What it means
یکصد (yek-sad) is one hundred, written as one word, pronounced as one breath. It is the formal and emphatic version of صد (sad), the plain everyday hundred. The two mean the same number but they do not sit in the same register. Sad is what you say when paying for groceries (صد تومن, hundred toman). Yek-sad is what you write in a contract, what a newscaster reads on the evening bulletin, what an official notice prints across the top of the page.
Sad vs yek-sad
English has the same split. A hundred is the everyday form, one hundred is the formal and emphatic form. You say a hundred dollars at the store, you write one hundred dollars on the check. Persian works the same way. Sad is the unmarked default, yek-sad adds weight and clarity. Newspaper headlines reach for yek-sad to make a number land harder: یکصد کشته در زلزله (yek-sad koshte dar zelzele, one hundred dead in the earthquake) hits differently than صد کشته.
You will also see yek-sad used to disambiguate. صد can be misheard as سد (sad, dam) in noisy speech. یکصد never gets mistaken. Legal Persian leans on it for that reason.
How to use it
yek-sad nafar dar tasadof zakhmi shodand.
One hundred people were injured in the accident.
یکصد نفر در تصادف زخمی شدند.
mablagh-e qarardad yek-sad million toman ast.
The contract amount is one hundred million toman.
مبلغ قرارداد یکصد میلیون تومان است.
yek-sad sal pish, hich kas in shahr ra namishenakht.
One hundred years ago, nobody knew this city.
یکصد سال پیش، هیچ کس این شهر را نمیشناخت.
Where it comes from
The compound is transparent, یک (yek, one) plus صد (sad, hundred), both ancient Persian words. The same construction logic builds دویست (devist, two hundred, from دو plus صد with a sound change) and سیصد (sisad, three hundred). Yek-sad keeps the two parts visible, which is why it reads as more formal: the structure is on the surface, nothing has fused away. The Persian Academy and official style guides prefer yek-sad in formal writing for this clarity.
In our voice
Both forms are correct, both are everyday Persian in their own registers. Use sad in conversation and at the bazaar. Reach for yek-sad when you write a formal letter, fill out a form, or want a number to sound serious. The shift between sad and yek-sad is small in size but real in feel, and it is the kind of register move that separates basic Persian from fluent Persian.
References
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