Adam Smith · 1723
5–8 unknown words per 100 at CET-4 level. Looking each one up will stall you; this one wants glosses that appear in place.
Coverage = the share of words in the text you already know. The research gives two lines: 98% before independent fluent reading is realistic, 95% for the instructional level that needs support (Hu & Nation, 2000). Proper nouns are not counted as unknown — recognising that it is a name is enough; this book has 807 of them.
CET-6 and up, or read it with glosses. 378,512 words in total; at 150 words a minute for a non-native reader, about 42 h 3 min to finish. Average sentence length 31.6 words — on the long side, so the load comes from sentence structure as much as from vocabulary. Reaching 95% coverage on this book (the floor for readability) takes roughly 14,000 word families.
The opening of An Inquiry into the Nature and Causes of the Wealth of Nations, word for word. Tap any word you don't know and its meaning here appears beside it — no sign-up, tap as often as you like.
The greatest improvements in the productive powers of labour, and the greater part of the skill, dexterity, and judgment, with which it is anywhere directed, or applied, seem to have been the effects of the division of labour. The effects of the division of labour, in the general business of society, will be more easily understood, by considering in what manner it operates in some particular manufactures. It is commonly supposed to be carried furthest in some very trifling ones; not perhaps that it really is carried further in them than in others of more importance: but in those trifling manufactures which are destined to supply the small wants of but a small number of people, the whole number of workmen must necessarily be small; and those employed in every different branch of the work can often be collected into the same workhouse, and placed at once under the view of the spectator.
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The numbers above are computed, but we haven't cut An Inquiry into the Nature and Causes of the Wealth of Nations into passages yet.
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These are already built, and closest in difficulty to An Inquiry into the Nature and Causes of the Wealth of Nations.
Ordered by unknown words per 100. Lower reads easier; to step up a level, pick a higher number.
Computed by Readii from the public-domain text (updated 2026-10-21), not estimated by hand. The Readii reading level is our own scale, anchored on unknown-word density; it is not the same system as Lexile or AR and does not convert directly. Unknown words are measured against the CET-4 ∪ Gaokao lists, so this is difficulty for a Chinese-speaking learner, not a native-child reading level.