three systems. In the lesser system, each name is ten times the last, and 兆 is 10⁶. In the middle and greater systems, the names climb much faster, and 兆 lands at 10¹⁶. No classical system yields 10¹² at all.
The million-million reading comes from a Qing mathematical compendium that discarded those three ladders and imposed a new one advancing by ten thousands. So the mainland's 10⁶ revives the oldest of the classical values and Taiwan's 10¹² follows the Qing reform. Both are old, and each has a documented lineage of its own.
There is a further turn. In 1948 the first meeting of Academia Sinica's academicians resolved to define 兆 as a million, drop 亿, and move to a thousand-based ladder. It was never enacted; the government moved to Taiwan and the proposal lapsed. The mainland adopted 兆 as a million in 1984 regardless. The two governments ended up on opposite sides of a question one of them had already voted on.
English did the same thing, and the moment it is credited to did not happen
This is not peculiar to Chinese. "Billion" meant a million million in British use and a thousand million in American use. The switch is often credited to a specific date: 20 December 1974, when Harold Wilson answered a written question in the Commons.
The citation is real. The record is there, the answer is Wilson's own, and it is a written answer of that date. What it says is not what it is credited with.
The question came from a member asking Wilson to commit ministers to the older, larger meaning. The answer opens with a single word: "No." Wilson then observes that the word "is now used internationally to mean 1,000 million" and that it would be confusing for British ministers to use it otherwise — a description of existing practice, not an instruction. He closes by accepting that it "could still be interpreted in this country as 1 million million" and asking colleagues only to avoid ambiguity.
The widely repeated gloss says government statistics would from then on use the smaller value. That sentence is not in the record, and the word "statistics" does not appear on the page. Wilson declined to defend the older meaning. He did not order a switch.
What to do when the number matters
Read the unit, not the character. If a figure is composed — 兆 followed by 億, or 万亿 spelled out — the composition tells you the ladder. Taiwan's budget line and the mainland's 万亿 are both self-proving in that way.
When converting between notations, keep the prefix and the numeral systems separate. The mainland instrument makes that distinction explicitly, and observing it dissolves most of the confusion. Our currency notation converter works in the numeral system rather than the prefix system, which is why it renders 10¹² through 亿 rather than reaching for 兆 — and, by accident rather than design, that matches what the 1984 note prescribes.
Where this comes from, and what will date it
The mainland table and its ninth note were read from the 1984 State Council instrument as published on two provincial regulator sites. Taiwan's assignment is from the standards bureau's own guide of November 2020 and the national measurement laboratory's table. Japan's prefixes and its 兆 numerals are from the measurement order as served by the government's own legislative interface. The budget figures are from each finance ministry's published documents. The Hansard answer was read from the parliamentary record.
Two limits apply. The classical values rest on two independent transcriptions rather than a scholarly critical edition. And we make no claim about Hong Kong: its regulator writes megahertz in Latin and uses 吉赫 for gigahertz, and no statutory table of Chinese prefix names could be obtained, so the honest answer is that we do not know rather than that it follows either side.
This guide will be dated by any revision to the mainland's prefix standard. Both Chinese-language tables predate the 2022 additions to the SI, which the mainland named separately in 2023 and which Taiwan's table has yet to reach. Japan's order already carries them.
